Microclimate inside Cultural Heritage buildings

Adriana Bernardi Abstract The problem of Cultural Heritage conservation is growing in importance. This is due to the increasing interest and sensitivity in this field, as well as the large awareness of the uniqueness of the works of art that have been preserved up to the present generation. Nevertheless, not much knowledge has been developed up to now on the non-destructive diagnosis of the environment in which the works of art are being exhibited and preserved. The people in charge of the restoration, the transport, the exhibition and, at a general level, the management of the works of art often don’t have enough background on and familiarity with the Microclimate inside Cultural Heritage buildings. Hence, this book has been written with the aim to fill this big gap in the world of scientific publishing and to provide the basis of microclimatology applied to Cultural Heritage conservation. The subject is being covered a simple way, rich in suggestions and important case-studies as examples. Part of the book is focused on the theoretical background of microclimatic monitoring, with an in depth study of the methodology and the experimental apparatus. An even larger part is devoted to the description of field surveys and practices in important European buildings. Microclimate inside Cultural Heritage buildings Dr. Adriana Bernardi. Graduated in Physics at the University of Padua (Italy) in 1980. Senior researcher at the Institute of Atmospheric Sciences and Climate (ISAC), National Research Council of Italy (CNR). Since 1980 she works at the CNR, in the field of Microclimatology, Environmental Sciences and Conservation of Cultural Heritage. Since 1982 she is participating and coordinating numerous national and international projects, a lot of them supported by the European Commission. She has been assigned as evaluator of proposal for the 5FP "City of tomorrow" and for the Italian Ministry of University and Research. She is the coordinator of the Working Group 4 "Energy and Environment" in the ECTP- FACH (Focus Area Cultural Heritage) supported by the European Commission. She is the official delegate of FACH Group inside the "JTI Energy Efficient Buildings" of the European Construction Technological Platform (ECTP) supported by the European Commission. She is the scientific responsible of a Spin-off project financed by Italian Ministry of University and Research for the building of innovative technology in extreme environments. She has published about 150 scientific papers and she is member of several international and national commissions established by the Ministry of Cultural Heritage and Environment concerning artworks conservation and pollution effects (e.g. French Ministry for the Lascaux Caves) and for the establishment of Italian Standards (UNI). She has taught numerous courses in several Italian universities and she is actually professor at the University of Ferrara in the field of Conservation of works of art. She obtained a European wide patent for an instrument called "Dew sensor". She is editor-in-chief of the Journal "Bollettino Geofisico", published by the Italian Geophysical Association (AGI). 21.9 T (°C) 23.3 11.3 22.7 11.1 22.1 21.5 20.9 23.1 23.0 € 60,00 SH (g/kg) 23.9 20.3 10.9 10.7 10.5 19.7 10.3 19,1 10.1 18.5 9.9 RH (%) 80 78 76 74 72 70 68 66 64 62 60 58 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 1 Adriana Bernardi Microclimate inside Cultural Heritage buildings Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 2 The translation of this work has been funded by SEPS SEGRETARIATO EUROPEO PER LE PUBBLICAZIONI SCIENTIFICHE Via Val d'Aposa 7 - 40123 Bologna - Italy [email protected] - www.seps.it Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 3 MICROCLIMATE INSIDE CULTURAL HERITAGE BUILDINGS Contents Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 7 2. Examples of microclimate analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 11 2.1. A guide to thermohygrometrical parameters . . . . . . . . . . . . . . . . . . . . . p. 11 2.2. Examples of microclimate analyses in buildings under examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 11 3. Microclimate analysis and control for the conservation of works of art . . . . . . . . . . . p. 3.1. Air conditioning and heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 3.2. The public . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 3.3. Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. a) Natural lighting: penetration of solar radiation . . . . . . . . . . . . . . . . p. b) Artificial lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 3.4. Changing the air and cleaning the rooms . . . . . . . . . . . . . . . . . . . . . . . . p. 3.5. Compatibility of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 3.6. Microclimate and biological proliferation . . . . . . . . . . . . . . . . . . . . . . . p. 3.7. Which buildings should be used: ancient or modern ones? . . . . . . . . . p. 3.8. Deplacement of works of art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 3.9. Human comfort and conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 3.10. Structural and non-structural passive measures . . . . . . . . . . . . . . . . . . p. 3.11. Showcases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 3.12. Aesthetics or conservation? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 3.13. Economical interests or conservation? . . . . . . . . . . . . . . . . . . . . . . . . p. 19 19 31 34 38 38 45 49 52 55 60 62 65 67 71 74 4. Environment-related processes and phenomena that involve works of art . . . . . . . . . . p. 4.1. Condensation on surfaces or in micropores . . . . . . . . . . . . . . . . . . . . . p. 4.2. Materials and thermohygrometrical stress . . . . . . . . . . . . . . . . . . . . . . p. 4.2.1. Principles of propagation of heat and vapour . . . . . . . . . . . . . p. 4.2.2. Equilibrium moisture content in materials . . . . . . . . . . . . . . . . . p. 4.3. Atmospheric stability, ventilation and turbulence . . . . . . . . . . . . . . . . p. 4.4. Deposition of pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 4.4.1. Deposition processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 4.4.2. Models and software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 4.4.3. Analysis of observed cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 75 75 80 80 83 87 89 92 99 100 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 4 5. Analysis methods and measurement techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 5.1. Main thermohygrometric parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 5.2. Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 5.3. Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 105 109 114 125 6. Professionals involved in conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 127 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 129 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 131 Appendix 1: Calculation of main thermohygrometric parameters and of their functional mathematical relationship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 139 Appendix 2: Use of the psychrometric chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 147 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 151 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 153 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 155 Index most important terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 157 Color tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p. 161 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 5 FOREWORD This book is meant to be a guide for all people who deal with the conservation of works of art. The treatment will therefore be simple and comprehensible to everybody, regardless of their specific academic training. The use of scientific terms and formulas has been reduced as much as possible in the treatment of microclimatic analysis. It has not been possible to eliminate mathematical formulas completely, because the analysis is a physical one and for some readers the few formulas that have been introduced might be useful; but the text can be understood even if one skips the detail of the formulas. The book does not aim to be a comprehensive guide to all the principles on which the conservation of works of art must be based from all points of view. It only means to be an element in a very complex mosaic within the sphere of the conservation of works of art in indoor environments such as museums, art galleries, archives, churches, etc. It intends to collect all the experience I have gained during the many years of my activity in the area of the physics of the atmosphere applied to the conservation of works of art. The field of conservation is quite vast and practically involves all aspects of science. All the professional people involved - physicists, chemists, biologists, architects, restorers, etc. - must give their specific contribution and work in synergy with the others, while remaining within their own specific range of expertise. Without delving too deeply into the physics of processes, for which the reader will be referred, in the text, to a more specific literature, this book will discuss the main causes of the alteration of environmental conditions, the methods of analysis used for their identification, and the possible solutions adopted or recommended in order to make an exhibition environment suitable for the conservation of the works of art contained in it. Creating environmental conditions suitable for conservation is a very complex task, also because during the last few decades the massive increase in the number of visitors made it necessary to install a series of superstructures (heating, lighting, etc.), giving rise to an environmental dynamic that is far from simple. As a result, the people in charge of the management of exhibition environments have a complex problem to tackle when coordinating the work of the various types of experts, because a great variety of abilities are required in each of the specific fields involved. 5 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 6 Here, as an example, we will consider some basic sets of problems: which temperature and humidity conditions ensure that the exhibited works of art do not undergo excessively frequent or marked thermohygrometrical cycles? How should we decide which materials are to be regarded as similar from the viewpoint of their coexistence in the same microclimate? How can we make an unsuitable environment become suitable? Which is the role of the public and what problems have been created by mass tourism? How is it possible to remedy atmospheric pollution, which has considerably contributed to the acceleration of degradation during the last few years? And so on. In the discussion of these problems, the author used a lot of the results of the studies that have or are being carried out by herself in various buildings in Italy and abroad: for details, the reader will be referred, within the text, to the specific literature. The technical terms are defined in the Glossary, to which the reader is referred by means of an asterisk. 6 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 7 1. INTRODUCTION Pieces of cultural heritage, which are so numerous all over the world and particularly in Italy, must be protected, because they belong to the category of “non-renewable resources”. During the last few decades, atmospheric physics, and in particular microclimatology, have turned out to be very useful in the preventive conservation of works of art, and have contributed to the comprehension and solution of some problems that are at the basis of conservation. The problem of conservation, as a matter of fact, has become much more complex during the twentieth century, because of the increase in atmospheric pollution and the resulting changes in the environmental conditions, and, in indoor environments, because of mass tourism. In the past, sightseeing was a privilege enjoyed only by a few people; in our times, the great cultural openness that has fortunately been achieved has also given rise to a series of new problems relevant to the conservation of the cultural heritage. “To conserve” means to create a suitable environment around a work of art. What does “suitable environment” mean? It is not easy to determine this in an absolute sense, for a work of art; but it is not impossible to seek an optimum compromise among the various existing problems, also allowing for the visitors’ needs. To conserve, therefore, means to plan anthropic and/or natural measures in order to act on the most easily identifiable causes of the alteration of suitable environmental conditions. This will make it possible to reduce restoration to a minimum: the latter is necessary when a work of art has already deteriorated , it is not a preventive operation, and can be avoided by creating around a work of art the environmental conditions suitable for its conservation. The new outlook is oriented to acting so as to prevent restoration rather than taking measures after a damage has occurred. Creating suitable environmental conditions that reduce the need for restoration is often a very complex problem, because a great number of variables are involved. The solutions that can be proposed in order to make the microclimate suitable for conservation are sometimes quite simple, sometimes not at all. However, the many studies carried out up to now have revealed that 7 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 8 much can be done to improve conditions, because the new methods and technology developed during the last few decades make it possible to identify the problems and their causes as soon as they arise. In physics it is understood that “no spontaneous physical phenomenon is reversible”, therefore “any action, including natural ageing, produces an irreversible change…”. In the field of the conservation of works of art, too, reversibility does not exist: restoring or cleaning a painting brings about a damage; storing, handling or exhibiting it is never completely non-damaging; no treatment can ever be totally reversible. So - aside from a very few exceptions - it is safe to say that all works of art, like living organisms, undergo a spontaneous, irreversible deterioration. The only possible action is to slow down this process. Its causes may be manifold. A material, because of its intrinsic nature, is chemically and physically transformed during its life. It is chemically and physically attacked by the components of the atmosphere (oxygen, water, pollutants, etc.) or by radiation, both natural and artificial . For instance, each person is a source of heat and vapour, and emits into the atmosphere approximately 0.05 kg/hour of water vapour. So, a crowd of people in a small room results in a great accumulation of heat and vapour (in relation to the number of people and duration of their presence). The work of art that is exhibited, seeking equilibrium with the atmosphere, “responds” to these variations by absorbing a part of the heat and vapour emitted by the visitors, and returns them to the environment after the visitors have gone away. These phenomena of absorption* and release of heat and vapour are called “thermohygrometrical cycles” and lead to a deterioration of the object that depends on their frequency and amplitude: they give rise to processes such as stress, changes in the structure of the material, conveyance of salts, deposition of pollutants (the solid, liquid, vapour and gas produced by natural or anthropogenic origin), etc. All these phenomena, which will be discussed individually in this book, must be considered as a whole in a microclimatic analysis that aims at identifying and defining the correct measures for the improvement of the environmental conditions. It is by now well understood by all those who deal with the conservation of works of art that all the physical, chemical or biological processes they undergo are in close correlation with the microclimate: the latter affects the onset of these processes and controls their speed of reaction or subsequent development. So controlling the microclimate is essential for preventing the deterioration of materials. 8 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 9 What is less known or only partly understood is that the study of the microclimate: - cannot absolutely be boiled down to a few numbers, in particular to two, i.e. temperature and relative humidity, as is done, unfortunately, in many cases; - the numerous variables that are involved and must be considered, and the complex phenomena determined by their distribution, must be examined at the same time and in their dynamic: spatial distribution and time evolution. Interdisciplinary work is indispensable: there must be an interchange between the people acting in scientific fields and those acting in practical fields (restorers, workers, etc), in order to achieve an overall view of the problems involved and to determine the specific environmental conditions that are most suitable. Moreover, a correct environmental management is essential, and must be based on a specific knowledge of the environment under consideration. To study a microclimate means to analyse the environment that surrounds a work of art, in all its moments, both in its natural temporal evolution (day-night, seasons, etc.) and in relation to external forcing (air conditioning, heating, lighting, opening of doors and windows, cleaning, admission of visitors, etc.), most of which are connected to utilisation. This makes it possible to take measures, to optimise the environmental conditions, and to try to achieve stability, as is desirable in any place that is intended to contain particularly important works of art. 9 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 10 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 11 2. EXAMPLES OF MICROCLIMATE ANALYSIS 2.1. A GUIDE TO THERMOHYGROMETRICAL PARAMETERS In order to be able to describe some examples of studies that have been carried out, it is necessary to briefly introduce the three thermodynamic parameters that are basic in microclimate analysis. They will subsequently be described in a more complete manner and finally, in the appendix, also defined physically and mathematically. These three parameters are: - Temperature (°C) - Specific humidity (or mixing ratio) (g/kg) - Relative humidity (%) Temperature (T) is a well-known, self-evident parameter. Specific humidity (SH) (or mixing ratio (MR)) represents the amount of water vapour present in a certain air mass. It is an independent variable, and does not depend on the temperature of the air. Relative humidity (RH) is the degree of saturation of the ambient air. It depends on both of the previous variables, decreasing when the temperature increases, and increasing when the specific humidity increases. A 100% value represents the saturation of water vapour in the air. 2.2 EXAMPLES OF MICROCLIMATE ANALYSES IN BUILDINGS UNDER EXAMINATION After having concisely defined the three main thermohygrometrical parameters required for analysing the dynamics of the microclimate of an environment where works of art are preserved, we will now present a few practical examples selected among the various studies that have been carried out. (The methodological approach of the analysis will be described more in detail in Chapter 5; in the text, these studies, and others, will be considered again, in order to point out the specific problems connected with the microclimatic analysis of an environment). In this chapter, our intention is first of all to give the reader an overall idea of what we mean by analysis of the microclimate of an indoor environment. 11 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 12 The first study we will describe was carried out when this method was first applied, and, though its findings are now obsolete because of some a posteriori changes that were taken, the study in itself has a didactic value due to the completeness of its analysis and the clarity of its results. It regards the microclimate analysis performed in the Sistine Chapel (Vatican) in the nineteen-eighties [43, 51]. The conditions in the Sistine Chapel, as we have stated, are now changed considerably, because after this study radical measures were taken in the chapel: it was restored and the structures of its conditioning and heating system were renovated. The situation we will now discuss is the one previous to these changes. We will consider its most significant points and the most interesting conclusions; for a complete description, the reader is referred to the articles that are cited. Let us analyse the evolution of the environmental conditions within the Sistine Chapel at the most significant moments of a normal day. The environment remains undisturbed throughout the night, so the indoor atmosphere attains a certain equilibrium with the walls. In the morning, the usual cleaning is carried out, then the doors Sistine Chapel - Rome (Italy) Fig. 1. Horizontal sections of the Sistine Chapel in Rome, representing the isolines related to temperature in three moments of the day: during the night (22:20) a quite homogeneous distribution can be observed; in the early morning (8:15), they can be noticed the maxima, along the walls, and the minima due to colder air intake from the doors opened during the cleaning operations; in presence of the public (12:45), they are evident marked thermal maximum in the center of the room. 12 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 13 and windows are opened to change the air. Air masses with a different thermohygrometrical content flow into the room: there are minimum temperature values at the centre and maximum values near the walls (Fig. 1). The walls, because of their high heat capacity, remain warmer than the colder ambient air that flows into the chapel from the outside. In this situation, the walls “respond” to these gradients* by seeking a new equilibrium with the different atmosphere around them and exchanging heat and vapour: this is one of the most stressful moments for the precious surfaces. From 9 o’clock on, the public starts flowing in. All the active systems (forced-air heating from grilles on the floor, lighting, etc.) are in operation. The presence of a constantly numerous public worsens the thermodynamic situation, because of the considerable addition of heat and vapour. The microclimatic conditions are inverted with respect to those of the moment of cleaning, and the maximum temperature values are at the centre of the chapel (Fig. 1, 2), while the minimum ones are along the walls. The drier air that flows along the walls highly encourages evaporation (Fig. 3), dehydrating the walls and leading to the appearance of little cracks. This is another highly dangerous moment, because of a series of processes, all of which are harmful to the conservation of the paintings. The thermodynamic equilibrium that the walls were previously trying to achieve is altered again, because the flows of heat and vapour between Sistine Chapel - Rome (Italy) 31 July 1984, h 10:30 Fig. 2. Horizontal sections of the Sistine Chapel in Rome, representing the isolines related to temperature and specific humidity in presence of public. They are evident marked both thermal and hygrometric maxima, due to the heat and vapour contribution from the visitors. 13 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 14 the walls and the ambient atmosphere are inverted; this condition of risk persists throughout the visit hours. A few hours after closing time, when all the sources of energy have been switched off and all the doors and windows have been closed, the microclimate of the chapel gradually returns to its natural condition of equilibrium. The gradients* between the walls and the indoor atmosphere disappear, and the thermal map becomes very homogeneous. All the physical processes tend to disappear, so that no more fluxes of heat an vapour exist: these are the optimum conditions for the conservation of the priceless artistic heritage of the Sistine Chapel. Sistine Chapel - Rome (Italy) Fig. 3. Evaporation rate (Er) measured close to the painted vault of the Sistine Chapel in Rome with a Piché vapour gauge simulating the evaporation of the masonry. The curve gives us an indication on the moment in which masonry could be in evaporating conditions. To notice, the relationship with the turning on of the lighting system placed close to the vault. As a second example, we will consider the microclimatic analysis carried out on Leonardo da Vinci’s Last Supper [48]. In this case, too, the main problem - which by now has been solved in a fairly satisfactory manner by the structural measures taken after the analysis - was the fact that entrance of the public was uncontrolled, and brought in heat, vapour and pollutants. Moreover, several openings in the room allowed a constant inflow of air masses with different thermohygrometrical characteristics and full of pollutants: in those years, the visitors’ buses were often parked in the forecourt of the building. The active heating and lighting systems were in op14 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 15 Koninklijk Museum voor Schone Kunsten - Antwerpen (Belgium) Van Dyck's room - 17 July 1999, h 14:00 Fig. 4. Horizontal sections of the Room I of Van Dyck of the Antwerp Museum, representing the isolines related to temperature, relative humidity and specific humidity in presence of public. They are evident marked both thermal and hygrometric maxima, due to the heat and vapour contribution by the visitors. To be noticed, the maximum of relative humidity low down in the centre, in correspondence with the maximum of specific humidity due to the vapour emission from the humidifier placed under a Van Dick’s painting. eration intermittently during the daytime, because they followed the schedule of opening hours. The Last Supper painting was therefore immersed in an environment that was subjected every day to changes - even sudden ones - in temperature, specific humidity and relative humidity: its surface had to respond to these influences with new thermodynamic equilibria, and this gave rise to stress and deposition of pollutants. At the Museum of Antwerp, Belgium (Koninklijk Museum voor Schone Kunsten), in the area for temporary exhibitions that was occupied at that time by works by Van Dyck [59], the attempt to keep a certain painting by that famous artist in the same conditions of temperature and humidity as those of its original place (a very humid church) led to the installation of two humidifiers right under the painting. The situation of the room in which the painting was exhibited was far from thermodynamically stable: there were variable gradients* due to the presence of a great number of visitors and to the use of active lighting and conditioning systems (the exhibition took place during the summer). The two humidifiers had been set to start when the relative humidity became lower than that of the original environment of the painting. Obviously this led to an inter- 15 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 16 mittent activation of the system, so the painting was subjected to constant thermohygrometrical cycles, therefore to stress (Fig. 4). The thermohygrometric daily trend of this room, in which the microclimate was controlled in an injudicious manner, is shown in Fig. 5. The overall result of the effects of the various factors - climate, visitors and humidifiers - was a considerable series of daily oscillations in temperature and humidity. In the Rubens room, the heating was thermohygrometrically controlled, but its intermittent operation resulted in continuous variations in the indoor thermohygrometric conditions. As you can see, for instance in Fig. 6, the temperature of the room changed by several degrees in a few hours, and this shows that the system was incorrectly set and/or that the control probes were incorrectly positioned. Koninklijk Museum voor Schone Kunsten Antwerpen (Belgium) - Van Dick's room Fig. 5. Daily trend of temperature and relative humidity in the Room I of Van Dyck in the Antwerp Museum; they are evident both thermal and hygrometric maxima, due to an incorrect environmental management. 16 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 17 In the three cases described, the constant attempts of the walls and works of art to reach new equilibria were the cause of the stresses undergone by the materials, as a result of a series of physical processes of absorption* of vapour and heat that will be described in the next paragraphs. These examples, in any case, give the reader an idea of the meaning of a dynamic parameter analysis and of the information provided by it. In the next chapters we will carry out a more detailed discussion of the various problems, and of the methods for identifying, reducing, and, when possible, eliminating them. Koninklijk Museum voor Schone Kunsten - Antwerpen (Belgium) Room I, Rubens - 10 February 1999 Fig. 6. Horizontal sections of the Room I of Rubens in the Antwerp Museum, representing the isolines related to temperature. Absolute values of temperature and distribution of thermal gradients vary within few hours. 17 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 18 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 19 3. MICROCLIMATE ANALYSIS AND CONTROL FOR THE CONSERVATION OF WORKS OF ART Controlling the microclimate, it is essential for the conservation of works of art; it is all the more important because works of art are made up of materials that are composite and/or sensitive to thermohygrometrical variations. Sensitive materials, such as organic ones (e.g. wood or paper) need stable conditions, in terms both of relative humidity and of temperature. The adsorption* or desorption* due to humidity or temperature variations leads to expansion or contraction, and eventually to the deterioration of a work of art. Quick thermohygrometrical variations, moreover, do not give the material the time to reach the new equilibrium values, because the latter change continuously. It is necessary, therefore, to control the microclimate of the environment so as to establish the most suitable conditions and inhibit variations, particularly the quickest ones. The variability limits of the thermohygrometrical parameters are often much more narrow than what one supposes, and the amplitude of the environmental variations is considerably greater than what one wishes. For instance, for the conservation of paper, it has been established not only that optimum conditions consist of not excessively high temperatures and medium relative humidity values, but also that the variations in relative humidity must not exceed 5% [74]. The analyses carried out in a great number of national and international museums, in more than 25 years of experience, have shown that in many cases this environmental variation is greatly exceeded every day and even several times a day, because of various physical forcing which we will now describe, and which are often due to an inappropriate management. 3.1 AIR CONDITIONING AND HEATING In the control of microclimatic conditions for a good conservation of works of art, a very sensitive point is the management of the air conditioning and heating system. In many cases, as a matter of fact, the heating systems have been made on the basis of the know-how and technology applied to the field of civil buildings. In this area, technicians have all too often been trained to solve the problem of “heating for humans” and are completely unaware of the problems of the conservation of works of art. When they are commissioned to install a system in a museum, they apply methods and techniques that are quite satisfactory for the needs of buildings for 19 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 20 civil use (dwellings, offices, etc.). The heating of rooms where works of art are to be exhibited, on the other hand, must be based on criteria that are different from those usually adopted for the benefit of human beings: it is necessary, above all, taking into account that any system, and its layout, rating and setting, may affect the spatial-temporal distribution of the environmental variables, particularly those relevant to the temperature and relative humidity. These are the two parameters that control the stress of a material. Their mean values, and above all their variations (and the amplitude and frequency of the latter), are the greatest cause of the deterioration and more or less accelerated ageing of works of art, and of all the other related chemical and biological phenomena. Unfortunately, in the decisions about the development of a system, all too often the well-being of humans takes priority, because “the public must be comfortable”, and because it is believed that the materials have similar tolerability limits. In many cases this is not true, as we will explain further on. For various reasons, controlling the thermohygrometrical conditions of an environment, when active systems such as air conditioning and heating are to be used, is a rather complex problem, much more than one can imagine. First of all, to achieve a homogeneous spatial distribution of temperature and humidity is far from easy: the inflow of hot/cold air or vapour from individual points gives rise to spatial gradients* that are often quite marked and result in stress for the materials: the latter seek new equilibria that constantly change in time and space. Furthermore, it is necessary to be extremely careful in checking the operation of the heating/conditioning/humidifying systems by means of probes set to the recommended thermohygrometric values, for two reasons. The first reason is that relying on control probes (usually placed in several points in the room) for controlling the system often leads to intermittent operation, therefore to the creation of constant thermohygrometrical variations. The second reason is relevant to the concept of “optimum values or ranges recommended for the various materials”. The utmost caution is needed when simplifying this problem by supplying a “range of acceptability”. Materials are quite different from each other, even if they belong to the same group and are loosely called, e.g., “wood” or “marble”. Their physical characteristics have changed over the centuries, and they have become more fragile, in relation to their history and vicissitudes. So, for a correct management of the microclimate in general and of active systems in particular, it is necessary to have a much broader outlook and to pay 20 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 21 attention to all physical, chemical and biological phenomena and processes that may require certain types of environmental management. We must absolutely not reduce the control of a heating/conditioning/humidifying system merely to two numbers that indicate the temperature and humidity values “suitable” for the materials that are exhibited. Another sensitive point in the development of heating and air conditioning systems is connected to the fact that in many cases the rooms themselves are works of art, because they are a part of a historical building. To act upon the structure in order to develop a system of this type is not always easy or simple, or even permitted by the authorities. Therefore whoever decides, at all costs, to install a heating or air conditioning system within a historical building will have to struggle with countless restrictions and difficulties, and the outcome will be that the system will be more likely to be suitable for human comfort than for the conservation of the works of art. In any case, the available solutions are often a “compromise between various needs”, and must therefore be studied for each specific building. First of all it is necessary to be acquainted with the specific microclimate of a building; then, as the changes are gradually carried out, efforts must be made to achieve results that improve the existing conditions, while moving in the desired direction, that of the completion of the system. Moreover, one of the main problems in the correct management of an environment is the fact that, for several reasons, the heating/air conditioning and lighting systems are operated intermittently. The result is a microclimate that is not suitable for the conservation of precious materials: it is difficult for an intermittent operation to achieve stable thermohygrometrical conditions, because during their periods of operation these systems suddenly emit an excessive quantity of energy that is subsequently dissipated into the environment. The main goal is always to achieve suitable thermohygrometrical conditions that are as constant as possible, both in time and in space. This must be obtained by using active systems as little as possible, and also by allowing for the local climate that determines the basic environmental conditions of the building: thermohygrometric cycles may lead to the crystallisation of salts (always present in building materials), and this may be an undesirable effect if, for instance, the surfaces are painted or frescoed [2]. So the stability of the environmental conditions is an essential point in the conservation of works of art. But which are the temperature and humidity values suitable 21 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 22 for the conservation of a work of art? There are some astonishing cases of works of art that have survived for thousands of years in particular, thermohygrometrically stable, conditions, for instance underground or immersed in water, theoretically not ideal conditions for the materials of which the works of art are made. Only after they were discovered and brought to the surface, in suddenly very different conditions, these works of art underwent an irreversible deterioration. An example is that of certain Egyptian tablets, made of materials, such as sandstone, that are highly sensitive to thermohygrometrical variations: they survived in an excellent state of conservation for thousands of years in very dry ground. Once they were unearthed and exhibited in museum environments not suitable for their conservation, or transferred to regions with a completely different climates, they underwent a gradual, irreversible deterioration, due to ample thermohygrometrical variations. As an example, we will now analyse some researches that have been carried out both in Italy and abroad, and that confirm what has been stated above (for details, the reader is referred to the relevant literature). In practice, heating/air conditioning systems are often constructed by placing the active elements in some points of the rooms (sometimes even under the works of art!), against the walls (e.g. fan coils or radiators) or on the floor (e.g. grilles for air inflow or heated coils). In these points, the thermohygrometric characteristics of the masses of ambient air become quite different from those of the surrounding air (warmer or colder, more or less full of vapour). Thus a distribution with maximum or minimum relative thermal or hygrometric values appears, and this gives rise to gradients* that are often particularly marked and variable in the time. In most cases, heating/air conditioning systems are equipped with fan coils, sometimes combined with humidifiers. It is rare, however, for the latter to be correctly adjusted so as to balance the effect of the temperature on the relative humidity and to keep it as constant as possible in time and space. In order to keep the relative humidity constant, it is possible to act on the two parameters on which it depends: temperature and specific humidity (or mixing ratio). If both these parameters are constant, as is desirable, obviously the relative humidity is constant too; but if one of the two parameters changes, it is necessary so act suitably on the other one so as to ensure that the relative humidity keeps a constant value, and this is not always technically easy. Sometimes it is simpler to act on the temperature, despite the fact that the latter, particularly for some materials such as organic ones, should be kept as constant as possible or should at least undergo only very slow changes. 22 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 23 Correr Museum Venice (Italy) Bellini room 15 March 1993 h 12:30 Fig. 7. Horizontal sections of the Bellini Room in the Correr Museum, Venice, representing the isolines related to temperature, relative humidity and specific humidity. Thermal distribution presents a maximum in correspondence of a fan-coil; the maximum in specific humidity isolines indicates the presence of a humidifier. 23 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 24 The use of humidifiers (which raise the specific humidity value of the air) placed in a particular point in the room is usually inadvisable, because in the areas near the points of vapour outlet the hygrometric levels are quite high, and give rise to conditions of dangerous unhomogeneity. This risk increases where the distribution of the system is unsatisfactory or if there is a failure. A situation like this was found, for instance, in the Correr Museum in Venice (Italy) [54, 58], where a heating/air conditioning system that was incorrectly distributed and operated intermittently (because of limitations due to the system itself) had been balanced by the temporary installation of a series of humidifiers in some rooms, next to the fan coils. The result (e.g. in the Bellini Room, Fig. 7) was that the relative humidity showed marked gradients*: the steps that had been taken were useless, because they were producing gradients* of vapour concentration, with consequences that were harmful for the paintings. The measurements performed over a long period (from 1992 to 2000) revealed various situations that depended on the season, on the positions of the sources of warmer or colder air and of the humidifiers, and on the management of the system, which, because of several complex problems, could not run in a regular manner. During the summer, minimum temperature values were present near the points of emission of conditioned (therefore colder) air. The corresponding distribution of the specific humidity, however, was characterised by the presence, in the same points of minimum temperature values, of maximum or minimum specific humidity values, depending on whether a humidifier was in operation or not near the air conditioner. The result, in any case, was a relative humidity with marked gradients*, despite the fact that the minimum temperature values and the minimum specific humidity values, when correctly adjusted, usually tend to achieve a mutual equilibrium, ensuring a stable relative humidity. The same situation is experienced in a room of the Diamanti Palace in Ferrara (Italy) where measurements were performed for one year (summer 2006-spring 2007) in a room of the palace [136, 137, 138, 139]. There, a humidifier is used to dampen the variation of the relative humidity inside the room. The aim of the museum manager was to maintain the thermohygrometrical conditions in front of the paintings constant, and precisely around 55%. The humidifier was positioned in a corner of the room, where a torn frescos is in its proximity. The study covered the control of 2 paintings from the thermohygrometrical point of view. They are positioned on the 2 walls opposite to the corner where the humidifier is placed. When the sensor regulating the humidifier detects a relative humidity below 55% the humidification of the air starts. In order to control the efficiency of the system and the cycles experimented by the paintings, a thermohygrometer was putted near the humidifier and precisely in correspondence to the adjacent frescoes. 24 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 25 The results of this study are shown in the Fig. 8. This Figure 8 proves in fact that this device is ineffective. In fact its functioning is irregular: continuous on/off occur and the device was not perfectly functioning (sometimes it didn’t start up notwithstanding that the value of the relative humidity was a lot less than 55%). The values of relative humidity of the 3 points controlled (2 paintings and frescoes) were very different with different cycles. So, the aim to control the relative humidity was on the contrary substituted by a extremely dynamic situation with variable gradients* in the room. In this way the thermohygrometric cycles of the material exposed increase in number and frequency. This is clearly evident in the Fig. 8 with a certain delay the air in front of the paintings follows the on/off of the device; as a consequence the materials adsorb and desorb the water supply. This is even more valid for the frescoes positioned in proximity of the humidifier. This situation is typical of a great number of museums. In fact the same situation was detected, also by the measurements done in 1995 at the Louvre Museum [54, 63], in the rooms where the Italian paintings are exhibited. The incorrectly distributed emission of heat and vapour from specific points gave rise to variable gradients*; the latter led to constant thermohygrometrical cycles in the air, affecting the paintings (Fig. 9). In general, the heating/conditioning of the rooms, in any case, is one of the most sensitive problems, because the management of the systems normally provides for an Diamanti Palace – Ferrara (Italy) Fig. 8. Trend of air relative humidity in front of the painting 1, 2 and the humidifier. 25 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 26 Louvre Museum, Italian Paintings – Paris (France) Fig. 9. Temporal trend of temperature and relative humidity in the Italian Paintings Room in the Louvre Museum, Paris. To notice the evident thermal cycles and relative humidity variations occurring in few days. Louvre Museum, Cour Marly - Paris (France) Fig. 10. Temporal trend of temperature in the Cour Marly of the Louvre Museum, Paris. To notice the daily fluctuations due to the setting of the heating system. 26 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 27 intermittent switching on, connected to opening hours. This is considerably harmful, because it creates constantly variable microclimatic conditions, with daily shocks and thermohygrometrical cycles whose consequences are obvious. The operation of the systems must be constant throughout the season in which it is used, and the thermohygrometrical level of the rooms must be set allowing both for natural conditions and for the use of the room (with the consequent possible input of heat and, Fig. 11. Examples of an air grid placed under the above all, vapour), so as to ensure that paintings. the relative humidity is as constant as possible, without nighttime interruptions. Measurements performed in several museums, on the contrary, clearly revealed the presence of daily oscillations, which in some cases defeated the effort of sophisticated systems operated during the opening hours, as, for example, in the Cour Marly of the Louvre Museum in Paris (Fig. 10). We must remember that works of art must not be kept near sources of heat or vapour, as, unfortunately, occurs in a great number of museums (Fig. 11). If no other solutions are available, it is better to place the sources of heat or vapour under the windows or at the centre of the rooms. This allows the masses of heated air to approach the works of art on the walls after having been mixed at least partly with the air masses that are present, so the stress undergone by the works of art is less intense, despite the fact that near the fan coils the gradients* may be quite marked (Fig. 12). The old heating system of the Sistine Chapel before its restoration in the nineteeneighties, for instance, consisted of very hot air coming out of two grilles on the floor at the centre of the room [44, 52]. The system was switched on only during opening hours. In the points of air inflow, the thermohygrometrical conditions were very different from those of the rest of the room. Near the grilles there were marked thermal or hygrometric maximum and minimum. These gave rise to dangerous gradients* that were eventually distributed all over the room, obviously leading to dangerous thermohygrometric cycles in the walls, and to the shifting and mixing up of air masses. For Leonardo da Vinci’s Last Supper, too, when measurements were performed in 1982-83, the heating was on the floor. The system [49] was switched on only during the winter and not all the time. During the periods of operation, the entire floor supplied an energy that, though becoming distributed in the whole room, heated up the 27 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 28 Koninklijk Museum voor Schone Kunsten - Antwerpen (Belgium) T Room - 13 February 1999, h 10:30 Fig. 12. Horizontal sections of the Room T of the Antwerp Museum, representing the isolines related to temperature, relative humidity and specific humidity. To notice, the maxima in correspondence of the radiators placed in the centre of the room. air from below. The heated floor was a source of vertical instability (physically defined in Paragraph 4.3), i.e. of a situation that promoted dangerous convection movements, resulting in transport of pollutants, deposition of dust, evaporation from the painted surface, etc. In order to solve the problem of an unsuitable environmental microclimate, the control of the thermohygrometrical conditions is often carried out by installing sophisticated, expensive thermohygrometrically balanced heating and conditioning systems, in which the relative humidity of the air is checked before the air is introduced into the environment. The use of these systems may sometimes turn out to be a double-edged measure: an unsuitable environment may thus be transformed into a suitable one, but how is it possible to guarantee the perfect, and above all constant, operation of these systems? A few hours of malfunction may be enough to subject the works of art to severe shocks and stress, leading to irreversible damages: the automatic monitoring systems may be set in an incorrect way, or lose their calibration, or break down. It is better, first of all, to become acquainted with the environment in its natural behaviour, which depends on its architectural structure and on the local climate, by 28 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 29 studying it thoroughly during its various daily and seasonal cycles. Only after having acquired an in-depth knowledge of the specific environment, one must endeavour to exploit, as much as possible, its potential for microclimate stabilisation, by adopting remedies that may be called “passive” and are less invasive, (for instance thermal insulation, protection from direct solar radiation, or use of materials, such as wood, that contribute to the preservation of a constant relative humidity), instead of immediately choosing the installation of sophisticated, expensive “active” equipment. The latter, because of an incomplete knowledge of the environment under consideration, is often designed and subsequently managed in an incorrect manner, with an excessive input of heat and/or vapour. This may be unimportant (except for the aspect of energy consumption) in a private dwelling, but it must decidedly be avoided in conservation environments, where an unnecessary input of energy and/or vapour always leads to undesirable results: any excess of energy and/or vapour may result in needless thermohygrometric cycles in the materials that are present. All this was evident also in the numerous microclimatic measurements performed for many years (from 1992 to now) in some rooms of the Uffizi Gallery in Florence (Italy) (Giotto, Leonardo, Pollaiolo and Botticelli Rooms) [17, 54, 61]. The management of the Gallery undertook to cooperate with the researchers, who, using the rooms as a laboratory for their study, endeavoured to find possible solutions for the improvement of present conditions, which are very difficult to manage, in all museums frequented by large numbers of visitors, at the same time particularly extensive and historical buildings. In the specific case of the Uffizi Gallery, the solutions for the improvement of the inner microclimate are not always easy to achieve, because of the complexity of the situation. The measurements mentioned above revealed, for instance, that the Giotto and Leonardo Rooms did not always have the constant thermohygrometrical conditions that the management of the Galleries has been trying to achieve with a sequence of provisions. The Giotto Room is not air-conditioned, while the Leonardo one is equipped with a sophisticated system since 1991; yet the thermohygrometrical conditions of the two rooms are not essentially different, as was to be expected after the work performed in the Leonardo Room. This is a consequence of a series of problems that are present, not least of which is the intermittent operation of the heating/air conditioning system: in both the rooms there are some points where, in some moments of the day, there are marked gradients* due to the inflow of thermohygrometrically very different air from the adjacent rooms. In the Giotto Room, moreover, the inflow of vapour from the humidifiers (which endeavour to balance the effect of the hot, dry air introduced from the ceiling by the central heating system) gives rise to localised gradients* of specific humidity. In the Leonardo Room, the gradients* appear not only because of exchanges with the adjacent rooms, but also because of the particular circulation of the conditioned air, due to the position of the intake grilles. These grilles existed before the installation of 29 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 30 Uffizi Galleries - Florence (Italy) Botticelli room - 18 February 1997, h 17:00 Fig. 13. Horizontal sections of the Botticelli Room in the Uffizi Gallery, Florence, representing the isolines related to temperature, relative humidity and specific humidity. To notice, the marked gradients correspondence of the humidifiers placed in the centre of the room. the new system in 1991, and had to be used, in order to limit work on the ancient structure. In the Botticelli and Pollaiolo Rooms, too, the heating systems are set in an unsatisfactory manner and compensated for by humidifiers. The gradients* that have been found are often quite marked, particularly near the humidifiers (this is true, for instance, for the Botticelli Room, fig. 13, where the humidifiers, fortunately, are placed at the centre of the room, far from the paintings on the walls). In any case, many of the systems are set in a random manner, leading to considerable variations both in temperature and in relative humidity; the latter reaches very high values (90%) because the setting of the humidification system does not correctly compensate for the thermal level (for details, consult the article that describes the study in all its aspects [58]). These marked oscillations in relative humidity lead to stress and to rather considerable deformations. In this regard, Toratti and Svensson [124], in the article quoted in the Bibliography, demonstrate that after even only one day of variations in relative humidity from 40 to 90% at 20°C, the wood samples show deformations that are quite noticeable and are different in the adsorption* and deadsorption* stages, thus confirming that this type of process is irreversible. (When the adsorption* and deadsorption* curves are not alike), the word “hysteresis*” is used: this phenomenon is typical of porous materials). 30 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 31 The damages due to the fact that the heating and air conditioning systems are switched off during the night and suddenly switched on during the daytime are among the most common ones. There exist, however, some situations that require technical solutions based on active systems, because the rooms are intrinsically not suitable for the conservation of very delicate works of art. Certain modern buildings whose structures have a low thermal capacity* could not be used for preserving works of art unless a controlled air conditioning and heating system were activated in them, respectively during the summer and winter. In fact, the external climate, severely affects the indoor microclimatic conditions. Certain areas of old buildings, too, would not be suitable for the exhibition of works of art without the help of active control system: this is true, for instance, for rooms that are affected by climate variations, such as the last floor of a building, which is in direct contact with the attic. Attics, in fact, if they are not adequately insulated or well ventilated, may not possess enough thermal inertia to reduce the external stresses connected with the alternation of the seasons and, above all, of day and night: these stresses may be felt also in the rooms below, leading to dangerous variations in the thermohygrometrical parameters of the rooms’ atmosphere. 3.2 THE PUBLIC A high concentration of public brings in a considerable amount of heat, vapour, carbon dioxide and other substances, and alters the natural equilibrium of the ambient air, giving rise to exchanges between the air and the walls. This problem is not as easy to solve as one might believe. Museums, exhibition rooms, churches, etc., are places designed to be used by the public, but undoubtedly any massive concentration of people should be avoided, when possible. Very often, nowadays, many rights are allowed to the public, and sometimes this is achieved to the detriment of the works of art: the environmental conditions attained after the installation of certain active systems, such as heating, air conditioning and lighting, are often unsuitable for conservation, and - we must add - not always indispensable for obtaining acceptable environmental conditions. An excessively large public may transform a room that is essentially suitable for the conservation of works of art into an environment that is characterised by considerable thermohygrometrical gradients*. 31 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 32 Let us consider two examples. 1) The natural emission of heat from people. It has been estimated that a person emits every hour about 150 W of heat. In cases of high number of visitors, for instance in the Sistine Chapel, we can calculate that a normal attendance (on the average approximately 6000 persons a day, concentrated in the few opening hours and staying on average for about 30 minutes) may lead to the introduction of approximately about 450 kW and this value may reach peaks of 1100, 1300 kW in the days of maximum attendance (about 15 000-17 000 people a day) [44, 52]. If a comparison is done with the emission of a lamp of 100 W, this emission corresponds to the opening of about 1500 lamps for 3-4 hours and to 4500 lamps during the peaks of presence. 2) The emission of water vapour due to the natural breathing and perspiration of people. It has been estimated that a person emits about 50 grams of water vapour per hour. This means that in the Sistine Chapel, we can calculate that a normal presence of public may lead to the introduction of approximately 150 kg/hour of water vapour into the environment; with peaks of 250-300 kg/hour in the days of maximum visitors. These simple calculations allow us to understand that the presence of the public, by itself, leads to the introduction of a considerable amount of vapour, which will be eventually adsorbed* by the walls; the latter, during closing hours, will tend to return the vapour to the air. This contention is valid also for the heat emitted by the human body. A massive presence of public during opening hours, therefore, results in a considerable alteration of the environmental microclimate, because of the appearance of marked spatialtemporal variations and of constantly variable thermohygrometrical dynamics. During opening hours, moreover, windows and doors are opened, the lighting system is switched on, and, in some seasons, the heating or air conditioning system is activated. A useful example of the microclimatic variations connected with a massive presence of public can be found, once again, in the study performed in the Sistine Chapel. Here it was observed that the presence of an excessively great number of people considerably alters the natural optimum microclimatic conditions due to a structure that supplies an environment suitable for the conservation of works of art. Another example can be found in the Marciana Library in Venice (Fig. 14) [19], where it was observed that the rise in the indoor temperature during the summer is chiefly due to the presence of readers in the library and to the heat they introduce, both directly and because of the lighting they use sometimes for reading. 32 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 33 Marciana Library - Venice (Italy) 11 August 1995, h 13:00 Fig. 14. Horizontal sections of the Marciana Library, Venice, representing the isolines related to temperature. To notice the thermal maximum in the centre of the room, due to the heat contribution from both the readers, and the reading lamps. To avoid big crowds and to limit the duration of visits in an environment is the first and simplest rule to be followed. In order to ensure that no visitor is denied the chance to enjoy the sight of a work of art, it is necessary, therefore, to find alternative solutions that are valid and satisfactory. A simple solution, which has already been adopted in some modern museums, is the projection, before the visitors’ access to the rooms or while they are waiting, of a film that explains and shows some details that cannot always be noticed during the actual visit, and then to limit the duration of the visit to a predetermined time. This solution, besides furthering the management of the environment and the conservation of the works of art, is also the most complete one from a cultural point of view, besides being the most instructive and ultimately most satisfactory one for the visitors. Obviously the projection should take place far from the rooms where the works of art are exhibited and not within those rooms, as is done sometimes, with the result of prolonging the duration of the visitors’ presence. It is advisable, in any case, to avoid as much as possible a considerable concentration of people, particularly in a small room. It is always advisable to spread the presence of people over a longer period. 33 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 34 3.3 LIGHTING All the environments (museums, churches, libraries, etc.) that may contain works of art or objects to be preserved must be equipped with a suitable lighting system, natural or artificial in relation to the time, season and architectural structure. To speak of lighting, however, means to speak of energy, an energy that necessarily is absorbed* also from the works of art. This energy absorbed by the material gives rise, within it, to phenomena that range from simple warming to more complex physical or chemical reactions. It is not our intention, here, to write a treatise on lighting techniques; but we have to take into account that the energy introduced by the lighting system is not a negligible aspect in the environmental equilibrium required for the conservation of works of art, so for a safe microclimate. All materials (solids, liquids, gases and vapours) when at a temperature greater than 0°K emit electromagnetic waves of different wavelength and frequency. Physics reveals that light energy is emitted by a source (the sun or a lamp) with a specific spectrum that depends on its temperature. This is expressed, for a “blackbody*”, by Wien’s law [86]: λm . T = constant = 2897.8 where T is the absolute temperature in Kelvin degrees (K) and λm is the wavelength of the maximum of the emitted radiation expressed in microns (μm). This means that, depending on the temperature, the spectrum of the emitted radiation is centred on a specific wavelength, λm. The sun, our main source of light, has a temperature near 6000 K. Its emission spectrum is characterised by a radiation centred around 0.5 μm, i.e. “visible radiation”. The Earth has a temperature of approximately 300 K and an emission spectrum centred around the infrared range (around 10 μm) (Fig. 15). The radiation that is used for looking at a work of art is obviously the one whose wavelengths are within the visible range (the reason why this range is given this name is selfevident). The other wavelengths cannot be used for vision, but are more or less energetic in any case, so they are harmful to the materials. The spectrum of the light emitted by the source must therefore be centred around the frequency band of the visible range, reducing, as much as possible, the frequencies relevant to the infrared radiation (IR) and to the ultraviolet one (UV), both of them harmful, though for different reasons. The exposure of the works of art to light causes 2 main degradation processes: photochemical and warming. The first is due to the electronic excitation which is the 34 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 35 Fig. 15. Spectra of the sun radiation (temperature about 6000 K), centered on the visible radiation wavelength (380 nm< λ<1mm) and of the earth (temperature about 300 K) centered on 10 µm, i.e. on the infrared radiations wavelengths (750 nm< λ<1mm) . (1 nm=10-9 m). basic process in the absorption* of light in the visible and ultraviolet bands. This causes a chemical change, modifying the mechanical and chromatic properties of the material until irreversibility is reached. This results in colour changes, i.e. alteration of the binders and organic pigments, and subsequent fading and yellowing. The absorption* of light in the infrared band causes an increase in temperature, with the same effects, is molecular excitation [80]. For conservation purposes, therefore, it is necessary, first of all, to endeavour to light the object with an energy whose emission is mostly within the visible range, eliminating, as much as possible, all the rest of the spectrum. We must remember that, even if the wavelengths of the radiation falling on the object are only within the visible range, the fraction absorbed by the material is subsequently transformed into heat. This means that an object that receives energy always tends to warm up. The use of a suitable lighting system is therefore one of the problems that must be tackled in a correct and competent way, and in any case the solution is not easy 35 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 36 Fig. 16. Solar radiation penetration in the Sistine Chapel during the months of February and May. Vertical section of a window (continuous line represents the penetrating beam at 9:00 and coincides with the one at 15:00; the dotted lines correspond to 10:00, 11:00 and 12:00). to achieve. To find the good compromise between the need of the observer and the needs for a good observation is the main aim to have to reached [62]. The warming undergone by a work of art always leads to undesirable consequences from the viewpoint of conservation. In fact the higher is the radiant flux and the absorbance of the material, the higher will be the heating of a surface. Let us consider the example of a wall painting. The warming takes place starting from the surface, then spreading to the inside. In many cases, the expansion coefficients of the painted surface are different from those of the underlying plasterwork, so the warming and cooling cycles involve constant and often sudden movements that are different in the two layers and inevitably give rise to cracks, or, in the worst cases, cause the painted layer to peel off. If we consider the example of a painting on wood, we may also notice deformations due to changes in the size of the material. On antique glass decorated with grisaille*, the latter tends to peel off the underlying glass, and the damage is irreversible [24, 25, 28, 108]. On any type of artefact, in conclusion, radiation, depending on its spectrum, supplies an amount of energy that is sufficient to act on the pigments and to give rise to local overheating, alterations of the organic binders, little cracks, dehydration, and conveyance of heat, vapour, pollutants, dust, etc. 36 16-10-2008 13:25 Fig. 17. Solar radiation flux R (KW/m2) in function of the time T (hours) on surfaces differently oriented, and for a latitude ϕ of 38°, 45° and 56° for the two solsitices δ=0. τ is a coefficient linked to the sky transparency. Position of the surface: H = horizontal; N = North; NE = North East; E = East; SE = South East; S = South; SW = South West; W = West; NW = North West [15] Pleiadi1_16102008:Pleiadi_eng_16102008 Pagina 37 37 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 38 a) Natural lighting: penetration of solar radiation For a correct environmental management, it is therefore important to study the environment in its complex, carefully considering its architectural structure and orientation. This makes it possible to identify the surfaces on which the solar radiation coming from the openings (windows, doors, skylights, glass ceilings, etc.) falls during the various hours of the day and during the various seasons, in order to reduce its direct impact. If we calculate, by means of suitable mathematical formulas, the trajectory of the sunlight with respect to the horizon during the day and during the various seasons [10, 14], and if we know the geometry of the structure, we can find out the trajectories of the radiation within the room (Fig. 16), so as to be able to determine the optimum position of the works of art. In particular, it is necessary to avoid subjecting the most sensitive artefacts, such as paintings on wood, or other organic materials, such as paper or photographs, to sudden inputs of energy [35, 44]. Moreover, we must keep in mind that there is a considerable difference in the quantity of energy received, depending on whether the surfaces are horizontal or vertical. As shown in Fig. 17 [15], in relation to the season and to their orientation, vertical surfaces are more or less affected by radiation, so the amount of energy they receive changes. These considerations make it possible to manage an environment, exploiting the energy coming from the sun as much as possible for lighting purposes, without using artificial lighting, whose spectra are characterised by a more or less great presence of wavelengths that are not useful for vision and are therefore undesirable from the viewpoint of conservation. Beside the solar radiation has a high content of UV and IR radiation so attention have to be taken into account to the incoming sunlight that have to be in any case filtered to eliminate this dangerous compounds. A simple use of curtains, even if it is not advised for other reasons in the museum (e.g. accumulation of dust), can help in reduce the incoming radiation. Measurements done in the room of Diamanti Palace (Ferrara, Italy) [138, 139] showed that a simple use of a light white curtain considerably reduces the quantity of incoming radiation. In the Fig. 18 one can see the difference in the quantity of lux measured on the surface (the painted zone was divided in 9 quadrants) of the painting with and without a simple white curtain placed in front of the window. b) Artificial lighting Artificial lighting and solar radiation, always represents an input of energy into the environment. It can give rise not only to thermohygrometrical cycles in the artefacts, but also to circulation of air in the environment, because of the convection cells that 38 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 39 Diamanti Palace – Ferrara (Italy) Fig. 18. Quantity of lux measured on the surface (the painted zone is divided in 9 quadrants) of the painting with and without a light curtain placed in front of the window. develop above the lighting body. These cells of warmer air convey to a higher level heat and suspended dust that may subsequently settle on the colder surfaces. For the conservation of works of art, no detail in the choice of the lighting system can be regarded as unimportant: the type of lamps (traditional bulbs, halogen lamps, halide lamps, neon lamps, etc.), their position (high or low, near the work of art or far from it), and the type of utilisation (constant or intermittent) are all important. When examining the characteristics of a lighting system, various factors should be considered, e.g.: 1. intensity of overall incident radiation (Watt/m2); 2. characteristics of source; 3. developed heat; 4. UV emission; 5. lighting mode; 6. exposure time; 7. system maintenance; 8. spectral absorption* factor of surface (e.g. shiny or dull surfaces); 9. tendency of material to be deteriorated by several wavelengths. Lighting, in conclusion, is a compromise between the needs of visual appreciation and those of a good conservation of the works of art. 39 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:25 Pagina 40 Fig. 19. Radiation spectrum of two kinds of lamps (incandescence (b) and halide (c)) compared to the spectrum of solar diurnal radiation (a) Some materials such as wood, paper, fabrics, paint and oil are very sensitive to light. The effects of light on materials can be quantified by means of mathematical formulas that consider the main following factors: 1. type of material; 2. energy content of radiation; 3. duration of exposure. A high input of light leads to an excessive warming of the work of art, so it is necessary to adjust the lighting needed for vision with great care, in order to reduce the incident energy as much as possible. In this book it will not be discussed the various available types of lamps (Fig. 19), because they are subject to a constant technological evolution, so the reader is referred to the specific lighting technique literature. We only underlay that they have different spectra, whose main components are the visible fraction, the infrared one (IR) and the ultraviolet one (UV). Each lamp has its own spectrum, and these components may be present in different percentages. Obviously, considering what we have explained above, it would be desirable for works of art an artificial light with a spectrum centred around the visible fraction and the other two (IR and UV) absent. 40 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 41 It is known, in any case, that the various types of radiation have different effects on the materials also in the visible fraction. The lower-λ radiation (violet, blue and the beginning of green), for instance, causes the colours to fade, but cannot be eliminated, because if some spectral components were absent, there would be a loss of overall chromatic performance in the lighting needed for the vision of the work of art. The UV radiation with an even lower wavelength gives rise to even more marked photochemical phenomena, particularly in the molecules of organic substances, but can be controlled more easily or even eliminated by means of suitable filters. The damage due to radiation, therefore, depends, above all, on the wavelengths that characterise the spectrum of the radiation. In the UV range, for instance, the following wavelengths, in decreasing order, are particularly harmful: UV-A (100-280 nm); UV-B (280-315 nm); UV-C (315-380 nm). In fact E=h•λ where E is the energy, h is the Plank’s constant and λ is the wavelenght. So, the main deterioration phenomena [1] that result from UV radiation (and, in the visible spectrum, from the violet and blue band, 380 - 440 nm) are the following: – – – – – fading of coloured pigments and textile fibres; crackling of paint; peeling of painted layers slackening of supports; loss of mechanical resistance of surfaces. The higher-λ types of radiation, particularly the IR one, produce a rise in the temperature of the surfaces. This rise is greater in the surfaces that have a high absorption* factor; the latter depends on the physical characteristics of the surface and of the material of which it is made. The main deterioration phenomena due to a temperature rise, some of which may have effects that are similar to the action of UV radiation, are the following: – thermal expansion; – mechanical tensions between different points; – alteration of objects that are sensitive to changes in relative humidity (e.g. wood, ivory, paper, etc.). 41 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 42 In conclusion, we can assert that almost all the lighting systems have negative effects on a work of art. This means that it is necessary to endeavour to dissipate as little energy as possible, while achieving the required illumination. The spectrum of the normally-used electric bulbs is shifted towards the longer wavelengths (i.e. towards the red range), so they emit a great quantity of IR radiation. They are therefore characterised by considerable heat dissipation, which is not useful for vision (200 lx are enought for a good vision). Halogen lamps are preferable, because their chromatic performance is pleasant; but they also dissipate much energy in the IR range. Other lamps, such as fluorescent ones, as a rule have a lower dissipation than the previously-mentioned types, but in their energy spectrum there is a great quantity of UV radiation, depending on the model. The illumination being equal, metal halide lamps dissipate less energy (approximately 1/7, in comparison with traditional bulbs, and 1/3, in comparison with halogen lamps) [13, 57], and their spectrum is fairly limited to the visible frequencies, with only a small margin of UV radiation that can be easily eliminated by means of a filter. Moreover, they light up gradually, thus reducing the thermal and hygrometric shock undergone by the works of art every time the lighting system is switched on. The research performed on Leonardo’s Last Supper [49] revealed that the intermittence of the lighting system, in which the light fell directly on the painting, gave rise to warming (Fig. 20) and evaporation (Fig. 21) every time the lighting was switched on, and to cooling and condensation every time it was switched off. The Last Supper - Milan (Italy) The Last Supper - Milan (Italy) 7 May 1982, h 16:00 8 May 1982, h 14:30 Fig. 20. Thermal distribution on the wall of the Leonardo’s Last Supper (Cenacolo Vinciano), Milan; to notice the thermal maximum due to the heating of the wall, following the sudden switching of a non suitable lighting system then in operation. 42 Fig. 21. Distribution of the specific humidity on the wall of the Leonardo’s Last Supper (Cenacolo Vinciano), Milan; to notice a slight hygrometric maximum due to the vapour emission of the wall, following the sudden switching of a non suitable lighting system then in operation. Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 43 Different authors gave their opinion on yearly and daily limits [121, 122] and a European group of scientists is establishing at this moment a common European normative (CEN). In the past, the total yearly limits of exposure to energy have been established. They are calculated in lux per hour (lx h)* multiplying the average illumination by the exposure time. The maximum total yearly exposure generally accepted until today obviously depends on the materials: a) for materials that are extremely or medium sensitive to light (textiles - in particular silk - rugs, tapestry, paper, dyed leather, feathers, lacquer, etc.), the limit is between 15 000 to 150 000 lx h/year; b) for materials that are moderately sensitive to light (varnished tempera and oil paintings, etc.), the limit is 600 000 lx h/year. In summary, the type of illumination to be chosen must have a minimum dissipation of energy, so as to minimise the release of energy into the environment and therefore into the works of art. It must have a spectrum concentrated on the visible wavelengths and must not have UV components, because of the effects of the UV radiation on colours; the IR radiation must also be avoided as much as possible, because it warms up the objects and is not useful for their illumination. Moreover, the spectrum of the light must be as similar as possible to that of sunlight, i.e., concentrated on the visible range and suitable for a correct chromatic performance (though sunlight, being filtered by the atmosphere, does not have a constant spectrum throughout the day). It is therefore extremely difficult to define the ideal spectrum for the illumination of an object, in terms of chromatic performance, because this choice is also affected by subjective aspects. The explanations we have given up to now, therefore, only mean to supply some useful pieces of information for the choice of the lighting system in a conservation environment, and do not aim at defining an ideal spectrum. From the microclimatic point of view, another aspect that should be considered in the creation and management of a lighting system is the position of the lamps. We have already explained what it means to create temperature gradients*, therefore also hygrometric ones, in an environment. In Paragraph 4.3 we will discuss the concept of “environmental stability”, i.e. the vertical distribution of the air masses in an indoor environment. The lighting system is formed of individual points of energy emission that seriously alter both the average quantity of energy within an environment (which changes with time) and its spatial (horizontal and vertical) distribution. 43 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 44 This is due to the fact that the energy released by the lamps causes the surrounding air to warm up and to become warmer than the ambient air. This gives rise, above the lamps, to the development of convection cells that rise towards the upper layers, resulting in vertical air movements between the lamp and the ceiling. Therefore, when deciding how and where to place the light sources, we should first of all consider the position of the works of art, and endeavour to reduce air movements as much as possible, or, if they cannot be avoided, to confine them to areas where there are no works of art. When considering these matters, in any case we should allow for the fact that the heat source - in this case the lamps - contributes to the determination of the thermal stability of the ambient air. A stable thermal profile is always desirable, because it limits vertical movements and consequently the transport of pollutants towards the upper layers. When possible, we should prefer indirect light, because it avoids also sudden thermal shocks at each switching on and off, besides preventing undesirable effects such as reflection by the protective glass. The choice of lamps with timing device depends completely on the evaluation of the “fruition” of the work of art. If adopting this type of lamp means that the work of art will be constantly subjected to the switching on and off of the lighting system, the consequences of the resulting frequent thermohygrometric shocks and cycles will certainly not be negligible. On the contrary, if the work of art is viewed only occasionally, this solution may be considered. The evaluation must allow for all the aspects we have discussed up to now. In this case, too, the basic conclusion is that what enables the person in charge to make the most suitable choices is a thorough knowledge of the environment and its specific problems. There does not exist a formula that is valid for every situation. The problems connected to the choice of the lighting for a work of art include a great number of variables that must be considered, and are not always tackled with the required thoroughness: the effects of unsuitable lighting can be seen quite often. A lighting technique expert can supply a series of indispensable pieces of information and suggestions, but only the operator in the specific field of conservation who has become aware of the whole problem is able to evaluate and consider the needs of a conservation environment, which are very different from those of civil environments. This expert operator, for instance, can determine the most suitable type of light beam, and in which points of the room (high or low) the lamps must be 44 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 45 placed. In conclusion, deciding which is the most suitable type of lighting to be adopted is not as simple as it seems, because of the interconnections with other phenomena and of the countless problems present in the environment under consideration, where any action inevitably brings about some consequences! 3.4 CHANGING THE AIR AND CLEANING THE ROOMS The first consequence of the fact that an environment is used for exhibiting works of art is the need to keep some doors open for letting people in and out. This involves the introduction of air masses whose thermohygrometric characteristics are often different from those of the air of the exhibition environment. The introduction of air having a different origin results in the mixing of air masses, creates thermal and hygrometric gradients* in the rooms, and stresses the exhibited materials, which are constantly subjected to environmental conditions that change in space and time. There are in literature a lot of mathematical formulae to calculate the air exchange in a room in function of this parameter. Anyway when using a following simple formula to calculate this exchange, we can already have an idea on the importance of a simple normal action we are used to do like an a opening of a window to exchange air, particularly on old building where the windows are often very large. This simple calculation was done during a study in the Diamanti Palace (Ferrara, Italy) [139]. Using a simplified formula which takes only into account the surface of the window and the volume of the room i.e. where n is number of total air exchange in one hour SL is the surface of the window (about 2.5 m2) h is the height of the window V is the volume of the room (about 600 m3), it has been demonstrated that one hour of complete opening of the windows causes a total air exchanged of about 2.5 times. This means that 20 minutes of opening it is enough to exchange almost all air in that room. This simple calculation shows how a simple daily action could abruptly change the thermohygrometric conditions in a room, with stress to materials as consequence. 45 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 46 Sainte Chapelle - Paris (France) 20 August 2002, h 16:45 Fig. 22. Horizontal sections of the Sainte Chapelle, Paris, representing the isolines related to temperature, relative humidity and specific humidity during the visiting time and the opening of the doors for the air exchange. Furthermore, the higher the number of visitors, the greater the need to change the air, and this is often done by opening the doors and/or windows in an uncontrolled manner (see for instance in the Sainte Chapelle in Paris [22], Fig. 22). Sometimes the change of air is achieved in a more indirect and continuous manner through ducts that directly connect the room to the outdoor environment, and in this case, too, air that has different thermohygrometric characteristics is introduced in the room in an uncontrolled manner. At the British Museum of London, for instance [12], in the two Egyptian art rooms where some measurements were carried out before the intervention which brought to the actual situation, two grilles in front of the door that interconnected the two rooms let in air that was directly sucked in from the outside. This air was detected by the measurements: though the route it covered before it reached the rooms partly reduced the thermal shock between the inside and the outside, the air was not treated thermohygrometically or filtered. The mass tourism of the last few decades and the constant, considerable presence of public, particularly in the buildings most attractive to sightseers, have worsened this problem, and it cannot always be easily solved by means of non-invasive passive measures. The presence of visitors and the opening of doors and windows also have another 46 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 47 Chiericati Palace – Vicenza (Italy) Fig. 23. Comparison among the number N (num/cm3) of particles of different diameter in Chiericati Palace, Vicenza (Italy), before and after the passage of the vacuum cleaner on the carpeting. result: the introduction into the rooms of pollutants, which settle on the surfaces and eventually blacken them. It is always advisable to filter the air before letting it into the room and also the air inside the room itself, though this operation may give rise to air movements between the intake and outlet points, thus promoting aerodynamic deposition. Despite this fact, the available filters are quite useful for eliminating the finest particulate, which is the most dangerous one, because it remains in the atmosphere for a longer time; but they do not avoid the introduction of pollutants brought in by people by means of their shoes and clothes. This might be partly controlled by a preventive cleaning at least of the soles of the visitors’ shoes before they come into the room. For instance, it is possible to purchase mats that are quite efficient in catching dirt: if they are kept clean, they may be a good preventive trap for pollutants carried by shoe soles. The fact that an exhibition room is visited by a great number of people also means that it must be cleaned often. Cleaning is carried out with more or less traditional methods, depending on the type of environment. In the simplest case, which is still the most common one, the floor is directly cleaned with water, giving rise to an increase in the specific humidity, consequently also in the relative one, with all the problems involved, among which a greater probability of condensation on the walls or in the pores within the materials. 47 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 48 Another custom is that of using traditional vacuum cleaners. The action of the motor causes air movements and raises the particulate (above all the larger-diameter one) that had settled on the floor during the hours of closing to the public. For instance, in the investigation carried out in Chiericati Palace in Vicenza (Italy) [16], much evidence was found of the effect of the use of a normal vacuum cleaner. As you can see in Fig. 23, after this appliance had been used for the daily cleaning operation, there was an increase in the atmospheric particulate: for larger-diameter particles, the concentration was found to be 10 to 20 times higher than that before the cleaning. The utmost care is needed in carrying out even this simple daily operation. Where works of art to be preserved are present, we must not maintain the daily habits of civilised life unaltered. Every day, the available technologies help us to safeguard the environment from these disturbing actions, which, in the long run, added to all the others, have destructive effects. We must keep in mind that any cleaning operation, though necessary and apparently simple, must be carried out without altering the conditions of the room in terms of temperature, humidity, turbulence, etc. Any action performed within a conservation environment, if it is not carried out prudently, may have harmful effects on the works of art to be preserved. Most of these actions (which in themselves seem harmless and necessary), since they are performed often, considerably contribute to the damage that may be undergone with time by the objects. Contrary to common practice, their frequency should be reduced as much as possible, and they should be carried out with due caution and with full knowledge of the facts, for instance using, when possible, suction systems equipped with pipes that directly expel out of doors the particulate sucked up in the rooms. Following particular care has to be taken at least: the filter in the vacuum cleaner has to be clean. In fact on average its use can bring in suspension again about one half of the previously deposited fine particles (diameters lower than 2.5 μm) when the efficiency of the filter is below 75% [123]. Another detail that must be considered is the use of rugs or carpeting in exhibition rooms or museums. This custom is absolutely not recommended, because it is a source of a great quantity of dust that is raised every time the floor is acted upon, either by the passage of visitors or by daily cleaning. A comparison between the particulate present in adjoining rooms in Chiericati Palace in Vicenza (Italy), some with carpeting and some without, revealed that the quantity of suspended particulate above all the larger-diameter one - is decidedly greater where carpeting is present. 48 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 49 3.5 COMPATIBILITY OF MATERIALS Very often a museum is set up keeping in mind aspects such as aesthetics, the relationship between the visitor and the work of art, the historical sequence, etc., but completely disregarding the compatibility between the materials, though, in terms of conservation, this should be the first aspect to be considered in deciding the distribution of the works of art in the rooms. It is known, as a matter of fact, that the conditions that are suitable for the conservation of a type of material sometimes are quite different from those suitable for another. Since works of art have a physical and chemical nature of their own, their needs with respect to their conservation may be completely different: each material has its own range, more or less wide, of toleration of changes in temperature and humidity. Therefore juxtaposing different materials means that some of them may not be in the environmental conditions most suitable for their conservation. The concept of “suitability” for a certain material may be quite different from that for another material. For instance, it is not difficult to imagine that the microclimatic conditions for a good conservation of wood or paper are much more restricted than those for stone or metals, and that new materials, e.g. film or similar materials, require completely different conditions. This may be true also within a single category of materials. Sandstone, for instance, is much more sensitive to changes in temperature and humidity than marble; a piece of marble that has just come out of the quarry and one that has already undergone, in its history, a series of thermohygrometric cycles that have weakened its structure will respond to environmental stress in very different ways. This applies even more to materials that are very sensitive to changes in temperature and humidity, for instance organic materials such as wood or paper: though we can mention wood in general terms, in actual fact there are many types of wood, and their age is a factor that may have altered their characteristics in terms of stability of the bonds between the fibres, capability of absorbing water, strength, etc. In the same way, a piece of stone that was originally very compact may have undergone such a great number and variety of thermohygrometric cycles or chemical reactions as to become quite porous, therefore more fragile and subject to deterioration. Analyses relevant to the physical and chemical characteristics of the various materials and to their response to environmental stress are carried out every day in the various research laboratories, but there is still much work to be done, particularly if distinctions are made between the materials on the basis of the transformations they have undergone with time. This scientific analysis is very difficult to carry out, and the extrapolations that can be performed for the practical purpose of the management of the environments are even more difficult. What is measured in the labora- 49 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 50 tory on samples that are often new, and in climatic chambers where certain physical and chemical processes are forced and accelerated, does not always correspond to the actual conditions of conservation, and the difficulty consists in having to understand how far these extrapolations can give the managers valid suggestions to guide their strategy [40] To state optimum ranges, for instance, of relative humidity (RH) for all exhibited materials may sometimes be risky. In paragraph 4.1, about condensation in micropores, we will explain that any material has its own distribution of pores that fill up with water at different degrees of RH, even below 100% [7, 42], depending on the diameters of the pores. Consequently, preserving, e.g., a RH lower than 100% in a room where a work of art is exhibited is absolutely not a guarantee that there will be no condensation in the microcavities of a work of art and this means thermohygrometric cycles, so stress, etc... Each material has its own more or less extensive range of tolerance of changes in temperature and humidity. It is necessary never to exceed the specific threshold of tolerance of each material, beyond which the process becomes irreversible and therefore destructive [96]. A careful study must be carried out on each material, in order to define the optimum conditions for its conservation; this is not easy, because of the variety of materials and above all because each piece has a different “history”. Therefore, to place different materials in the same room, though it is aesthetically more pleasant because it is less monotonous, may be risky from the point of view of their conservation. This shows how useful it is, for anybody who must act and make decisions in the area of conservation, to be acquainted with the physical and chemical characteristics of the most frequently present materials. All materials, in theory, need to be classified on the basis of certain criteria and catalogued in a simple and compact manner from the point of view of their physical and chemical characteristics (both the intrinsic ones and those attained after years of stress), in order to facilitate the management of their conservation; but a classification like this is extremely difficult and its practical use is risky. To determine exact numerical limits of tolerance based on a classification like this may be very dangerous if the user is not expert enough to understand its consequences: for instance, deciding at what temperature or relative humidity an exhibition room must be kept is far from simple. If several types of materials, such as wood, cloth, paper, stone, etc., are placed in the same room of a museum, as is often the case, it is possible that some of these materials are not in optimum conditions. A correct strategy, on the contrary, is that of juxtaposing two different materials, one of which has a wider range of tolerance than the other, af50 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 51 Cologne Cathedral (Germany) Fig. 24. Graph of the glass, iron and lead temperature trend on a glass of the Cologne Cathedral, due to the solar radiation heating. ter having adjusted the environmental parameters to the requirements of the latter. To adjust them to the requirements of the former would be harmful. This applies both to the indoor and outdoor conservation of objects. Let us consider the example of the Parthenon in Athens. This building is outdoor, so it is subject to environmental conditions that cannot be controlled and are characterised by considerable thermohygrometric variations. In the reconstruction of the building in Athens, iron bars and staples were used to fasten the marble blocks of which it is formed. The expansion of the iron (whose expansion coefficient is different from the marble one) and the transformation processes due to oxidation produced such a severe damage in the marble that the iron was replaced with a more suitable material, in this case titanium. A technique used in the past was much more judicious: the iron staples were fastened to the stone blocks by sinking the iron part that penetrated the stone into a lead pad, whose ductility allowed it to absorb the expansion of the iron, thus preventing the stone from getting damaged; or lead was used directly to join several pieces of the same material. Another interesting example illustrates a critical problem for the conservation of the European Cultural Heritage: the case of weathering of the paintings (grisaille*) on the glass tesserae of ancient stained-glass windows, such as the famous ones in a great number of European cathedrals (Fig. 24) [22, 34, 35, 76, 79, 87, 91, 101, 104, 51 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 52 105, 107, 108, 132]. The loss of grisaille*, a particular material used for painting mainly the lines, is the main cause of deterioration. The grisaille* is a powder of lead silicate glass mixed with a pigment (copper or iron oxides) and an organic binding agent such as water and egg white and heated to 650-700 °C. Temperatures did not allow the substrate to melt. In this way it has been shown that a permanent stress exists at the glass/grisaille* interface due to different thermal expansion coefficients of the 2 materials, glass and grisaille*; this could explain the formation of micro-cracks in the grisaille* and in the underlying glass, observed in the irreversible weathering phenomena. A similar case of the association of materials that are different both in their expansion coefficients and in the degree of swelling in case of absorption* of water (a phenomenon which can appear both outdoor and within many buildings), is that of mosaics: because of the different responses of the tesserae to environmental stress, in many cases the tesserae gradually come off, as has been observed, e.g., in the façade of the Orvieto Cathedral (Italy) [46]. Another example is the use of metal brackets for fastening the wooden planks of the support of a painting, where a series of stress come out when the environmental conditions change [77, 127, 128]. The different degrees of expansion of the materials resulting from changes in temperature and humidity give rise to (often severe) stress in the structure of the wood, sometimes leading to the appearance of cracks in the paint film above. 3.6 MICROCLIMATE AND BIOLOGICAL PROLIFERATION Another element that should be evaluated when trying to determine the microclimatic conditions most suitable for an environment is biological proliferation. In this book, whose approach is strictly physical, we will not expatiate on microbiological considerations, which should rather be left to the experts in this field. It is essential, however, once more to underscore the importance of a complete, interdisciplinary analysis of the environment, and the need to consider all the involved factors at the same time, even if they belong to different branches of science. In indoor environments, a good strategy is that of endeavouring to create thermohygrometic conditions that are stable and appropriate to the range of tolerance of the materials in hand; but in this case too there are some exceptions. As a rule, an environment that is defined as “suitable” must not cause the works of art to undergo large and frequent thermohygrometric cycles; in some materials, however (for instance paper), conditions of excessive stability and high relative-humidity values may promote the growth of microorganisms. It is known that the main factors that promote mi52 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 53 crobiological growth or speed up its process are temperature, humidity, light, nutrients, dust, etc., and all these factors are present in the management of an exhibition environment. So the prevention and control of biological deterioration are strongly determined by the environmental conditions. For instance, it has been demonstrated that the combination of low values both of relative humidity and of oxygen (achievable, e.g., in a showcase) decreases the biological activity on the objects [129]. For example it has been demonstrated [3, 75, 99, 123] that at room temperature (around 25°C) on a susceptible material mould growth varies importantly in function of the relative humidity of the air. For example at a relative humidity of 60% a few thousand of days are needed to observe growth, at 70% at least 150 days and only few days at 90%. It is known [117] that the growth of various species of bacteria and fungi depends on the quantity of water available to them, expressed by a number called “water activity” (aw)*, which is an index of the quantity of the water that the material contains in relation to the activation and growth of the microorganism cells. The term ‘water activity’ (aw) describes the (equilibrium) amount of water available for hydration of materials; a value of unity indicates pure water whereas zero indicates the total absence of ‘free’ water molecules. On the consequence, pure water, aw is 1, whereas for materials it is usually between 0.6 and 0.99: the highest values are typical for the development of microorganisms, and the lowest ones are sufficient for the development of fungi: not all species of microorganisms need the same conditions for their development. Control of water activity is very important as low water activity prevents microbial growth. Growth of most bacteria is inhibited below about aw = 0.91, but the absolute limit of microbial growth is about aw = 0.6. Obviously the aw value depends on the material in hand, and the water content of the material depends on the relative humidity of the air (paragraph 4.2,3) and on the temperature. For instance, in pure-cellulose paper, at RH=100% there is approximately 16% and 18% of water content in the material, respectively at the temperatures of 20 and 30°C, while at RH ~ 60% there is approximately 7.8-7.9% of water content at both temperatures. In paper, a particularly sensitive organic material, biological degradation is quite severe and appears easily. The biodeterioration of cellulose is caused by the enzymes released by fungi and bacteria. It is known that at high RH values (for paper, e.g., approximately above 75%) microorganisms that cause weakening and breakage develop. The thermohygrometrical conditions most favourable to the life of insects are T=2224°C and RH=70-80%: these conditions occur very easily in the middle Europe environments where works of art are stored or exhibited It is therefore extremely important, in the management of an environment, to en53 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 54 sure that the temperature and humidity values do not reach these thresholds. Otherwise microorganisms introduced into the environment either directly or by means of the dust that comes in through the openings or is carried in by the visitors will grow, encouraged by the environmental conditions, and will give rise to irreversible processes (in paper, for instance, brown areas and/or spots appear). The most immediate solution for preventing this mishap may be that of causing the relative humidity of the environment to decrease to a point where biological proliferation is stopped. A measure of this type, however, must be taken with the utmost caution: an excessively dry environment may cause the evaporation of the water molecules within the structure of the materials, particularly the organic ones, and alter their structure. The final effect therefore might be a destruction of the materials. In the case of paper, an excessively dry environment might result in the complete fragmentation of the objects as soon as they are handled. Moreover, comparatively high temperatures may promote a quicker development of microorganisms: metabolic processes speed up between 20 and 38°C, temperatures that are often present in exhibition rooms. This, however, does not mean that above and below these values biological activity stops completely: there are some microorganisms that can easily tolerate both very high and very low temperatures. Furthermore, the limits recommended for a microclimate acceptable to the public are not always in accordance with the need to limit biological proliferation, particularly if the public is very numerous. For this reason some precious sites (such as the caves of Lascaux, France or Altamira, Spain, or certain archives) have been closed to the public, because of the conditions, harmful for preservation [126], that had been reached during the years in which the sites were open to the public. In addition, there are some environments in which it is difficult to control the microbiological growth because of the particular position of the objects to be preserved. This is the case, for instance, of ancient windows with or without a protective glass [95, 115]: their particular interface situation involves changes in the thermohygrometric parameters that are rather marked, cannot be easily controlled, and are very often combined with temperature and humidity values within the range of the optimum conditions for biological proliferation. There is a great number of studies on this subject [89, 110], and several solutions for achieving an efficient remedy have been proposed [24, 25, 26, 27, 62, 63, 104, 106, 108, 114], but many subjects must still be studied [22] and followed up in order to reach a satisfactory solution. 54 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 55 3.7 WHICH BUILDINGS SHOULD BE USED - ANCIENT OR MODERN ONES? Both of these choices have positive and negative aspects. Ancient buildings are intrinsically suitable for conservation, because their structure (very thick walls, and therefore great thermal capacity*, helps to keep the indoor atmosphere in conditions of thermohygrometric equilibrium, reducing the external climatic stresses, at least the highest-frequency ones. Everybody has noticed that inside a massive building in summer season one has a feeling of greater coolness, though there is no conditioning system, while during the winter one feels less cold than outdoor even if there is no heating: the wall structures have a “memory” of the previous season, accumulating thermal energy during the summer and gradually releasing it into the environment during the colder season. This means that during the summer the heating due to the thermal wave connected to the daytime oscillation does not immediately reach the inner surface of the walls of the building; the walls accumulate it, and transmit it in a more subdued manner in the subsequent period. The contrary effect is present during the winter: it takes some time for the walls to release the internal energy they have accumulated during the previous period into the colder external environment, and this gives a feeling of greater comfort indoors. The time of accumulation or release of this thermal energy depends mainly on a series of physical parameters of the structure and on the local climate. Some studies on the propagation of thermal energy in a compact grassy soil that has a high thermal capacity* (comparable to that of the massive structures of an ancient building) have demonstrated that a few tens of centimetres of thickness are sufficient to eliminate, almost totally, the daily solar cycle [8], whereas at that depth the effect of the seasonal cycles, more gradual and therefore less harmful in terms of shocks on the material, is still felt. This means that where active internal systems, such as heating or air conditioning, are absent, within ancient buildings the thermohygrometric changes in the atmosphere are slower and more gradual, and their amplitude and frequency are lower. This mitigation does not occur in modern buildings, which are usually built with walls that are thin and therefore have a low thermal capacity*, and are often also equipped with glass panels, in order to use natural daylight without resorting to artificial lighting; so the preservation of a stable indoor atmosphere requires the use of active systems. These active systems, as we have already explained, are not easy to achieve a good performance, particularly in terms of obtaining a homogeneous distribution of the air masses, i.e. the absence of temperature or humidity gradients*. 55 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 56 Though it is expedient, as a rule, to endeavour to exploit ancient buildings as much as possible, since their structure facilitates the control of the microclimate, in many cases there is a considerable disadvantage: when the buildings are works of art in themselves, it turns out to be difficult to make some of the changes required to ensure more comfort in the rooms while respecting a correct management in terms of conservation, particularly if there are great masses of visitors and if the buildings are equipped with active systems such as lighting, heating, etc. For instance, a study performed in the Leonardo Room of the Uffizi Gallery [17] revealed that, though a sophisticated system of introduction of thermohygrometrically controlled air had been installed, as we have already stated, the constraints on the structural work made it necessary to use old channels that were not suitable for this purpose and partly thwarted the efforts to achieve a greater control of the indoor microclimate. The fact that ancient buildings are often good containers for the exhibition of works of art is confirmed also by some studies performed in the British Museum in London some years ago before the actual restructuring. The structure of the museum is ancient, so it has excellent basic characteristics. The analysis was carried out [12] in two areas that were managed in completely different ways at that time. The first one was that of the two Egyptian Rooms, where a great number of forcing factors are present: the main ones were the penetration of air from the outside, a roof that is transparent to sunlight, a great number of visitors, and a heating system with the intermittent introduction of air at a very high temperature from grilles on the floor around the room perimeter in front of the cases. All this gave rise to severe variable gradients*, with a RH that in some points drops even to 18-20%: these minimum values may be extremely dangerous for the exhibited objects (mummies, sarcophagi, etc), which are made of materials that are highly sensitive to thermohygrometric variations (Fig. 25). The second area was the Special Exhibition Gallery: here the absence of a heating system, of sunlight (because the roof was not made of glass) and of the penetration of air directly from the outside, plus the low concentration of public, determine a distribution of the thermohygrometric parameters with very slight gradients* both spatial and temporal, making the gallery suitable for the conservation of the exhibited works of art (Fig. 26). In other words, the second area, which belongs to the same architectural structure as the first one, was found to be an excellent environment for conservation, in spite of the presence of active systems (lighting, indirect heating, etc.). The only difference consists in the management of these active systems, which in the second case was completely different from that of the first area. In conclusion, in a structure that is essentially suitable for conservation, what creates conditions that were dangerous for the conservation of the exhibited objects is the incorrect management of the environment. 56 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 57 British Museum, Egyptian Rooms - London (UK) 25 February 1988, h 18:00 Fig. 25. Horizontal sections of the First and Second Egyptian Room in the British Museum, London, representing the isolines related to temperature, relative humidity and specific humidity. British Museum, Egyptian Rooms - London (UK) 24 February 1988, h 10:40 Fig. 26. Horizontal sections of the Special Exibition Room in the British Museum, London, representing the isolines related to temperature, relative humidity and specific humidity. 57 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 58 Chiericati Palace – Vicenza (Italy) Fig. 27. Horizontal sections of the Chiericati Palace, Vicenza, representing the isolines related to temperature and relative humidity, before the restructuring interventions (30 January 1987) and after the realization of the Museum (22 September 1987). An ancient building, however, is not always inherently suitable for conservation. There may be areas that need particular attention because their microclimate is less stable. For instance, this applies to the case of works of art placed on the last floor of the building, under the attic, where the microclimate is much more affected by the external climate. So in the management of the positioning of the works of art, it is necessary to endeavour to place the ones that are most sensitive to thermohygrometric variations in areas where the microclimate is naturally more stable, such as the inner rooms of the building, placing the less-sensitive objects in the external rooms or on the last floor. This was observed in 1987 in Vicenza when a new painting gallery was created in the museum of Chiericati Palace, on the last floor of the building [16]. Before the renovation works began, microclimatic analyses were performed, and as a result of their findings a series of measures to be taken before the introduction of the delicate works of art was recommended. The measures suggested (insulation, manage- 58 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 59 ment of the openings, lighting with low energy dissipation, double door for the entrance of visitors, vinyl carpets for catching dust, etc.) considerably stabilised the environmental microclimate (which was previously characterised by considerable thermohygrometric gradients*: see Fig. 27), and made it suitable for the conservation of works of art by drastically reducing the thermohygrometric cycles, therefore the stress undergone by the materials, and the deposition of pollutants. The series of changes that had been suggested, however, left a problem unsolved. An effort had been made to exploit the potential of the building, by recommending measures that were as “passive” as possible and reducing the “active” ones to a minimum, also because of the limited availability of funds. However, the environmental conditions of the building, which were excellent from the point of view of the conservation of the paintings, were not equally satisfactory as regards the conditions of temperature and humidity required for the comfort of the visitors, which are quite different. In this case, the use of passive methods was not sufficient to meet all needs, and a more expensive and delicate solution was suggested for the attic. In this building, since all the rooms were to be used for exhibition purposes, it would have been more simple to change the internal distribution of the works of art in the museum, placing the paintings not under the attic, but on the ground floor, where the microclimatic conditions were more stable, therefore more suitable. The materials that are less sensitive to thermohygrometric changes, for instance coins, that were exhibited on the ground floor, might have been transferred to the upper floor, because these materials allow a less restrictive management of the environment. The Sistine Chapel is another example of an optimum conservation situation that has become complicated because of use. The building, which is ancient and has thick walls, has good microclimatic conditions, as we have already stated, when undisturbed [44, 52]: a few hours after the closing of the Chapel, with no active systems in operation and no visitors present, the microclimate returned to its natural, excellent conditions. Particular attention has to be taken when ancient buildings, even if in good conditions, are particularly humid because of the external climate. This is the case, for example, of ancient buildings in central Europe, in particular cold and humid regions. There, during the winter season, the relative humidity inside the ancient building can reach very high values. In this case the use of humidifiers, sometimes associated to ventilation systems, increases the exchange between materials and atmosphere. If not attentively applied, these solutions could cause high damage for the rising of the thermohygrometrical cycles due to the greater gradients* and increase at the same time the deposition of pollutants due to different processes (see paragraph 4.4) 59 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 60 On the contrary, modern buildings, though allowing unlimited changes in their structure, have considerable limits in terms of conservation, due to the particular constructive concept of modern architecture: because of their low thermal inertia, they need sophisticated systems for controlling the indoor microclimate. These are not easy to achieve, if a sufficiently homogeneous distribution is required; moreover they require a careful upkeep, because even the slightest malfunction (which is not a rare occurrence) may lead to undesirable consequences, i.e. to sudden shocks for the works of art, with damages that are often irreversible. Consider what happened a few years ago in the Pinacoteca of Brera (Milan, Italy), when the heating system suddenly broke down. In that case in fact very heavy damages on the paintings occurred when the air conditioning broke down during a week end when nobody controlled the situation. A lot of discussion is still taking place concerning this subject taking also in consideration the costs and the consumption of energy. 3.8 DEPLACEMENTS OF WORKS OF ART The increase of works of art fruition during last few decades has also often involved the shifting of works of art from their places of origin, where they had found a thermodynamic equilibrium, to places used for temporary exhibitions, in many cases in completely different microclimatic conditions. The immediate consequence of this action is that the works of art are forced to quickly achieve another equilibrium with the new environment, obviously leading to thermohygrometric cycles and stress for the material(s). The mere fact of shifting a work of art from stable conditions to other, different (even if equally stable) conditions may result in irreversible damages. Let us consider, for instance, a painting on wood that is moved from an environment with constant temperature and humidity to another environment where the thermohygrometric conditions are equally stable, but different. The new equilibrium that the painting tends to achieve more or less quickly subjects its structure to a stress that is proportionate to the difference between the conditions of the two places. The consequences may be irreversible, because they may cause the wood to warp or crack. Moreover this may occur in different ways, depending on whether the two sides of the painting have different characteristics of transpiration, therefore of exchange with the environment. Paintings on wood usually have one side painted and the other bare, so the two sides have different coefficients of shrinking and warping; sometimes the unpainted side is treated with plaster and glue, wax or other materials [70]. In order to prevent or limit the harmful effects on the paintings, where there are cracks, materials that are more rigid and have different expansion coefficients are sometimes inserted; this did not always give good results, and sometimes actually worsened the neg60 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 61 ative consequences. For instance, with the so-called “metal staples” used to “tack” the parts that had come unglued or were cracked, the results were so negative that these devices had to be removed [77]. Sometimes “wooden staples” are used, but in this case it is necessary to know the coefficients of expansion of the two woods, since they may be quite different. These considerations are valid for the displacement of works of art not only to rooms used for temporary exhibitions, but also to ateliers where they are restored. It is not a rare occurrence, in museums, to see newly-restored works of art that soon show rather evident signs of deterioration and require a new restoration after a short time. In this case, the work of art, besides changing environment (because it is brought to an atelier where the conditions are certainly determined on the basis of the comfort of the restorer), is often subjected to an intense and concentrated lighting for the restoration. Obviously lighting is necessary, but it is important for the restorer to be aware of the delicacy of the operation and to endeavour, while keeping a satisfactory intensity of lighting, to equip the atelier with the most suitable type of lighting, despite the fact, as we have explained in the paragraph on lighting, that this is far from simple. A further problem may be caused by the transportation of the works of art from a museum to another. There are many specialised firms that, besides considering safety problems, also take care of the climatisation of the works of art during this delicate time. What is not always clear is that the work of art does not merely have to be conveyed in constant thermohygrometric conditions, maybe on the basis of parameters that have been “recommended” in a slipshod way by some publication or determined by non-specific regulations: it is necessary to be careful to ensure that the work of art is stored in a container that has been regulated on the basis of the thermohygrometric conditions of the original environment, and that the conditions of new environment, if possible, are similar. Moreover, the container must be opened by specialised personnel following a precise procedure, and considering the environmental conditions. If the opening operation is performed incautiously, this may thwart the efforts made during all the procedure. If the opening operation is performed incautiously, this may thwart the efforts made during all the procedure. Summarising schematically a procedure of moving works of art has to take into consideration 3 fundamental steps: - the measure of the thermohygrometric conditions of the environment where the works of art are stored since a long-time; - the case where the object is stoked during the transport has to be conditioned at the same thermohygrometric conditions; 61 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 62 - when the object arrives at the new destination where the thermohygrometric conditions are different, if it is not possible to put it in a conditioned case for the exhibition, which is the case at most occasions, it needs to be brought towards the new conditions very gradually in function of the difference between the two environments and the dimension of the object The same has to be done when the object returns to the original place. This means that the operators have to pay attention to each step and in particular they have to regulate in a correct way during each step the time dedicated to the works of art to reach the new equilibrium without submitting them to irreversible stress. This is not a simple operation because it needs a thorough knowledge of the response time of the different objects in function of the materials and geometry. So this matter is more complicated that we think, because the control of the environment and the reaction of the materials are difficult and the objects under consideration are not worthless pieces of household furniture, but unique works of art, so they need particular manoeuvres that are quite different from those that are customary in everyday life [65]. 3.9 HUMAN COMFORT AND CONSERVATION Exhibition rooms are designed to allow the public to visit them and enjoy the sight of the exhibited works of art. We must not forget, however, that these rooms are first of all containers of works of art: the latter must be kept in conditions that ensure that they do not deteriorate and can be preserved for a long time. This aspect, however, does not always take priority in the choices that are made: in many cases, unfortunately, these choices are based more on the needs of people than of those of the exhibited materials. In all scientific domains, both in Italy and abroad, the main environmental factors to be considered in conservation environments (museums, archives, etc.) have been picked out, but there is still much debate about the solutions and limits to be proposed to the people in charge, for a correct environmental management. Two of the main parameters to be considered are temperature and relative humidity. The limits of these two parameters, for preventing deterioration processes, are particularly difficult to determine for each material, even within a single species of material (for example wood, paper, ivory, etc). It is reasonable to wonder whether at present we are in a position to supply the people in charge with limits for all the materials contained in an exhibition environ62 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 63 ment. The studies performed up to now have not been able to give complete replies. As a matter of fact, recent studies carried out in laboratories [98] have demonstrated that, in some types of materials, comparatively great variations in RH do not result in irreversible damages. The dimensional variations undergone by the materials during these experiments were within what is called the “elastic deformation range of the material” where the process is reversible. These findings, however, apply to the specific samples of materials - usually new - that are used in the experiments: are they valid also for all the types of materials of which works of art are made, and which have each aged in a different way? Scientific approaches and everyday practice often yield deeply different results, so a datum obtained in laboratory experiments must be correctly interpreted by whoever means to use its numerical conclusions. The reader has undoubtedly understood that changes in temperature and relative humidity cause a deterioration of the materials, particularly of the most sensitive or hygroscopic ones, because they give rise to stress, which in turns results in a change in the dimensions of the materials. Thompson perceived this as early as 1976. The reader has also understood that materials last much longer if they are preserved at suitable values of temperature and humidity. For instance, photographic materials last much longer if they are kept at very low temperatures [97]: it has been demonstrated that the new materials used at present, such as acetates, if preserved at a relative humidity of 20% and a temperature of 2°C, last about 1250 years, whereas if they are kept in the typical conditions recommended for a museum, 60% RH and 21°C, they last only 30 years. It has been found, finally, that for each specific object (which may be formed of several materials) it is possible to find the limits within which it does not undergo irreversible damages. Is human comfort - which takes priority in far too many cases, because the people in charge believe that the materials have similar limits of tolerance - compatible with the conservation of materials? If this were true, the curves that represent the wellbeing of human beings and that of the materials should have some overlapping areas within which it might be possible to determine limits acceptable for both. In the past, environmental parameters were determined on the basis of criteria in which human comfort took priority. With time, however, many works of art appeared to be deteriorated and had to undergo several restoration operations. It took many years and a great number of researches for people to understand the causes of this and to try to find the most appropriate solutions. Human comfort is defined as the psychophysical condition in which a person is satisfied with the microclimate. The main factors that affect the feeling of comfort of the 63 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 64 subject are the temperature of the ambient air [68, 100], which regulates thermal exchanges by convection; the radiant temperature of the surfaces, which regulates the radiative exchanges; the relative humidity of the air, which affects the rate of evaporation of water from the body to the environment; and the relative speed of the air with respect to the subject, which regulates the exchange of heat by convection and evaporation. The temperature and relative humidity of the air are therefore two of the most important parameters. The conditions of “human comfort” in terms of the values of these parameters are well-known, and have been specified in the standards of the [4]. What are the “safe zones” for the material? Let us consider the example of paper, and make a comparison between the limits for the conservation of paper and those for human comfort. For this purpose we will use the limits of tolerance for temperature and relative humidity given by two authoritative sources: Coppedé [67] and the Dutch Institute “National Archives”. We will compare them with the limits determined for human comfort [5] for winter and summer, in a condition of mild phys- Fig. 28. Comparison between the human well being with a typical summer and winter clothing (normative ASHRAE 55-81) and the well being conditions for paper, following two authoritative sources (Coppedé, 1991 and National Archive Institute – The Nederlands). 64 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 65 ical effort, such as that of a visitor slowly walking through a museum (Fig. 28). If there were conditions acceptable both for human comfort and for the conservation of the materials, the areas should overlap, at least partly [23]. In this case, on the contrary, there is no overlap. The person in charge for the management of a museum or exhibition building, therefore, will have to decide whether he/she prefers the comfort of the visitors or the conservation of the objects. In our case it is not even possible to try for a compromise, but it is necessary to choose. Various types of interests often lead to the choice of preferring human comfort! Once again, we can conclude that the problem of meeting the needs both of human beings and of the materials is not simple and cannot be simplified into general rules. In this area, the specific characteristics of the situations and a thorough knowledge of them are the basis for the achievement of a suitable microclimate. For this reason what describe in the paragraph 3.5, i.e. the organisation of the exposed materials in a museum, becomes much more important and clear. 3.10 STRUCTURAL AND NON-STRUCTURAL PASSIVE MEASURES All passive-type measures, i.e. limited ones that are not too invasive in their realisation, always reversible and without the use of energy supply, underlie the achievement of an environment dedicated to conservation. In order to reduce risks, they must be carried out before resorting to active measures, which are more invasive and less easy to control. Passive measures are taken in order to create an environment that varies very slowly, in a natural manner, so the works of art can adapt to it without undergoing dangerous stress. When an exhibition environment is being created or renovated, sometimes there are comparatively simple structural measures that can greatly simplify the subsequent management of the environment, in terms of conservation. As we have pointed out in the previous examples relative to Chiericati Palace in Vicenza and to the British Museum of London, in the same building there are areas that are less easy to manage in terms of conservation, because of their structure and position, and others that are easier to control. In a building destined for the exhibition of works of art, it is very important to endeavour to take all possible measures for simplifying its management, to find the most suitable position for the works of art on the basis of their sensitivity to thermohygrometric variations, and to limit, as much as possible, all exchanges with environments whose air masses have different thermohygrometric characteristics, in particular to limit exchanges with the outside, since the outdoor environment is always characterised by a high thermohygrometric variability in relation to the weather. 65 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 66 For instance, in the renovation of Chiericati Palace in Vicenza [16], the two doors for the entrance and exit of the public were reduced to one door surrounded by a two-door system in which the second door opens after the first one has been closed. This system considerably cut down the exchange of air masses with the outside, and the only room that was found to be slightly affected by the influence of the outdoor environment was the one where there was the door. It was inevitable for this room to undergo the greatest variations in temperature and humidity, so it was recommended to use the room for stone materials, which can withstand these variations without getting damaged. The indoor circulation of air may also affect the distribution of the thermohygrometric gradients*. Knowing how the air currents are distributed within the rooms may be useful in determining which areas are most dangerous, therefore in finding the best position for the most delicate pieces. A work of art placed between two doors that face each other will undergo (also because of the passage of visitors) a greater thermohygrometric stress than another work placed outside this circulation of air. The philosophy of choosing passive measures should be preferred, because these measures improve the natural environmental conditions (to a degree that depends on the available structure), cutting down to a minimum the subsequent active measures that may still be required. This happened on the last floor of Chiericati Palace, where, after a series of structural passive measures, the conditions were still not suitable for human comfort. For this purpose it was necessary to act on the attic, because the passive measures alone were not able to ensure the desired conditions. As a matter of fact, the final situation was ideal for the conservation of delicate works of art such as paintings on wood, but was still uncomfortable for the visitors. At that point, the only solution was to take more effective active measures. Ceiling insulation is one of the most common passive measures aiming to limit the exchanges with the outside environment. As we will explain in detail in paragraph 4.3 (about the physical treatment of “atmospheric stability”), a cold ceiling (possible during the winter or the night) cools down the air in contact with it, starting a downward motion: the colder air, which is heavier, descends towards the floor and is replaced by warmer air that rises towards the ceiling, creating convection cells and increasing the conveyance of pollutants. A good insulation of the ceiling, on the contrary, helps keep the air in its natural conditions of atmospheric stability, inhibiting any vertical movement. Among the preliminary passive measures, we should mention the management of the windows, which are often one of the main causes of sudden or constant ex66 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 67 changes with the outside. As a matter of fact, windows cause an exchange of considerable air masses with the outside, either because they let in draughts (this occurs frequently, above all in ancient buildings) or because they are managed incorrectly: it is not rare to see some windows that are kept open in museum rooms because an employee finds that the air is “too stuffy” or not right for the conditions previously defined as “human comfort”. The windows of a controlled environment in which works of art are exhibited, particularly if they are made of very sensitive materials, should not have any chinks that allow an uncontrolled passage of air from the outside. They should also be well insulated, otherwise their thermal inertia, being lower than that of the adjacent walls, would cause them much more quickly to reach equilibrium with the more variable temperature conditions outdoor. For instance, if the surface temperature of the windows becomes lower than the dew point of air, the windows become preferential surfaces for the condensation of the indoor vapour. Moreover, windows should not permit the introduction of pollutants from the outside, or the direct intrusion of sunlight, which is very harmful to works of art if it falls directly on them, both because of its thermal effects and because of the effects of some particularly damaging wavelengths (e.g. ultra-violet) as already explained. Many technical solutions are available in the market for solving a lot of these problems both of insulation and of penetration of sunlight, in the latter case by eliminating the most harmful components of the radiation. 3.11 SHOWCASES Sometimes the microclimate of an environment is absolutely unable to meet the needs of the good conservation of a work of art or it is too fragile to support the minimum variations, impossible to eliminate completely in a frequented environment. In these cases, the solution may be to create a micro-environment that is easier to control, such as that of a showcase. This solution, however, is not always simple or decisive: constructing an efficient showcase is not easy or inexpensive. In this field, too, there is still much debate and specific European normative in is still in progress. Many technological solutions are available, but the physical, chemical and biological parameters that must be controlled are quite numerous. Moreover a good showcase is always very expensive, so that only particular works of art can be displayed using this technological equipment. What type of showcase should be preferred [31]. A completely closed one ? A glass one? A ventilated or non-ventilated one [64, 130]? The lighting must or can be internal or external, near or far? What type of lighting should be installed? As we have 67 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 68 already stated, glass is not transparent to infrared radiation, so a glass showcase, keeping in almost all the heat, will cause the object to heat up more than it would if it were outside the showcase [59]. We have already explained that the direct lighting of an object involves the transformation of the absorbed fraction into an infrared radiation. The latter remains trapped within the showcase, because glass does not allow the infrared radiation to come out, so the temperature within the showcase is bound to rise: the showcase acts like a “greenhouse”. It is easy to imagine what the consequences will be. It is advisable, therefore, if there is not the financial possibility of purchasing a sophisticated showcase, to choose a showcase built with different materials transparent to infrared radiation and at least one of the surfaces made of a material that can exchange heat and vapour with the outside more easily. For instance, a wooden back or bottom that can easily exchange excess heat and vapour with a healthy wall in contact with it might answer the purpose. In any case, there are some simple systems for keeping a showcase in constant conditions of humidity, for instance that of using silica gel, which buffers the relative humidity: after it has been conditioned to predetermined RH values, silica gel allows the release or absorption* of excess water vapour, so as to keep the RH constant. It must be checked periodically and regenerated, if necessary. For this reason it has to be positioned in a easily reachable position inside the case. The recommended quantity to be placed in a showcase ranges from 1 to 25 kg/m3 of air as reported by Thompson in 1977 [121], but until today the quantity needed has not been precisely established, also because every type of material and its thickness influences the performance. For example a statue in wood needs a lot of time to respond to solicitation in all of its mass and can also itself be a buffer. There is still a debate about it and new products of silica gels came out. In fact for example Art-Sorb proposes 0.5 kg/m3 for its products. There are a certain number of formulae to calculate this needs of silica gel. Over the last years Weintraub and Tetreault [123], elaborated this formula: Q= (Ceq • Δrh) • V • (N • t) / (MH • F) where : Q = Kilograms of silica gel required. Ceq = Concentration of water vapour at saturation in the silica gel (g/m3). 68 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 69 Δrh = difference between the external relative humidity (expressed as 0<rh<1) and relative humidity within the exhibit case. V= The volume of the case, expressed in m3. N= The number of air exchanges per day. (Thomson used N= 1 air exchange per day for a typical moderately sealed exhibit case). t= time expressed as the maximum number of day that exhibit case should remain within an acceptable range of relative humidity. MH = (g/kg RH(%)) is the moisture buffering capacity of silica gel within the specific relative humidity range of use, taking into account the hysteresis. F= The acceptable maximum range of RH fluctuation within the exhibit case. The variable MH, the moisture buffering capacity of silica gel, is the parameter that defines the kind of silica gel and its quantity (Table 1). In fact it represents the “specific reserve of water”, i.e. the quantity of water in grams that has to be gained or lost for a variation of 1% of relative humidity. Table 1 A comparison between the performances of the different products of silica gel is showed in Table 2, The new products require less quantity and are more efficient in function of the needs. In fact they are more specific in function of the range of relative humidity that has to be controlled. An example of application of these new products following the formula of Weintraub and Tetreault has been made in a recent study of the remains of Petrarca the famous Italian poet [135]. An inspection to study the conditions of these remains was done by a multidisciplinary scientific team at the occasion of the 7th century anniversary from the poet’s birth. The Petrarca’s tomb is outside in front of the cathedral of Arquà Petrarca (Padova-Italy), the town where the poet died. Pe69 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 70 Table 2 trarca himself at his death expressed the desire to be buried in this place and in this way. The microclimatic conditions inside the tomb were very bad for the conservation of the remains (constantly more than 75-80% of relative humidity). So, it was decided to put the remains into a glass box where a certain quantity of Art Sorb was put inside to control the microclimate inside the tomb. As it is evident from the Table 2, which shows the adsorption capacity at 25°C, the Art Sorb is the best for the microclimatic conditions inside the tomb and the quantity of the product was calculated using the upper formula in function of the volume of the box and the time foreseen for the next reconnaissance. The quantity of Art Sorb and Regular Silica Gel have been put in comparison for the volume of the box containing the Petrarca’s remains. The box, installed far from all the walls of the tomb, was built in glass and sealed with silicon. Glass is perfectly sealing, but not the silicon so an Petrarca's Tomb - Arquà Petrarca, Padova (Italy) Fig. 29. Quantity of 2 different kinds of Silica Gel in function of the time calculated for the stabilisation of the microclimate inside the box containing the rest of Petrarca. 70 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 71 air exchange of 1-2 month was taken into consideration. (Fig. 29). Till today no danger of production of internal pollutants occurred. Showcases can be also an excellent means of preventing the passage of dust from the outdoor, therefore its deposition upon objects. Besides, they allow a careful filtering of the air that is introduced, so the air can be kept clean within this limited volume. The production of internal pollution from the objects themselves or from the material composing the case itself is another big problem, but for the chemical aspects we refer to more specialised publications [39, 88, 96, 122, 123, 131]. Furthermore, for certain materials and in certain particular conditions, the air in the showcases can be replaced by other inert gases that do not interact with the materials (e.g. nitrogen), in order to prevent oxidation and biological activities. This might also solve the problem of disinfection for all aerobic biological species. The use of showcases, however, is not free from problems. Great attention must be paid to the formation of mould. Where there is no ventilation at all, if the RH levels become too high, this may promote the proliferation of micro-organisms. Moreover, though sealed showcases can solve several problems, for instance the control of RH or safety, they can give rise to other problems, such as the concentration of harmful chemical substances (e.g. sulphur or nitrogen oxides, formaldehyde, etc.) produced by processes that take place directly in the showcase, or by an action of the materials of the showcase or of the object contained in it [33]. It is not a trivial or simple matter. For this purpose, national and international standards are being drawn up to regulate the technical specifications and quality of the showcases for works of art that require particular care. 3.12 AESTHETICS OR CONSERVATION? In the management of a museum, an exhibition room or an atelier, the location of the works of art must be considered, so as to allow for the various requirements of conservation, but also to avoid useless expense. As we have already mentioned, in designing or managing museums some decisions are taken on the basis of criteria that have nothing to do with conservation. This occurs particularly when the people entrusted with the realisation of an exhibition project do not normally work in the area of conservation and are unaware of its problems and requirements. Sometimes we can see museums in which much attention has been given to the aesthetic aspect of the exhibition and to the chronology of the historical sequence or archaeological discovery of the exhibited objects, rather than to the requirements of conservation. 71 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 72 Giotto room, Uffizi Galleries - Florence (Italy) Fig. 30. Trend over time of the air temperature on the wall and on the Giotto painting placed in the centre of the Giotto Room in the Uffizi Gallery, Florence, when the heating system is on. Giotto room, Uffizi Galleries - Florence (Italy) Fig. 31. Trend over time of the air temperature on the wall and on the Giotto painting placed in the centre of the Giotto Room in the Uffizi Gallery, Florence, during a day of closure. Note how, during the days of opening (fig. 30), when the heating system is on, the temperature of the painting follows more closely the trend of the temperature of the environment air while it is less pronounced during the days of closure (Fig. 31). In all these elegant philosophies, where is the safeguard of the works of art? Which aspect takes priority in the choices that are made? Let us analyse a very common, simple example: the use of panels for supporting works of art, particularly paintings, that are placed in the exhibition room at a certain 72 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 73 distance from the walls or in a central zone. The works that are in this position are exposed to all the microclimatic variations experimented by the environment of the room (Fig. 30, 31), e.g. the opening of doors or windows (resulting in the introduction of air with a different thermohygrometic content), the lighting of lamps; the presence of visitors, etc. This choice would be justified if the walls of the room were unhealthy, or had an extremely low thermal capacity* so that affected by all the influences of the external environment. It would be a much better choice to use healthy walls with a high thermal capacity*, such as those of an ancient building: here the walls would absorb the variations in temperature and humidity, acting as a natural filter for the most frequent stresses (Fig. 32). Moreover the panel to support works of art are often constructed with material very different from the material of the work of art. If the material is highly conductive, like metal, the Fig. 32. Painting hanging on a museum wall. To panel responds very quickly to the environnotice their distance from the surface of the walls. mental variations transmitting them to the work of art. Besides if the work of art is fixed on it, the panels experiment stress and strain different from those experimented by the work of art with dramatic consequences. What about the works of art, for example wooden sculptures, that are placed at the centre of a room for aesthetic reasons or for a better relationship with the public, and that are therefore exposed to draughts? For works of art that are particularly sensitive to thermohygrometrical variations (for instance, wooden sculptures), a position like this is the most unsuitable place from the point of view of conservation, because it is the most subject to thermohygrometrical variations. Moreover in some exhibitions, in order to prevent visual monotony, there is an alternation of works made up of different materials. In a choice like this, though a marble work can tolerate the thermohygrometrical variations that do not exceed the limits required for the conservation of paintings, the opposite is not true. Therefore the decisions relevant to the choice of the acceptable thermohygrometrical range must always be based on the most restrictive case. This can be applied to materials whose “safety zones” overlap; but what can be done for those materials whose limits do not overlap? (Fig. 33) 73 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 74 Optimal range of RH for different materials Fig. 33. Comparison among relative humidity ranges as suggested in the literature, taking into account various factors: a better conservation of different materials; human well being; restriction of physical, chemical and biological phenomenon. To be considered that there are no RH “ideal” values, but every choice is a compromise. These considerations lead the reader to understand that also the choice of the positioning of the precious objects is a non neglectible aspect, with possibly heavy consequences in terms of conservation. Moreover no general guidelines can be given because every solution is particular and related to the specific conditions of air, walls and materials involved. 3.13 ECONOMICAL INTERESTS OR CONSERVATION? And last but not least, what to say about the financial aspects of the management choices? In some cases, the decision to limit the number of visitors or, in extreme situations (such as those of the caves of Altamira and Lascaux) to close the sites completely to visitors is very unpopular, but indispensable. The total closure is decidedly criticised; controlling the number of visitors sometimes involves lengthy queues that some people dislike, to the point of giving up the visit. This means a financial loss that is unwelcome to the managers or owners and is often not accepted by them, in spite of the fact that not laying down limits may be harmful to the conservation of the works of art. For instance, the massive inflow of visitors that can still be observed in the Sistine Chapel, moreover concentrated in a few hours, is certainly not a good thing for the conservation. The more or less simple alternative solutions (regulating the flow of the visitors and the duration of their stay; or creating a fixed course that compels the visitors to proceed towards the exit under the pressure of the visitors behind them who want to come in, etc.) may clash with the financial interests involved in the exhibition. The result is that the most sophisticated solutions achieved after a restoration and based on sophisticated active system involve decidedly higher risks. 74 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 75 4. ENVIRONMENT-RELATED PROCESSES AND PHENOMENA THAT INVOLVE WORKS OF ART 4.1 CONDENSATION ON SURFACES OR IN MICROPORES The condensation of water on the surface and within the materials is a very important problem in the field of the conservation of works of art, because water is one of the basic elements in the deterioration processes of the materials from a physical (change of state of water vapour), chemical (chemical reactions) and biological (internal growth of microorganisms) point of view. In the past, in the field of conservation of works of art (and not only..), the calculation of the time of wetness gave an idea of the quantity of time that the condensation occurs on a surface. From the fundamental theory, the time of wetness (TOW) refers to the period of time during which the atmospheric conditions are favorable for the formation of a surface layer of moisture. For the purposes of the standard this has been defined as the time period during which the relative humidity is in excess of 80% and the temperature is above the 0°C. This parameter was defined for metal or alloy surfaces, but it is often extrapolated to other kind of surfaces. This means that the value of RH 80% and 0°C are referred to the air characteristics. It has to be observed that this definition doesn’t consider the temperature of the surface, which is fundamental for the formation of the dew and also the condensation phenomena internal to the surface is totally forgotten. The information that come out from this kind of elaboration of data can furnish a picture of the whole phenomenon of condensation on a material very far from the reality. So a more precise approach is necessary for this complicate and important phenomenon. Keeping this phenomenon under control is far from simple, also because of the possible presence of salts that alter the chemical equilibrium. In porous and particularly hygroscopic materials, such as organic materials, the condensation cycles result in severe stress. In materials that are not very hygroscopic, e.g. metals or glass, condensation is the cause of important deterioration phenomena such as corrosion [79]. For instance, the formation of water on glass is particularly harmful to the glass, whose deterioration processes are all correlated with the presence of water. Condensation occurs on a surface when the temperature of the latter is lower than the dew point of the surrounding air, i.e. lower than the temperature at which the air 75 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 76 becomes saturated and condensation can take place spontaneously on a surface. This phenomenon can be easily observed on a surface exposed to the external climate, because of its spontaneous cooling by radiation, particularly during clear nights: as soon as the temperature of the surface drops below the dew point of the air, droplets begin to appear on the surface: this is dew. To quote an example from everyday life, let us consider the formation of droplets on a glass pane that is colder than the air of the environment. Whereas a wall usually tends to absorb the condensation water that settles on it, thus making the phenomenon less evident, when a glass pane is in the conditions that may start condensation, the condensation process may proceed to the point where rivulets of water appear and eventually flow onto the surfaces below. It is more frequent for the phenomenon of surface condensation to appear on the surfaces of the outer walls of buildings that are most subject to climatic variations and stronger radiative cooling. Condensation is less frequent on internal surfaces, because the walls, particularly if they have a considerable thermal capacity*, are less affected by the outdoor climatic oscillations. For instance, during the autumn, the internal surfaces, because of their inertia, undergo a slower surface cooling process than the external surfaces, so it not easy for their temperature to drop below the dew point of the air, starting the condensation process. The presence of atmospheric particulate on a surface may promote condensation: this process depends not only on the hygroscopicity of the surface, but also on its degree of contamination by pollutants [10304]. The origin of these pollutants may be natural or anthropic, so the types of particles may be different; but all of them act, more or less efficiently, as “condensation nuclei”, promoting the formation of droplets and consequently accelerating the deterioration processes correlated with them. This phenomenon is more efficient more hygroscopic the nuclei are, so that RH lower than 100% are enough to start the condensation. In fact the condensation of pure water on a surface takes place when the RH reaches 100%. This is not only true in the presence of pollution but also in micropores. In fact the condensation phenomenon follows different physical laws. The reason for this is that the saturation RH value (at which the condensation process begins) depends on the radius of curvature of the water surface, as established by Kelvin’s equation [7, 20, 42, 57]. The latter determines, in function of the radius of curvature, r (negative for a pore), the saturation RH value expressed by the following exponential curve: RH = 100 • e 2 σV/rRT 76 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 77 Fig. 34. Example of a open pore (a) and a close one (b). in the open pore, the opening radius (ro) is equal to the pore radius (rp); in the close pore, opening radius (ro) is minor the the pore radius (rp). where σ is the surface tension of water, V the molar volume, R the constant of gases and T the temperature in Kelvin degrees. The total porosity of a material can be primary (present at the origin) or secondary (coming from the following transformation or alteration). Moreover, there are two types of porosities: the “open” ones, in which the pores are half spheres and the radius of the opening coincides with that of the pores, and the “closed” ones, in which the opening is smaller than the diameter of the pore (Fig. 34). In the former, the condensation process is reversible, that is to say the evaporation follows the same route as the condensation; in the latter the same route is not followed (this is called a process with hysteresis*). In practice, because of Kelvin’s equation, in micropores there is a critical radius to which there corresponds a certain RH value (lower than 100%) at which condensation or evaporation is started; in open pores, the two radii coincide, so the process is reversible, whereas in closed pores radius of pore and opening do not coincide, so the process is not reversible: for condensation the critical radius coincides with that of the micropore, but for evaporation it corresponds to that of the opening of the micropore. So in the inner closed pores, where the radius of the opening is smaller than that of the micropore, the equilibrium RH values for evaporation and condensation are different and the two processes are not reversible. Allowing for the fact that in the condensation or evaporation process a different equilibrium RH value corresponds to each radius of the water meniscus (Fig. 35), the condensation/evaporation cycle in micropores takes place with the following stages. a) The condensation process begins when the increasing RH of the air reaches the equilibrium value relative to the radius of the micropore. At this point, the mi77 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 78 Fig. 35. Graph of the Kelvin equation, connecting the meniscus radius of the water in the pore and the relative humidity, for two temperature values, 0° and 30°C. cropore is filled up immediately: the radii of curvature of the water menisci while the micropore is being filled up are smaller than the radius of the micropore, therefore they are in equilibrium with lower RH values than those already reached for the onset of the process. b) Contrariwise, in the evaporation process the critical radius at which the process is started is the one of the opening of the pore. When this equilibrium RH is reached in the air, the evaporation of the water from the micropore takes place at once, until the cavity is completely empty: the radii of curvature of the water menisci during the emptying out are in equilibrium with RH values higher than the one that has been reached. The geometry of porosity, therefore, is an essential element in determining the moment of the condensation and evaporation and even their possibility, and for this range of porosity these processes take place at RH values lower than 100%. Considering the distribution of the pores in the materials, the condensation process within a material takes place in a gradual manner, involving pores with increasingly large radii, while the pores with a smaller radius are already be filled of water. The smaller pores (those with a diameter of less than 0.1 μm) fill up with water by condensation before the air saturation (RH=100%) is reached. In practice, pores with a diameter between 0.1 μm and approximately 10 Å (molecular limit of water) fill up with water at 30%<RH<100%. 78 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 79 Moreover, since the internal capillary vessels sometimes have radii that are smaller than those of the pores to which they are connected, many pores may remain full of air and devoid of water after the capillary vessels have filled up. Another aspect of the condensation process that must be kept in mind is the presence of hygroscopic salts in the materials that form the works of art or on the medium on which they have been made. These salts may considerably anticipate the condensation process, in comparison to what may be expected on the basis of the Kelvin effect alone. Also the ventilation is another aspect that heavily influences the rate of evaporation or condensation. Let us consider for example an evaporation above a surface. Under a surface of water the saturation occurs when vapour pressure in the water equalize the saturation pressure in the air. If the air further away is not saturated (more dried air, i.e. lower relative humidity) a diffusion of vapour occurs from the surface towards the zone with a lower molecular concentration of water (Dalton Law), i.e. toward a zone with a lower pressure. This means that the evaporation is favoured and the more the difference of pressure is high, the more his evaporation takes place. This means that also the temperature has an important role. Moreover also the direction of the circulation of the air is important: the evaporation increases more if the air flux is perpendicular than parallel to the surface. Also the atmospheric pressure influences the evaporation which increases when the atmospheric pressure decreases and viceversa. Finally we can write the following relationship [82]: Mw = C • (esat - ev) • (760/p) (g/m2 h) Where Mw is the quantity of evaporated water, the esat and ev are the vapour pressures expressed as mm Hg and C is a coefficient of superficial evaporation which contains the influence of the air velocity. For example at an air velocity of 2 m/sec, with parallel direction to an evaporating surface, the empiric value of C is 42 g/m2 h mm. In conclusion it is easy to understand how important it is to preserve environmental conditions (T, RH) whose characteristics avoid the condensation-evaporation cycles that determine mechanical stress and salt migration within the materials. In the conditions usually present in exhibition environments, particularly in our climatic regions, where the seasonal cycles are marked, this goal is not always easy to achieve. Stress, however, can be avoided or at least reduced by endeavouring to keep the RH conditions as stable as possible. 79 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 80 4.2 MATERIALS AND THERMOHYGROMETRICAL STRESS A material immersed in an atmosphere whose conditions of temperature and humidity have changed tends to achieve equilibrium with the atmosphere, by releasing or absorbing heat and vapour. This phenomenon is less immediately visible and is revealed later on by its effects, i.e. damages, often already irreversible, in the precious object. For instance, the change in the dimensions of a piece of parchment can be up to 3-4% when there is a transition from very low relative humidity values (15-20%) to very high ones (85-90%) [120]. This explains the irreversible damages undergone by certain show-pieces when they are brought to regions with very different climates. 4.2.1 Principles of propagation of heat and vapour a) Heat flow Heat is the energy that is transferred from a warmer body to another colder one. In physics the phenomenon of heat transmission may take place in the following three modes [36, 118]: Conduction. This is the gradual release of thermal energy between points of a body that have different temperatures, or between two bodies that touch each other. This energy release is proportional to the existing thermal gradient*, according to a coefficient that is typical of each material and is called thermal conductivity*. Convection (advection). This type of heat transmission is characterised by mass transport. It is typical of fluids (e.g. the phenomenon of ebullition or the transport of air masses between two areas having different temperatures). Like conduction, it is proportional to the thermal gradient*. Irradiation. This is a phenomenon of energy transport that does not require a propagation medium like the previous ones and depends on the temperature of the irradiating body. The thermal emission is characteristic of each body, and is proportional to the fourth power of its temperature, besides depending on another series of coefficients, directly correlated with the body and its nature (Stefan-Boltzman’s law). For the first mechanism, the conduction, the heat flow F, from a surface and towards it, is proportional and opposite of the thermal gradient* of the air in front of it, and may be expressed as follows (Fourier law): 80 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 F 13:26 ∝ - KF • Pagina 81 dT dn where dT/dn is the temperature gradient* along the perpendicular n to the surface and KF is a coefficient, which depends on the properties of the body (thermal conductivity). Convection is the second mechanism which intervenes in the heat balance of a body. In fact a great quantity of heat can be transported towards a surface or removed from it. It is proportional, like the first to the difference of temperature, and precisely: F ∝ KF • (Tsurface – Tair) where KF is a convection coefficient. The efficiency of this exchange depends on the typology of the flux, i.e. if the flux is laminar or turbulent. The last one rises the efficiency of the heat exchange. The calculation of the Reynolds number determine if the flow is laminar or turbulent. Reynolds number is the ratio of inertial forces to viscous forces. Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, producing random eddies, vortices and other flow fluctuations. In this last condition the quantity of heat that can be exchanged greatly rise. Frequently conduction and convection are considered as the main processes responsible for the exchange of heat. This is true if we consider exchanges between air and bodies, but if we take into account also the exchange between bodies this is no longer true. In fact an important contribution to the heat balance is due to the third mechanism, the irradiation. This mechanism, sometimes the most important, is often forgotten. The radiative emission from a body follows this law: F ∝ T4 The heat exchange is almost always the combination of these three mechanisms where the flux exchange is a function of the temperatures of the bodies and the air. Consequently, the radiative exchange which depends on the fourth power of T can play a very important role and has to be taken into consideration. This explains why a person (but also an object) experiences a bad sensation in a building where the walls are a lower temperature. It exchanges his thermal flux with the walls, transferring his heat to the colder body. 81 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 82 For the upper mechanisms bodies immersed in environments that undergo for example an increase in temperature or are directly hit by radiation (solar or artificial) are subjected, in turn, to heating. The superficial layer of the body is the first one to warm up, and the propagation of heat in the layers below takes place with a delay: the temperature of the layers below depends on their distance from the surface and on time. Moreover, the quicker the fluctuations in temperature, the greater the difference between the superficial layer and the inner one. All this involves stress, therefore a dimensional variation called strain*, cracks and then eventually deterioration. The slower the fluctuations, the deeper their penetration in the material, which has more time to adapt to the new conditions. For a correct conservation, it is necessary, therefore, to limit the cycles and to endeavour to achieve conditions that at least have very slow fluctuations. For instance, the thermal stress due to the daily cycle of the sun is much more harmful that that due to the seasonal cycle. So the thicker the walls of the buildings (as in ancient ones), the greater the thermal inertia, and the greater the elimination of the daily thermal oscillations within them. b) Vapour flow Like heat flow, also a difference of concentration of vapour cause a flow Fv which depends on the vapour concentration gradient* in the air. It is expressed as follows [118]: . . dmr Fv = – ρ Kw dn where dmr/dn is the gradient* of the mixing ratio mr (see the definition in the Appendix) along the perpendicular to the surface n, ρ is the density of the air and Kw is the coefficient of vapour diffusion along direction n. Like heat flow, Kw is not constant in the air: it takes on the value of the molecular diffusivity of vapour in calm air and that of diffusivity of turbulent motion in air. When a difference of concentration of molecules of water occurs in the free space (for example from the opening of a humidifier or a dehumidifier) or near a surface (for example if the surface is condensing or evaporating) a flux of molecules of water start in the direction where the concentration of molecules is lower. This case transport of water from a point to another and other phenomena linked to the deposition of pollutants (see Chapter 4). 82 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 83 4.2.2 Equilibrium moisture content in materials The matter is very complicated because the effects of the changing environmental conditions on the real objects depend not only from he particular material, but also from the dimension, geometry, propagation of the temperature and humidity in the internal layers and time, etc. Moreover the dimensional variations consequent to these exchanges with the surrounding air are function also from the structure of the materials, junctions between different materials in the works of art, constrains, etc. For this reason in this chapter only a general treatment of the subject will be done. For a deeper discussion the author send to a more specialized bibliography [77, 81, 82, 83, 93, 94, 127, 128]. Relative humidity is a very important parameter, together with temperature, as regards the stress undergone by the materials; so it must be kept as constant as possible in an environment where works of art are preserved. We have explained in the previous paragraphs that changes in the temperature and in the vapour concentration in the air lead to changes in relative humidity. Moreover, as we have explained in the paragraph about condensation in micropores, the adsorption* of humidity in materials is a function of the relative humidity, so when the relative humidity changes, the internal water content of the materials changes as well. The materials, particularly if they are very sensitive to changes in RH, like organic ones, undergo dimensional changes, therefore stress, cracking, etc. For instance, it was demonstrated that cracks may appear in the paint film that covers a wooden structure when the indoor humidity exceeds 20% [77]. Materials contain “free” water that occupies their cavities without forming any bonds, but also water that is “bound” by particular chemical bonds. For instance, the absorption* of water by paper causes a swelling that may result in the breakage of the hydrogen bonds* or the loosening of the van der Waals forces*, thus reducing mechanical resistance. Variations in relative humidity alter these internal equilibria. The water content of organic materials, for instance wood [57, 81], is usually defined as follows: M= mw- md . 100 md (%) where mw = mass of wet wood, md = mass of dry wood. So, the hygroscopic materials have the capacity to exchange their internal water with the atmosphere. This happen by means two contemporaneous phenomena: the evaporation of the water molecules from material to the external atmosphere and the 83 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 84 capture of the water external molecules from the material. A difference between the velocities of these two processes cause a variation of the contents of water into the material: this latter rise when the adsorption* phase prevails; on the contrary decrease when the desorption* prevails. When these two velocities are equal a situation of “equilibrium moisture content” happens: the material neither loss, nor gain water: the material is in conditions of equilibrium with the surrounding air; this hygrometric content is defined as EMC (equilibrium moisture content), i.e. the mass of water contained per mass unit of anhydrous material in a state of equilibrium [82]. It has been noted that the relative humidity of the air and the humidity of the material are two parameters totally different and don’t clash numerically, so when the equilibrium is reached the value of relative humidity of the air is different from the water content of the material. To reach the equilibrium is not an instantaneous phenomenon, but it needs time, sometimes days or weeks, in particular in thick objects [127, 128]. In fact this equilibrium between object and air is difficult to reach in continuously changing environmental conditions. Inside the thickness a certain gradient* of humidity will be always present, causing stress and strain of the material, so weathering in the time. In the wood, for example, during the drying, the most humid zone is the central part of the piece; on the contrary when adsorbs* the concentration of the water is higher Table 3 84 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 85 in the superficial layers. To calculate the EMC a lot of models continue to be built and different experimental formulae exist. This fields is continuously in evolution because a lot of work have still be done. However it is evident that the EMC, as already said, depends of a lot of parameters, function of the material, but the two most important parameters that influence the content of the water inside a material are the relative humidity and the temperature, and in particular the EMC: 1) rises with the rising of the relative humidity, and decrease with the rising of the temperature: 2) has a stronger dependency from the relative humidity, than the temperature. The most frequently models cited are those of Giordano [82, 84]. The Giordano’s model is based on empirical data and it was built in particular for the wood. Giordano wrote that the most used table adopted from numerous authors is the Table 3 where average results are reported. Following this model the EMC (expressed in % of the anhydrous weight) could be on average approximately calculated from this ratio EMC = UR/5 (%) For normal temperature and relative humidity of the air between 30 and 80%. This formula put in evidence that the EMC depends exclusively by the relative humidity. Fig. 36. Trends of temperature T (°C), relative humidity RH (%) and equilibrium moisture content EMC (%) in the Diamanti Palace in Ferrara (Italy) 85 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 86 From the table it is evident that variations of relative humidity between 10 and 95% and temperature between 0 and 40 °C brings to variations of EMC between 2 and 27%. It can be also noted that in this ranges of variation if the relative humidity is fixed, the variation of EMC for the only effect of the temperature variation is very modest, about 1-2%. Vice versa fixed the temperature the variation of EMC due to the variation of relative humidity is about 24-25%. In practice, in the range between 30 and 80% the EMC depends almost exclusively from the relative humidity, as is evident in the Fig. 36. This explains the similar pattern of the 2 described models. The second one, the Henderson’s model, has a more complex formula that the previous, where the EMC is a function of both relative humidity RH (%) and temperature T (°C): where A (°C-1), B (°C) and C are three coefficients specific for each material, the second one (B) is the most related to the absorption* curve and C is a dimensionless exponent lower that 1. For example, for a wood these coefficients could be approximately A= 0.247 (°C-1), B= 95.83 (°C) and C= 0.667 Diamanti Palace Ferrara (Italy) Fig. 37. Comparison of the EMC calculated by means the two models for the painting 1. 86 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 87 Table 4 Based only on environmental measurements, in these very simplified conditions, a comparison between the two models have been made during a case study [137, 139] on two paintings present in the Diamanti Palace (Ferrara). The results showed that the patterns are similar, but not the absolute values. Anyway, the difference between the 2 models are approximately constant, where the Giordano model furnishes the greater values of EMC as we can see in the Fig. 37 and in the Table 4 where the monthly average values during one year are reported. 4.3 ATMOSPHERIC STABILITY, VENTILATION AND TURBULENCE Another piece of information that is useful in the analysis of the microclimate is the vertical thermal profile of the air, i.e its atmospheric stability. This information makes it possible to understand phenomena such as the vertical conveyance transport of pollutants in the air, their diffusion and deposition, etc. In the physics of the atmosphere, the environmental atmospheric stability [118] is defined by using as a reference what takes place in a small air mass that is shifted adiabatically (i.e. quickly, without any exchange of heat with the surrounding environment) from its original position to a different height. The cooling (or warming) undergone by this small air mass when it is adiabatically raised (or lowered) is -1°C/100 m (1°C/100 m) in dry air (i.e. not allowing for the water vapour present in the air). Along the wet adiabatic* the temperature decreases less, because of the contribution of the latent heat of condensation* of vapour. 87 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 88 It is possible, therefore, to distinguish 3 types of atmospheric stability (Fig. 38), on the basis of the behaviour of the little air mass that is shifted from its original position. a) The thermal gradient* present in the atmosphere is less than -1 °C/100 m. After the lowering and adiabatic* warming, the temperature of the little air mass is higher than that of the sur- Fig. 38: Atmospheric stability: · neutral profile (dT/dz= -1°C/100m), rounding air, so the mass tends to re· unstable (dT/dz > -1°C/100m), turn to its initial position, because it is · and stable (dT/dz < -1°C/100m). lighter than the surrounding air. The atmosphere that has this vertical gradient* is called “stable”, because it is in stable equilibrium in relation to vertical movements. Special cases of stable atmosphere are “isothermia” (null gradient*, i.e. same temperature at different heights) and “inversion” (increase in temperature with increase in height from the ground). b) If the gradient* is -1°C/100 m, the little air mass remains always in equilibrium with the surrounding air, because, though it warms up or cools down, it is always at the same temperature as the air at that height. This is called a “neutral” equilibrium. c) If the thermal gradient* of the atmosphere exceeds -1°C/100 m, the downward movement of a little air mass causes it to reach an adiabiatic-warming temperature that is lower than that of the surrounding air, so the colder, denser air mass goes on moving down, and in this case the equilibrium is called “unstable”. As these definitions make clear, when there is atmospheric instability (produced, for instance, by a floor heating system or present above a lighting system), the air masses are mixed with the upper and lower layers and convey the suspended particles; the latter, as a result of various processes that will be described in the next paragraph, settle on the surfaces of the works of art. If, on the contrary, the atmosphere is stable, the movements of the particles are inhibited and the deposition processes are limited. In undisturbed indoor environments, there is a natural stratification of the air, because air tends naturally to become distributed with a temperature that increases with height, therefore in conditions of atmospheric stability. Getting to know the speed of the air near the surfaces may be very important in the interpretation of deposition phenomena. The speed of deposition of pollutants when the conditions of the other atmospheric parameters are constant may vary con88 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 89 siderably (even by several orders of magnitude), depending on the speed of the air flow and on the diameter of the particles [134], and may become null for the largest particles (diameters exceeding several tens of µm). Moreover, getting to know the trajectory of the air masses and the areas subject to the greatest turbulence may help us to understand how and where the air masses having different thermohygrometric contents get mixed up, and how pollutants are conveyed in the various environments. It may be useful, finally, to identify the areas where air flows at a higher speed or where there is a greater turbulence, in order to position the most precious works of art away from the areas that undergo the greatest heat or vapour exchanges and deposition. A modelling of the circulation of the air masses within a room may be useful for improving its environmental management. 4.4 DEPOSITION OF POLLUTANTS Surfaces, whether they be those of frescoes, paintings or statues, are blackened by the deposition of natural-origin particles, such as those that form the soil, or of anthropic origin ones, such as those produced by various types of household, vehicular or industrial combustion [32]. In particular, the introduction of pollutants in an exhibition environment occurs because the particulate matter suspended in the atmosphere is directly carried in by the public, or penetrates through the openings required for the entrance and exit of people and for changes of air. These particles, which are different depending on their origin, subsequently settle as a result of several physical processes [11, 45, 48, 50, 53, 102], blackening the precious surfaces (Fig. 39), and even deteriorating them chemically or biologically if the environmental conditions allow the triggering of certain chemical processes. In atmosphere the motion of the particles is determined by the action of a certain number of the external forces: m • (dvp/dt) = Σ Fi where m = mass of particles, vp = velocity of the particle and Fi are the different kinds of forces: gravity force, phoretic forces, electrostatic force, etc. The forces present in this formula are connected with the different deposition processes. The thermohygrometric conditions of the environment, particularly at the interface between the air and the surfaces, considerably affect the behaviour of the suspended particles and of the gases, their deposition on the surfaces and their ad89 Pleiadi1_16102008:Pleiadi_eng_16102008 (a) 16-10-2008 13:26 Pagina 90 (b) Fig. 39a-b. Example of cleaning of a Leonardo’s Last Supper particular (Milan - Italy) and example of chemical weathering of the glass due to the reaction between pollutants and water on an ancient stained glass window in (Sainte Chapelle, Paris (France)). (c ) Fig. 39c. Example of chemical weathering of the grisaille* due to reaction between pollutants and water on an ancient stained glass window (Sainte Chapelle, Paris (France)). 90 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 91 sorption*. In fact water vapour is a key pollutant [31] because even if it is not directly involved in the deterioration, it often has a major influence on adverse effects from others pollutants Hence reducing the RH minimizes the adverse effects of pollutants and consequently the deterioration process. Reducing the temperature also reduces the rates of deterioration from pollutants. Reducing both relative humidity and temperature also decreases the emission rate of materials [123] Deposition is a very complex phenomenon closely linked to the microclimate. Many books and articles in the literature contain a detailed treatment of this topic [57, 78, 83, 111, 112, 116, 118]. Here we will only highlight the physical quantities that play an important role in the determination of the deposition processes and in their effects on indoor environments used for exhibition purposes, in relation to the existing microclimate. In museums, galleries, libraries and archival collections some air quality targets have been established For a preservation target of 100 years, the maximum average concentrations for some key pollutants (NO2, SO2, O3, Fine particles (PM2.5)) have been proposed to be [123]) not more than 0.1 µg m-3 (ppb). The deposition processes are the cause of the accumulation of particles on a surface. They are classified as gravitational, inertial, phoretic, Stefan’s flow, etc., and their efficiency depends on the diameter of the suspended particles and on the microclimatic conditions, which may promote or inhibit them. Some deposition mechanisms may occur simultaneously, in synergy or opposition with each other. As a rule, the deposition process is described in terms of the number of particles that settle, N, depending on their diameter, d, and on their concentration in the atmosphere, C. The empirical proportionality constant is the deposition velocity* (called also “mass transfer coefficient”), vdep(m/s or m/h). So for each deposition mechanism and particle diameter, the number of particles that settle on a surface, N, is proportional to the concentration of the particles in the atmosphere, C, as expressed by this simple relation: N(d) = vdep . C (d) The parameter that is fundamental, but more difficult to calculate, is the deposition velocity*, vdep , which is a measure of the rate at which a pollutant reaches the surface of a material. It is closely connected to the microphysical state near the surface. It can be positive or negative in function of the direction of the flux. This parameter depends not only on the diameter of the particles but also on parameters such 91 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 92 as the temperature and vapour content of the air and/or their gradients*, on the turbulence of the air and but also on the characteristics of the surface that determine the latter’s capturing efficiency (the particles may stick to the surface or bounce off it), etc. For each range of particle diameter, the microclimatic conditions determine the value of vdep. In order to reduce the deposition of pollutants on a surface, besides reducing their concentration in the air by acting on their introduction into the environment, it is necessary to create microphysical conditions that can inhibit the process of deposition of the fraction that in any case remains for a longer period in the air, i.e. the submicronic particles. In fact the most harmful particles for the artworks exposed in a museum are those having a diameter between 0.01 and 1 mm, like carbon black, salt, sulphate and nitrate compounds, etc. The transport and deposition processes of these particles are totally correlated to the microclimatic conditions. 4.4.1 Deposition processes The deposition processes of pollutants are generally classified [55, 56, 57, 69, 78, 111, 112, 118], on the basis of the diameter of the particles, d, into these 6 main groups: • Brownian deposition (d<0.01 µm), connected with molecular agitation; • phoretic deposition: - thermophoretic (0.01<d<1 µm), connected with thermal gradients*, - diffusiophoretic (0.01<d<1 µm) and Stefan’s flow (independent of d), connected with vapour concentration gradients*; • inertial deposition (d>1 µm; maximum efficiency for diameters around 4/5 µm, where dynamic forces (connected with movement) prevail over viscous ones (connected with air friction); • gravitational deposition, effective for large particles (d>1-2 µm); • electrostatic deposition (independent of d), connected with particle charge; • photophoretic deposition (independent of d), connected with the solar radiation hitting the particles. We will now briefly describe each of them. a) Brownian deposition This type of deposition is due to the Brownian movement, defined as the irregular motion of the particles due to the thermal bombardment by the air mole- 92 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 93 cules, whose agitation corresponds to their temperature. The Brownian deposition is due to random impacts with the air molecules. All the particles suspended in the air are subject to Brownian deposition, but its efficiency depends on their diameter: a small particle (d<0.01 µm) may be affected by these impacts, which may cause it to cross the layer of air to which it belongs, particularly if the turbulence is very moderate. This type of deposition prevails decidedly in particles whose diameters are from 0.001 to 0.01 µm, and becomes practically negligible for d>1 µm. The deposition velocity*, vdep , ranges from 10-2 cm/s for smaller particles to 10–5 cm/s for particles exceeding 1 µm. Within the larger-diameter ranges, other deposition phenomena gradually gain predominance. In the largest particles, Brownian deposition is not very effective. In gases whose mass is similar to that of air molecules, the process is called “molecular diffusion” towards a surface. The shift of particles that are diffused by Brownian movement depends on the temperature of the air and on the time: it increases when both increase, but decreases with the increase in the viscosity (friction) of the air. b) Phoretic deposition Thermophoresis When temperature or vapour concentration gradients* are present, two diffusion processes can be observed: the first is called thermophoretic, the latter diffusiophoretic. These processes are particularly interesting in the study of the deposition of pollutants in indoor environments, where gradients* are frequently present near the surfaces. The range of diameters of the particles involved in this type of process belongs to the kind of pollutants that linger in the atmosphere for the longest time. Literally, thermophoresis means transport (phoresis) due to temperature, though it is more correct to say that the transport is due to a temperature gradient*: the presence of temperature gradients* causes submicronic particles (0.01<d<1 µm) to be transported towards the wall or away from it, respectively if the temperature of the surface is lower or higher than that of the surrounding air. The thermophoretic process is due to the fact that when there is a temperature gradient* in the atmosphere, a particle undergoes stronger impacts from the air molecules in the warmer zone, and less strong impacts from those in the colder zone. The resulting net total of the forces (i.e. the sum of the forces due to each impact with a molecule) is greater in the warmer part and smaller in the colder part, so a particle of pollutant suspended in the air advances in the direction where it receives less impacts, i.e. towards the colder zone (Fig. 40). 93 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 94 The thermophoretic velocity, vt, is defined by means of the following relationship with the temperature gradient*: vt ∝ dT dn where n is the distance from the surface. The efficiency of thermophoresis is highest in particles whose diameters, d, are 0.01 to 1 µm, and becomes negligible when d > 1-2 µm. Diffusiophoresis and Stefan’s flow Fig. 40. Scheme of the termophoretic deposition. R1 is the resultant on the particle of pollutant of the strengths due to the less intense molecular bumps (o), for an inferior molecular agitation due to a lower temperature (T<); R2 is the resultant of the of the major molecular agitation due to a higher temperature (T>). The difference R between the two resultants represents the force resultant among all the bumps received by the particle, and determine its movement. The expression “diffusiophoresis” means transport due to diffusion: the deposition of pollutant on a surface is also due to a diffusive transport connected to the presence of vapour gradients*. The latter may be due for example to a condensation process on a surface or within its micropores. When a diffusiophoretic process is mentioned, in many cases this does not mean only a process due to diffusiophoresis alone, but the net result of the combination of diffusiophoresis and another process called Stefan’s flow, which takes place simultaneously with the diffusion process. Let us see how this transport process takes place during the condensation of vapour on a wall. The mechanism is rather complex, because it is the sum of a great number of microphysical phenomena connected with diffusion and with the vapour pressure gradients*; so a dynamic vision of the phenomenon is preferable and more easy to understand. 94 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 95 The general process of molecule diffusion takes place when the molecules of a fluid, A, immersed in another fluid, B, have a different concentration in two points. The molecules in excess in the first point get scattered in the fluid until they reach the second point and fill the gap. After a sufficiently long time, A is distributed evenly in B. In particular, in the case of a condensation process on a surface, vapour (fluid A) is “scattered” towards the condensing wall, because in front of it there is a lower concentration of vapour due to the fact that the vapour molecules are adsorbed* by the wall. If we consider the process in terms of pressure, this “disappearance” of vapour molecules may be interpreted as a sub-pressure (from 1 to several hPa) near the wall. When there is a difference in pressure between a point in a fluid and another one, it is known that a hydrodynamic flow sets in and tends to compensate for this difference in pressure. This flow, called Stefan’s flow, is a hydrodynamic flow, not a transport due to diffusion: what is transported is the entire mass, formed of air molecules, vapour and suspended pollutants. The consequence of the onset of Stefan’s flow is the accumulation of dry air molecules near the wall (unlike the vapour ones, these molecules are not adsorbed), so another flow, a “diffusive” one, relevant to the dry air molecules, sets in, in the direction opposite the surface, as an effect of the different concentrations of dry air near the wall and at a distance from it. So two “diffusive” flows, moving in opposite directions, set in: one of vapour and the other of dry air. Each particle of pollutant suspended in the air therefore undergoes the action of two opposite “diffusiophoretic” forces, and the net shift due to the diffusion processes alone is the resultant of the two transports. Since the molecular mass of air is approximately 29, much greater than that of vapour, which is 18, the net shift will be in the direction of the diffusive shift of dry air. Consequently, as a result of the two diffusion processes alone, the particle would move away from a condensing wall. But the presence, at the same time, of Stefan’s flow determines the final direction of the flow of pollutants towards the condensing wall, as a resultant of the different transport forces. In fact, it has been demonstrated mathematically [111] that hydrodynamic flow has a velocity (called Stefan velocity vs) that is much higher than the equilibrium between the two diffusive flows (vd), so the resulting velocity (vR) turns out to be: vR = vs + vd = 0.8 • vs In conclusion, Stefan’s flow turns out to be predominant (80%) over the flow due to the equilibrium between the two diffusiophoretic processes (20%). So the contribution of Stefan’s flow, when particles are conveyed as a result of a condensation 95 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 96 process prevails over the contribution due to the mutual diffusion of vapour and air. Stefan’s flow is therefore the main cause of the transport of pollutant particles, and has the same efficiency for each type of particle, independently of its diameter, up to a size of several µm. On the contrary, the efficiency of the two involved processes in relation to particle diameter is different: the maximum efficiency of diffusiophoresis is in the diameter range between 0.01 and 1 µm, and tends to approach zero for larger particles, whereas Stefan’s flow is practically independent of the size of the particle, since it is a transport of the total mass. In conclusion, pollutant particles settle on a surface when there is a condensation process, and move away when there is an evaporation process. c) Inertial deposition Inertial deposition, also called aerodynamic capture, is connected with the impacting of the particles conveyed by the circulation of the surrounding air, or is induced by inhomogeneity of temperature near a surface. It is the most important active mechanism of removal of particles in the atmosphere. Inertial impaction occurs when, as a result of the turbulence that usually exists in the atmosphere or of weak convection currents that cause the air to slide along the walls, a particle, because of its inertia, leaves the air flow that is transporting it and impacts against an obstacle. In practice, inertial deposition may occur for several reasons, for instance: - because of friction near a surface that generates turbulence; - because of differences in pressure (due to air draughts, e.g. when windows or doors are opened); - because of a thermal inhomogeneity that generates convection movements (e.g. as a result of the presence of a heating or lighting system). This mechanism becomes effective only for particles whose diameter exceeds 1 µm (d>1 µm), and reaches its maximum efficiency when the diameter of the particles is about 4-5 µm. Inertial deposition depends, obviously, not only on the type of particles, but also on the turbulence that is present, the speed with which the air slides near the surface, and the characteristics of the surface. 96 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 97 d) Gravitational deposition This is the deposition of the largest-diameter particles and dust because of their weight. It is the dominant process for large particles. The sedimentation rate in still air is proportional to the square of the diameter of the particle (Stokes’s law) [118], with an aerodynamic correction factor (Cunningham factor) for the smallest particles. The sedimentation rates are in the order of, e.g., 10-6 cm/s for diameters of about 10-2 µm, and in the order of 1 cm/s for diameters of about 20 µm. The process becomes important particularly in still air: when there are convection movements Stokes’s law is not valid, because turbulent dragging is much more effective, and the particles remain suspended for a much longer time, following the movements on a microscale level. This process is effective particularly when the particles are micronic (d>1-2 µm), while it is negligible when they are submicronic. Obviously the deposition process chiefly involves horizontal surfaces (Fig. 41). e) Electrophoresis This type of deposition is similar to gravitational sedimentation, because it is due to the presence of a force field. However, since in this case it is electrophoresis due to electrostatic and non-gravitational forces, there are some differences between the two forces: Coulomb force may be either positive or negative like the Coulomb law establishes: q .q F=K• 1 2 2 r Fig. 41. Example of gravitational deposition (Ara Pacis, Rome). To notice the more evident blackening of the horizontal surfaces, for example in correspondence of the nose, of the shoulders, of the arms or of the folds of the draperies. where K is the Coulomb’s constant (8.99x109 (Nm2/C2), q1 and q2 are the charges of particles 1 and 2 and r is the distance between the particles. The force is attractive, when F is negative, hence when the charges are opposite. In the atmosphere, bodies easily become electrically charged, for several reasons: for instance, the ionisation of the 97 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 98 particles, which causes them to become electrostatically charged, may be brought about by UV radiation or by impacts between particles and molecules. The atmospheric particulate is highly charged, chiefly as a result of random impacts between ions and particles. Electrostatic deposition on a surface does not depend on the diameter of the particles. So it can occur for any particle type and size, provided the charge of the particle has opposite sign with respect to the wall. It is the more active the slower the flow. Electrophoretic deposition is a process that may become important at low RH values: in humid environments, the conductivity of air is greater, and the surfaces are covered with a water film that reduces the phenomenon. Electrostatic capture is typical of dry environments, where the electrostatically charged particles can preserve their electric field for a long time, so they are attracted by a surface and stick to it. f) Photophoresis When particles are hit by a beam of intense light, they are caused to move in the direction of the propagation of the beam: the photons* exert a force on suspended particles. Photophoresis is therefore a process that occurs when pollutant particles are driven towards a painting by a beam of direct and very intense light. Like electrophoresis, photophoresis is independent of the size of the particles, but the intensity of its force varies in relation to the diameter: the photophoretic force with which a particle is driven is proportional to the cube of the diameter, for small particles, and gradually becomes proportional to the diameter, for larger particles. Not all scientists believe that this description of the phenomenon is realistic. Another extensively accepted explanation is that the movement of the particle is caused by a thermal unbalance due to the absorption* of photons* in the area of the particle that is in front of the radiation. This phenomenon is the more evident the larger the particle. The air molecules in front of the warmer area of the particle are warmed up more than the opposite ones. The resulting phenomenon can therefore be regarded as transport by thermophoresis. A universally accepted theory does not exist yet. g) Adhesion of particles to surfaces The efficiency of deposition processes depends not only on the microclimatic conditions, but also on the conditions of the surface. Impact is not inevitably followed 98 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 99 by adhesion: a particle may simply bounce back or be easily brought back into suspension in the air by the current that flows along the surface. The fact that the particle hits the wall does not mean that it is captured. As the reader can easily understand, only in two cases adhesion always takes place: in gravitational deposition, the particles remain on the surface until a mechanical action (such as the passage of visitors, the use of a vacuum cleaner, etc.) raises them from the floor; in electrostatic deposition, Coulomb’s force is very intense between bodies with opposite electric charges, so adhesion to the surface is certain. Capturing efficiency, in conclusion, depends on the physical and chemical properties of the surface and also of the particles, on the conditions of the surface, such as roughness, wetness or dryness, and also on the nature of the adhesion forces. These forces are very intense particularly if the surfaces are damp. They are attractive forces that are proportional to the diameter of the particle and inversely proportional to the distance between the particle and the surface. Moreover, since particles are different not only in their composition, but also in their size and shape, the total blackening effect varies in relation to the particles that are involved. Their size, as we have already stated, ranges from a few hundredths of a micron (microparticles) to several tenths of a micron (giant particles). Their shape may be spherical, like that of the silicate particles emitted by the combustion of coal; irregular, like that of pollen; long, like that, e.g., of hairs coming from clothes or animals. Some of them are opaque (e.g. carbonaceous particles) or semi-transparent. The total blackening effect they give rise to is therefore the sum of shape, size and transparency factors. For instance, mass and trasparency being equal, the surface is more uniformly blackened by smaller particles, which get distributed in a more homogeneous manner on the surface, than by larger particles, which leave considerable gaps. 4.4.2 Models and software A lot of work was done by multidisciplinary teams during international projects (e.g. EU project: IMPACT, contract EVK4-CT2000-00031) in developing models and software tools to define and forecast the air pollution in museums, galleries and archives and the corresponding damage to collections. These models try to predict the average concentrations of pollutants inside buildings, based on the knowledge of some parameters such as ventilation rate, construction materials, surfaces characteristics for absorbing pollutants, etc., and external concentrations of pollutants. The deposition velocity is calculated in order to determine the rate at which the different materials absorb the pollutants. This kind of information can help the museum managers and designers to control and minimise the indoor concentration of pollutants, 99 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 100 so reducing the risk of damage of the objects. This more specific information can be found in specialised literature [29, 30, 123] or in the previous cited EU programs. 4.4.3 Analysis of observed cases Many studies carried out in museums have revealed various phenomena of deposition, some of which are self-evident, while others are less easy to understand, because they require knowledge of the environmental dynamics. Dust is constantly raised by turbulence and by the convection movements, which are present particularly if there is an environmental instability (e.g. where a floor heating system is present). In this condition the warmer air near the floor tends to rise towards the higher layers, following trajectories determined by the conditions of the walls and ceiling. The equilibria between the air, the floor, the walls and the ceiling must be evaluated carefully. If, for instance, the indoor atmosphere of a room is stable and inhibits vertical movements, there may be several situations that lead to conditions favourable or unfavourable to deposition. We will mention a few examples. a) The temperature of the ceiling is lower than that of the air. This means that the air masses that are in contact with the ceiling are cooled by conduction, therefore they move towards the lower layers, creating convective cells*. The result of this is that particles are transported, particularly the ones smaller than 1 µm, which remain in the air for a longer time; then a process of inertial deposition begins on the surfaces as soon as these air masses flow near them. Depending on the temperature of the walls, the vertical movements take place in the following ways: 1) The temperature of the ceiling is lower than that of the air and the walls are warmer (for instance, because a lighting system directly aimed at them has been switched on). The upward movements take place near the walls, the downward ones at the centre of the room. This phenomenon always involves an inertial impaction of the particles, then their inertial deposition on the walls, while the thermophoretic deposition is inhibited because the ceiling is colder than the air. 2) The temperature of the ceiling is lower than that of the air, and the walls are colder too. The air moves down, along the walls. In this case there is both an inertial deposition and a thermophoretic one on the walls. b) The temperature of the ceiling is higher than that of the air or equal to it, and the temperature of the walls is in equilibrium with that of the air. In this case, no vertical movement sets in, so there is no transport of particles and no deposition process, except the Brownian or electrophoretic ones. 100 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 101 The particles that have settled, particularly the larger-diameter ones, are raised again at every passage of visitors, as has been shown by various researches performed, e.g., in the Uffizi Gallery [17, 54, 61], in Chiericati Palace in Vicenza [16], in the Sala dei Giganti in Padua [53], in the Correr Museum in Venice [54, 58], and in the Museum of Antwerp [60]. In the latter, for instance, it was found that the passage of groups of visitors resulted in a tenfold increase in the concentration of larger-diameter particles in the air (Fig. 42). Another particular case of non-equilibrium shows that thermophoretic or inertial deposition respectively play a preponderant role, depending on the situation. Let us consider a ceiling that is apparently homogeneous, but in actual fact formed of different materials, because its structure contains some supporting beams. The areas with and without beams have a different thermal capacity*, so they warm up and exchange heat with the environment in different ways; therefore the efficiency of the thermophoretic and inertial deposition processes is different in the various areas. This is clearly revealed by the blackening that can be observed on the ceiling after a certain time. The circulation of air towards the ceiling and therefore the transport of particles is the same, but the difference in the warming of the areas with beams and of those without leads to a difference in the degree of blackening (Fig. 43). In this case, the difference of blackening is due to the different efficiency of the thermophoretic deposition. Koninklijk Museum voor Schone Kunsten - Antwerpen (Belgium) Fig. 42. Temporal trend of the number of suspended particles N (num/cm3) with different diameters d at the Antwerp Museum. To notice, especially for larger diameter particles, the maxima of concentration in air, corresponding to the central hours of the day, when Museum is open. 101 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 An inverse process can be observed in everyday life above a radiator where there is a ledge that has been built as a “protection” against blackening! The temperature of the wall above the radiator is the same all over its surface, so the efficiency of the thermophoresis is the same; but on the edges of the ledge, where turbulence sets in for geometric reasons, two black smudges appear: here, as regards the blackening, the inertial deposition prevails on the thermophoretic one, which is distributed in a more uniform manner. 13:26 Pagina 102 Fig. 43. Example of ceiling showing a blackening due to the termophoretic and inertial deposition. The different degree of blackening is linked to the different effectiveness of the two phenomena in zones with different mass (and consequently thermal capacity) of the upper material. The clearer zones, in fact, correspond to truss over the ceiling. Summarising the most important points, we can state that, as a rule, in a museum: 1) during the hours in which the museum is open to the public, there is a considerable increase in particles with diameter d>1 µm (Fig. 42); 2) there is no daily cycle for particles with approximately d<0.5 µm (Fig. 42); 3) during the night, particularly in conditions of atmospheric stability and if there are no turbulent movements, as may be the case in an undisturbed indoor environment, the particles with d>1 µm settle in a short time (a few hours or less, depending on their diameter (Fig. 44); 4) during the daytime hours in which the museum is open to the public, the increase in the concentration of particles, particularly the larger ones (Fig. 42 and 44) is caused by people treading (particularly if there are rugs or carpeting that promote accumulation), by the turbulence generated by heating/conditioning/humidifying systems that involve the recirculation of forced air, or by cleaning operations, particularly if they are carried out with the methods normally used in households. Sometimes solving the problems connected with the presence of certain pollutants in the atmosphere is not as difficult as one supposes. In any case, the most simple and commonplace methods should always be preferred to the more complicated solutions, which are not always easy to manage, particularly in everyday life. This is demonstrated by the example described below. 102 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 103 Correr Museum – Venice (Italy) 17 March 1993 Fig. 44. Comparison among the number N (num/cm3) of particles with different diameter in the Bellini Room of Correr Museum, Venice, during the daytime and the night. During the opening hours, a sensible increase of the major diameter particles is noticed, as they are brought in suspension by the passage of the public itself. This difference in concentration daytime/night is not observed for the smaller particles, having a longer residence time in the atmosphere. The concentration of important pollutants* such as ozone (O3), was measured within several exhibition environments (e.g. Correr Museum in Venice) [54, 58]. It is known that O3 is a pollutant that must not be present in rooms where works of art are exhibited: the main authorities [5, 36, 120, 121] lay down these limits: British Museum Library Nat. Bureau of Standards G. Thomson J. Tétreault 0 µg m-3 25 µg m-3 0-2 µg m-3 0.1 µg m-3 0 ppb 13 ppb 0-1 ppb 0.05 ppb Ozone is often present in museums [71], and has very harmful effects on the materials [118], particularly if they are organic. Damages attributed to its presence include: - breakage of the double bonds in carbon chains and consequent alteration of organic material; - loss of colour; - increase in the oxidation rate of silver and iron; - sulphatation of silver and copper. The measurements performed for two summers, in 1997 and 2000, at the Correr Museum in Venice showed that the ozone concentration was unacceptable (peaks up 103 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 104 Correr Museum – Venice (Italy) Fig. 45. Comparison between the ozone concentration in the Correr Museum, Venice (Italy), before and after the restructuring interventions, in the years 1997 and 2000. Notwithstanding the drastic reduction of the average indoor concentration of ozone, the presence of peaks due to the opening of the windows still occurs. to 70 ppb) before some steps were taken to improve the tightness of the shutters and frames. After this, the concentration improved (values under the instrumental threshold of 5 ppb), disregarding the fact that there still persisted a condition of incorrect environmental management, because every day the windows are opened for a change of air, allowing the penetration of air masses with a higher ozone concentration, harmful for the conservation of the works of art. This occurs particularly during the summer, when the outdoor ozone levels, as a rule, are decidedly higher, in average from 20-300 μg/m3, (Fig. 45). The description above leads us to the conclusion that the concentration of particles and gases in the air coming from the outside into a room can be more easily reduced by means of filters or other suitable devices. This is not true, however, for the considerable fraction that is present in any case in the indoor atmosphere (because it is introduced by the opening of doors and windows, or by the public), and that is subject to deposition processes. So a correct management of the environment and microclimate can also solve, or at least minimise, the problem of the deposition of pollutants on the surfaces, which cannot undergo an unlimited series of restorations or cleaning operations. In the control of the pollutants within an indoor environment, what is essential - after the complex situation of the microclimate has been understood - is, once again, a correct management. 104 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 105 5. ANALYSIS METHODS AND MEASUREMENT TECHNIQUES By using non destructive methods, the exchanges between the atmosphere and an artefact can be studied, in order to identify the causes of the alteration of the microclimate. This way, it is also possible to give useful advice for the improvement of the existing environmental conditions and the achievement of a certain microclimatic stability [18, 23, 53, 56, 59]. The problems that may be revealed by this method of analysis are, as we have explained in the previous chapters, of different types: the management of the conditioning and/or heating system, the lighting to be adopted in order to prevent shocks for the works of art, the overcrowding of visitors, the organisation of the opening of doors and windows, the introduction and transport of pollutants, etc. In order to determine whether a microclimate is suitable for conservation and to identify the causes of its variation experimental data are needed over a long time. The methods include several stages and proceed simultaneously in several directions in order to achieve a complete analysis aimed at a very accurate diagnosis of the environmental conditions and at the identification of the causes of its variations. This analysis foresees both short-term ad hoc measuring campaigns and longterm continuous measurements carried out with the appropriate instruments. This will provide the elements to plan the most appropriate interventions to mitigate the changes in temperature and humidity and to reach more suitable environmental conditions. After this diagnostic first phase to improve the microclimate, a second routine phase has to be foreseen. The methods for the first diagnostic phase are based on the essential activities described below. 1) Measuring campaigns The measuring campaigns can be realized in two ways in function of the environment and the goal that the operators want to reach: the first one, more complete but more expensive in time and money, is for environments which contain very precious works of art or which are particular. The second one, more easy to realise and less expensive but giving a less accurate solution for the indoor problems, is sometime sufficient to solve the main problems. 105 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 106 The most complete microclimatic analysis needs to follow the steps described below: - In order to obtain a very fine and sufficiently detailed and complete spatial-temporal analysis of the dynamics of the microclimate and of the processes induced within the environment, measuring campaigns must be carried out for several days and in several seasons, including in each period, if possible, at least one day in which the room is closed to the public and not disturbed by any active system. This makes it possible to have a reference on the basic conditions of the environment. A subsequent comparison between these conditions and the ones altered by the operation of the active systems allows the measurers to understand the phenomena under way and to identify the causes of perturbation. For a few days, during the seasons that are most significant for the local climatology (for example in Europe winter and summer ad hoc measuring campaigns are carried out, detecting, both manually and in a continuous manner, the main thermohygrometric parameters: temperature (T) of the air and walls, specific humidity (SH) and relative humidity (RH) of the air, and dew point spread (DPS) of the air. These manual measurements are performed approximately every two hours throughout the day, with the readings beginning before opening and ending 1-2 hours after closing. - The manual measurements are carried out at predetermined intervals throughout a day, at the same height from the floor (usually about 1 metre), by manually shifting an electronic psychrometer (paragraph 5,2) to positions previously identified on a grid The grid is determined by tracing, on a horizontal section of the environment, a series of equidistant perpendicular lines whose intersections are the measuring points. To these points other points have to be added in order to put in evidence singular situations like outdoor air intrusions (e.g. windows or doors) or the use of devices (e.g. heating or conditioning units). This method makes it possible to reveal the spatial-temporal dynamics of the parameters within the environment, in particular the gradients* in the air and near the walls, therefore to identify the processes under way in the various moments. - During the same measuring period, the temperature and humidity values are simultaneously and continuously recorded: a) along one or two vertical profiles, in order to obtain information about the vertical distribution of the air masses and the thermal stability of the indoor atmosphere. The fact that the measurements are continuous makes it possible to detect the trend of these parameters also in the moments in which the environment is not easily accessible (nightime or periods of closure): these moments are very important, because they contain information about the environment when it is undisturbed. 106 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 107 b) outside in order to follow the external climate and understand the influence that it has on the internal microclimate. - Measurements are also performed with an anemometer in the internal free air and near the walls, in order to evaluate the air flow that skims the surfaces. The concentration of particles in the atmosphere possibly in function of their diameters is also measured, in order to study the different deposition processes that involve the works of art, and their relative importance in function of the microclimatic conditions present in the room. In a less complete analysis some continuous recording of temperature (air and wall) and humidity at significant points of the rooms is done instead of manual measurements. Of course the determination of the number of measurement points depends from the specific goals. 2) Data processing The collected data are then processed by means of specific programmes and formulae, in order to obtain the parameters that have not been measured directly. Suitable software is used to plot the spatial-temporal distribution of all the measured and/or calculated quantities. In particular for the manual measurements the spatial distribution of the parameters is ascertained by interpolating the measuring points on the grid so as to obtain maps with isolines (equal-value lines) for the different parameters traced with the frequency linked to the resolution* of the used instruments that usually are 0.1°C for temperature, 1% for relative humidity. 3) Data interpretation All the processed and plotted data make it possible to have an overall view of the environmental dynamics in the various moments, in other words of the spatial-temporal trend, both vertically for studying the atmospheric thermal stability and horizontally for revealing the gradients* in space and variations in time, therefore the exchanges between the atmosphere and the artefact. At this point in the analysis, we are in possession of most of the information required to evaluate the suitability or unsuitability of the microclimatic conditions and to identify precisely the causes of perturbation. 107 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 108 4) Other analyses - Modelling A further elaboration can be supplied by the mathematical modelling of some phenomena on the basis of experimental data. The purpose of this is to simulate several possible problems and situations, and to predict or calculate the effects of certain phenomena, such as: thermal or hygrometric stress (e.g. the contribution of energy due to the absorption* of solar or artificial radiation or for the on/off of the heating/cooling systems); air circulation within a room, with the consequent identification of the trajectories of the particulate or of the greater turbulence that may promote exchanges and deposition; comparison between the concentration within the museum and out of it; etc. For instance, the study of the thermal wave propagation on objects that have different geometric shapes and are hit by a periodic fixed radiation (e.g. a lamp), or by a moving one (e.g. the sun), may be useful for understanding the consequences of the use of intermittent lighting. The latter may be more or less harmful, depending on the material, number of cycles, intensity of the light source, etc. - Local climate analyses Another aspect that should not be underestimated is the analysis of the local climate, in terms both of the actual climatic conditions of the environment which influence the indoor microclimate and of the knowledge of the local circulation, for the identification of the place of origin of the air masses or rain [47, 51], and of their content of pollutants. Therefore it is advisable also to carry out purely meteorological investigations, which turn out to be quite useful in completing the picture for the interpretation of the causes of the perturbation of the indoor microclimate: the local climate provides information both about the external forcing factors (e.g. solar radiation) that affect the indoor climate and about the air masses with different thermohygrometric content and concentration of pollutants. For instance, it is possible to identify the place of origin of the winds that may be the cause of the concentration of pollutants detected around the works of art, or the relationship between the concentration of pollutants and the weather conditions that determine the trajectories of the air masses [41, 47, 50]. This further investigation would be particularly useful if it were carried out (as is advisable) together with other experts, such as chemists, biologists, etc., who can supply the complementary information for a correct environmental diagnosis. All the information defined in the points above is used for an overall view of the phenomena and a more thorough and detailed knowledge of the causes of perturbation, and for supplying all the elements needed to understand the state of the mi108 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 109 croclimate that is present. Only an overall view of the situation can give the information required for taking measures for improving the conservation conditions, where necessary. This type of analysis, performed with non-destructive methods and based on the dynamic study of the environment, can reveal the physical processes that affect a work of art and the stress it undergoes. Only this thorough and detailed knowledge of the environment and of the boundary conditions makes it possible to define the causes and effects, and to identify the correct improving measures to be taken. After this first phase to obtain the knowledge of the environment and the subsequent interventions to improve it, a second routine phase has to be foreseen. In the most critical points identified during the first phase some simpler and cheaper devices have to be installed to monitor continuously the environment. These devices have an alert function. When the measurements recorded by these devices show to deviations from the established values a more thorough investigation has to be done again to understand what is happening. The approach that has been described requires, besides the knowledge and application of certain basic physical concepts, also a series of ad hoc instruments for reading the thermodynamic quantities and, above all, their variations both in space and in time, with an accuracy* that makes it possible to extrapolate the information needed to understand the processes under way. Conventional meteorological instruments used outdoor are not suitable for this type of analysis, therefore in the last years a series of ad hoc instruments and technologies has been developed and is still being improved, for the specific purposes and needs of this sector. 5.1 MAIN THERMOHYGROMETRIC PARAMETERS In the general description of the methods to be applied for microclimate analysis, we have mentioned several physical variables and the physical processes connected to them. In order to allow the reader to thoroughly understand all this, we must define these variables and explain their physical meaning. Microclimate analysis is based on the study of the spatial-temporal evolution of these thermohygrometric parameters. The latter will be described below and physically defined in Appendix 1, referring the reader to specific texts [57, 112, 118] for a more in-depth analysis. 109 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 110 a) Temperature (T) Temperature (T) is a physical quantity with which all of us are familiar in everyday life: it indicates the level of heat or cold, giving an objective evaluation of our sensations. Thermometers and thermometric scales are also a part of our day-to-day experience: the temperature scale commonly used in Europe is the centigrade or Celsius one, which assigns the value 100°C to boiling-water temperature at a standard atmospheric pressure and the value 0°C to melting-ice temperature. The absolute thermometric scale places its zero value at –273.16°C, called absolute zero temperature, and uses as a measurement unit the Kelvin (K), quantitatively identical to the centigrade degree. b) Vapour pressure (ev) The vapour pressure (ev) is the pressure exerted by the vapour molecules present in the air mass. The saturation vapour pressure, esat, is the pressure exerted by the vapour molecules at the maximum concentration allowed by the temperature of the air mass. This means, in other words, that an air mass at a certain temperature can contain at most a certain quantity of vapour, and not more than that. Beyond this concentration, vapour cannot exist in that state and the molecules condense. This process is dynamic, not static, because in the air mass under consideration there takes place a constant exchange between the molecules that pass from the vapour state to the liquid one and vice versa; however, the maximum quantity of vapour molecules contained in the air mass at a specific temperature is constant and is called saturation vapour pressure. c) Specific humidity (SH) and mixing ratio (MR) Specific humidity (SH) and mixing ratio (MR) are others parameters that are essential for understanding the physical phenomena under way. Both, they express, in simple terms, the ratio of the number of vapour molecules to the total number of dry plus vapour (SH) or only dry air molecules (MR) (see appendix 1). They are very similar and furnish similar information. Some scientists prefers to use the first, some other the second. The SH and MR are independent both of temperature and of volume, so they remain unchanged in any thermodynamic process where no variations in the vapour content such as evaporation or condensation occur. In practise, in a certain air mass formed of dry air and vapour, SH or MR represent the amount of vapour present in it, expressed in grams. It is therefore a quantity that hygrometrically identifies the air mass under consideration. 110 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 111 These parameters are very useful for understanding the processes that change the hygrometric characteristics of the air mass. A change in SH or MR of the air mass contained in an environment makes it possible to realise that something is happening, therefore to look for its causes, because only a contribution of vapour from a source (e.g. a humidifier) or a suction (e.g. a dehumidifier) can cause a variation in this parameter. The saturation specific humidity, SHsat, or the saturation mixing ratio, MRsat are the maximum amount of vapour that a certain air mass at a certain temperature can contain. The vapour molecules added to that air mass will therefore condense. d) Absolute humidity (AH) Absolute humidity (AH) is a parameter that is often used, but equally often mistaken for specific humidity. It is physically quite different from the latter, and the information it can supply is much less useful bring to mistakes in the interpretation of the phenomena, at least for the investigation of the causes of environmental perturbation. The absolute humidity expresses the ratio of the quantity of water vapour contained in the atmosphere to the volume of the air mass, which is a variable quantity, because of a possible compression or expansion of the air mass. This variable is still used in place of SH. As the reader can notice, in spite of its being called “absolute” (a word that is often misleading), AH, in comparison with SH, is far from being unchanging in the physical processes we intend to study. It depends on the vapour mass, like SH, but also on the volume of the air mass that contains it, which is a variable that depends on temperature. If the temperature rises, the volume of the air mass increases; if the temperature drops, the volume decreases; this is due to the well-known state equation of perfect gases, pV = nRT. If an air mass warms up (e.g. because a heating system is switched on), it tends to expand, and its volume changes, therefore its AH changes too, but not the vapour mass contained in it. Therefore, whereas in this context the AH changes, the SH does not, and the latter is a parameter that makes it possible to identify the air mass, because the vapour content is one of its characteristics. This way the SH in practice constantly identifies the air mass even if the latter, in its movements, undergoes variations in the other parameters that characterise it. For this reason, the SH and MR, in meteorology are considered a tracer of the air masses, because thy makes it possible to identify and follow them along their trajectories. e) Relative humidity (RH) Together with temperature, relative humidity (RH) is the most important parameter in the study of the behaviour of materials. The variations in this parameter underlie the stress undergone by the materials and therefore their deterioration. 111 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 112 RH expresses the degree of saturation of vapour in the air, and depends both on the temperature and on the quantity of vapour present in the air. In practise, it indicates how far the air is from saturation, i.e. from the condensation of the vapour contained in it. Since, as we have already explained, saturation depends on the temperature of the air, RH indicates how much vapour is contained in the air mass, with respect to the amount that the same air mass at the same temperature might contain if it were saturated. In practical terms, this explains why, if the temperature is kept constant, when the vapour content of the air increases the relative humidity rises. Vice versa, when the vapour content is constant, an increase in temperature leads to a decrease in relative humidity. So in the environment small variations in vapour or temperature may take place, but if the relative humidity is to be kept constant, as is advisable, the variations in both these parameters should compensate for each other. f) Dew point (DP) In practise the dew point (DP) indicates the temperature that must be reached by the air mass to become saturated, and at which the condensation process begins in the air or on a horizontal surface in thermal equilibrium with the air. It is the parameter that must be monitored in environments, because if, for some reason (e.g. the switching off of the heating system), the ambient temperature drops below the dew point, condensation begins on the air or on the surface in equilibrium with the air. g) Dew point spread (DPS) When the dew point has been defined, the dew point spread (DPS) is a self-evident quantity: it indicates which difference in temperature is required for spontaneous condensation, in other words by how many centigrade degrees the air must cool down in order to make it possible for condensation to take place in free air. h) Wet-bulb temperature (Tw) or psychrometric temperature If the bulb of a thermometer is kept wet by wrapping it in a damp gauze bandage, the temperature value that is read is different, because of the water evaporation that takes heat away from the bulb. The temperature (which is also called dry-bulb temperature) and the wet-bulb temperature are the basic parameters in the measurement of the thermodynamic characteristics of the air that are performed by means of a very precise instrument called psychrometer. In actual fact, Tb depends on the degree of air saturation, therefore it is a function of the hygrometric content of the air. In fact the drier the air, the more water evaporates, the more latent heat is required and the lower is the temperature detected by the bulb. 112 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 113 The wet-bulb temperature (Tw) is also defined as the equilibrium temperature reached by water when it evaporates in the atmosphere where there is a moderate ventilation (3 to 5 m/s). From the measurement of these two quantities, T and Tw, it is possible mathematically to obtain all the other main thermohygrometric parameters that are useful in microclimate analysis (see Appendix 1). i) Comparison between Tw and DP When both the parameters of the air and surface temperature, DP and Tw, are known, it is possible to understand whether a surface is in the evaporation or condensation stage. We have already stated that the DP is an important parameter for studying condensation on a non-porous surface: for condensation to begin on a surface, the latter must cool down to the DP of the air. Instead Tw represents the lowest temperature that the surface of evaporating water can reach, so for evaporation to begin on a wet surface, the latter must reach the Tw of the surrounding air. The following relationship is always valid: DP ≤ Tw ≤ T and only with RH=100% : DP = Tw = T The condensation temperature (DP) cannot be reached through evaporation: both the DP and Tw values are reached through isobaric (constant-pressure) cooling up to the saturation point, but the DP is reached without changes in the vapour content (expressed by SH or MR), whereas the Tw is reached through the addition of water, therefore by increasing the SH and thus anticipating air saturation. Therefore a comparison between the three temperatures of the surface, DP, Tw and T, explains what kind of phenomenon is taking place. 113 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 114 5.2 INSTRUMENTS Microclimatology was developed on the basis of the fundamental concepts of the physics of the atmosphere, so the instruments of microclimatic analysis are in close correlation with those used in the field of the physics of the atmosphere or meteorology [3, 38, 71]. However, there are significant differences between the instruments for detecting environmental phenomena indoors and outdoors. The phenomena that are detected in indoor environments have more narrow operational limits and sometimes a greater sensitivity to thermodynamic phenomena. The classical physics of the atmosphere uses instruments that must work outdoor, therefore must be weatherproof and able to detect wider-range phenomena than those that appear in a room where works of art are preserved. However, the physical principles on which the instruments required for an indoor microclimatic analysis work are the same. What is a “measurement”? This term denotes a set of operations whose purpose is to extract, process, transmit and record signals that supply quantitative information on the values of the quantities under consideration. The measurement can be made both in a direct and indirect way, applying specific physical laws that link the unknown variable to others directly measurable. In order to obtain quantitative information that helps us to understand the processes underway, the measurements must be performed by means of instruments suitable for the specific case and properly calibrated with specific samples. The three fundamental compounds in a measurement system are: 1) the sensors which are the basic elements being “sensitive” to the parameters that have to be measured and able to transform this “sensibility” in a signal . This signal can be - analog when the output is the direct measure of the parameter being measured; - digital, normally binary, when the information is a series of digits following a determined logic 2) the analog / digital converter(A/D).Today almost the all of the instruments have the first type of output and then they are properly transformed in digital with suitable analogical/ digital converters(A/D) which normally amplify the signal 3) the indicator which gives the operator the numerical value in appropriate units. But measures of the same parameter made with different devices give different results due to the presence of various kind of errors. To a measure is an operation 114 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:26 Pagina 115 more difficult than the people could think! In particular, this difficulty becomes grater, higher is the precision that the operator needs! Moreover, the periodical operation of calibration is another aspect very important in the use of the technical devices. A typical example is the use of the hair hygrometer in museums: the traditional thermohygrographs. This instrument is still extremely widespread, because it is easy to use, though it is not very sensitive, reliable or precise, and requires a very frequent upkeep: at least once a month it must be calibrated and the hair regenerated. In many cases, this procedure is not followed. It is very rare for museums to monitor their instruments with this degree of care and precision, considering the frequency required for these operations. So the traditional, and by now obsolete, method based on the use of a hair thermohygrographs placed at the centre of the room or in some corner of the museum cannot supply reliable information about the variations in environmental conditions. Nowadays much more reliable instruments are available: they supply the same information, require a less frequent calibration and are much more sensitive. However a remark have to be done. Frequently the operator buy compact instruments for the detection of the main thermohygrometrical variables. When he use this kind of apparatus he have to be very careful to its technical characteristics. Frequently the firms gives as characteristics of the devices (accuracy*, time constant*, etc.) those of the sensors inside the box. This in realty is not true. The real technical characteristic is that of the whole apparatus which is far from that of the internal sensors. Table 5 We will now describe the main instruments used in the analysis of the microclimate of an indoor environment. 115 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 116 a) High-resolution thermometers for continuous air and surface measurements The temperature is the first fundamental parameter that has to be measure. This measure is strongly influenced by different variables like for example contact with the surface or the time response. In general the introduction of the sensor to do the measure perturbs the initial conditions of measure because itself absorb a certain quantity of heat. On the consequence the measure of temperature will be more accurate minor will be the thermal capacity* of the sensor. For this reason the kind of sensor, the method of measurement and the modalities of installation have to be done with a lot of attention. The most used temperature sensors in the microclimatic field are: Thermoresistance* Thermistors* Thermocouples* The thermoresistance called also RTD (Resistance Thermal Detector), are sensors formed of an electric resistor that changes when the temperature changes. The element can be platinum, nickel or tungsten with different resistance values. The most used is the platinum and for this reason they are called Platinum resistance thermometers or Pt100 that have a resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C. The relationship between temperature and resistance is approximately linear over a small temperature range. Also the Pt1000, more precise of Pt100 begins to be used in specific applications. The thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. The sensitive element in this case is a semiconductor whoes resistance increases with increasing (PTC) or decreasing (NTC) temperature. They have a small dimension in comparison with the RTD and a greater sensibility. The last ones are the thermocouples. Its functioning is based on the Seebeck effect. This effect occurs when 2 metals are connected. In the presence of a temperature difference a small current flows between the junction. In function of the kind of metals used they are differently classified (Table 5). They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is precision*; system errors of less than 1 °C can be difficult to achieve. They are more hardy than the previous two. For this reason they are often used in outdoor environment. 116 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 117 For microclimatic studies, the air temperature is measured by the most sensitive sensors, i.e. the thermoresistences and the thermistrors. In fact it is suitable for environmental studies, choose sensors which restricted ranges, but with a high accuracy* and precision*. The measurements performed with thermoresistences and the thermistors can be very precise and quick: the specifications declared by the manufacturers for these devices include an uncertainty* (often called also accuracy*) of ±0.1°C and a time constant* of about 1 s. In environmental climate analysis, another essential information is that of surface temperature, which is important in the analysis of heat flows, therefore of thermohygrometric cycles, in the study of the surface condensation process and in the analysis of deposition processes. For surface temperature measurements, also called “contact” temperature measurements, as a rule the sensors that are used are thermoresistance* (Pt100) or a thermistor*. Surface temperature measurements are quite tricky. They may alter the surface temperature; they may measure a temperature that is half-way between that of the air and that of the artefact; or they may be affected by energy coming from the outside. Moreover, the sensitive portion of the sensor is very delicate, so it is fitted in a casing in order to protect it, and the casing may affect the measurement, particularly in transient situations. Another very important factor is the way the thermal contact is made. Therefore, surface measurements that are performed inadequately or interpreted without allowing for the operating modes may easily lead to incorrect results. This type of sensor (Pt100 or thermistor) is used also in the analysis of vertical thermal profiles for studying indoor atmospheric stability. The thermohygrometric characteristics of the surrounding air undergo quick and frequent variations, particularly in rooms frequented by a great number of people, such as exhibition and reference rooms. In order to analyse the dynamics of the phenomena it is necessary therefore to have prompt, precise responses, so in this case too these sensors can answer the measuring needs. It is necessary, however, to pay attention to the casings that protect these sensors: their presence may cut the quickest temperature variations or cause a certain delay in their detection, because of the casing apparatus. Obviously the manufacturer’s declaration about the uncertainty* of each instrument usually refers to the sensitive portion and not to the overall device, i.e. the sensor and its casing, so the declared specifications may turn out to be different from what is subsequently found in reality. It is expedient, therefore, always to be very exacting when choosing an instrument on the basis of current needs and of the environment to be analysed, and extremely wary in using the instrument. 117 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 118 The accuracy* of the output of these instruments is linked not only to the accuracy* of the temperature sensors that can be very high with the actual technology, but also depends to the final accuracy* of the system of measure which includes next to the sensor (termoresistences or termistors) the electronics used to process, transmit and record signals that supply quantitative information on the values being measured. So, it is often not enough to read the characteristic of the sensor (accuracy*, etc..) declared by the suppliers on the data sheet to be sure that the variable measured in the field has the declared precision. In fact to know the real error at the and of the chain of measurement the whole device composed of sensor and electronics part has to be calibrated. The more accurate the measure the more difficult it is. b) High-resolution electronic psychrometer The thermohygrometric parameters of the ambient air and of the air that flows over the surfaces under consideration must be detected by means of an instrument called “electronic psychrometer”, which has a high resolution and must have a forced ventilation at a constant speed between 3 and 6 m/s. It is the classical instrument for a very precise manual measurement. The humidity measurement is performed by means of the reading of two thermometers (usually thermoresistance* or thermistors* with a 0.1°C accuracy and a 1 s time constant*) placed one next to the other: one is dry and the other has its bulb wrapped up in a cloth soaked with distilled water, and they respectively obtain the T and Tw values. The psychrometer is the most accurate instrument above all for high RH values, though in these extreme conditions it is necessary to watch out for the possible deterioration of the entire apparatus that surrounds the two sensors. On the contrary it is not advisable to use it under 20% RH and in environmental conditions under 0°C. In the case of the manual-type measurements described in Chapter 5, the method with which the psychrometer is used must be correct, the operator must try to interfere as little as possible with the instrument and be very careful in choosing his/her position with respect to it. In order to allow a comparison between these very accurate data, the measurements are performed with the same psychrometer: thanks to its speed of response, the instrument can be shifted quickly from a measuring point to another one. This prevents intercalibration errors between different instruments. In practice, therefore, the only limit is the resolution* of the instrument that is used. Moreover, in the evaluation of gradients* the important information is the difference between the values that have been read, so also any systematic instrumental error becomes negligible. The accuracy* of this instrument is mainly linked to the accuracy* of the temperature sensors used and of course, as described, to the calibration of all the system, sensors plus electronic system. 118 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 119 Because the measure is done with a forced ventilation, this measure is representative of the relative humidity conditions of the environment around the device, not of the precise point of measured. In fact during the functioning a certain quantity of air is aspired in function of the dimension of the sucking fan and geometry of the body to reach the 3-5m/sec of velocity. During delicate measurements, in particular if associated to other interdisciplinary measurements on the same point, it could interfere with other measures. This emerged during an interdisciplinary study is in progress at the Lascaux caves where the auFig. 46. Picture of the psychrometer built ad hoc installed in the thor is involved. The measure of Lascaux caves (2008). relative humidity have be done near the wall and it could negatively interfere with the microbiological measures and with the measure of the contents of the water into the most superficial rock. For this reason this device (Fig. 46) was studied at hoc to drastically reduce the dimension in order to have the required velocity of the air inside the sucking tube, but at the same time moving a minimum quantity of the air. Moreover less of 0.2÷0.3% of accuracy* is reached by means of very accurate and opportunely calibrated temperature sensors. c) Capacitive sensors for relative humidity measurements The reading of the relative humidity of the air is another very delicate operation, particularly if it is done in certain conditions, for instance in highly polluted environments or where there are very high relative humidity values, e.g. in a cave. For measurements to be performed in a continuous manner or over a long period or outdoor, the sensors that are used most frequently are those based on the response of capacitive elements whose dielectric* portion is quickly sensitive to variations in the relative humidity, called “capacitive sensors”. One of the two faces of the condenser is made of porous material in order to permit the water molecules to reach the dielectric*. 119 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 120 Sainte Chapelle - Paris (France) Fig. 47. Comparison between direct (Dew Sensor) and indirect measurements (difference between temperature of the surface and dew point of the air): (a) test in laboratory; (b) data from field test in Sainte Chapelle, Paris (France) (VIDRIO Project). If we assume that the dielectric* constant linearly varies with the change of relative Humidity RH we obtain: e = e0 + α • RH where α is a constant function of the polymer used as dielectric* and e0 is the dielectric* constant at RH=0. So the capacity of the sensor is expressed by: Csensor = e • K = e0 • K + K • α • RH where K depends of the geometrical characteristics of the sensor. 120 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 121 The used polymers are generally characterised by high sensibility to water vapour, low hysteresis* and good chemical stability. However they are easily perishable and they can’t support high temperatures where combinations of different polymers are used. Their uncertainty* (or accuracy*) declared buy the constructors is in the order of 1-2%, in reality at least 2-5%, and their time constant*, depending on the model, may range from 10-15 s to a few minutes, particularly if the sensors are equipped with a protective cover, as is almost always the case. Their operating principle is simple: the dielectric* film of a capacitor* absorbs water molecules, changing the electrostatic capacity* of the capacitor*. What is measured is the variation in the dielectric* constant. The sensors are fairly quick and have a low hysteresis* at a RH between 5-10 and 90÷95%, while at extreme values they become rather unreliable. They have some problems: a high drift*, particularly if they are working in rather polluted environments; and a need for frequent calibrations (every 5 to 12 months, depending on the environmental conditions in which they work). d) Measurement of surface condensations The knowledge of the formation of surface condensation is another important measure for the conservation of works of art. Actually few devices give this information in a precise way, in particular if the surface is transparent, so that the light can interfere in some way with in the measure. Normally the superficial condensation is found in an indirect manner. It consists in fact in the calculation of the condensation conditions using the physical parameters measured with the microclimatic sensors, in particular surface temperature and dew point of the air (calculated using air temperature and relative humidity). The condensation phenomenon occurs when the difference between surface temperature and air dew point reaches zero. Hence, the indirect measurement of condensation is affected by the errors included in the measured values (temperature and relative humidity) that spread through successive mathematical operations. Taking into account the accuracy* of the measurements of temperature and relative humidity, the error in the calculation can been evaluated, but the real error of the measurement is in practice greater than the one calculated for a lot of reasons not linked to the simple accuracy* of the sensors. This have been clearly put in evidence comparing the indirect measure with a direct measure by means an innovative dew sensor built during the EU VIDRIO project (Fig. 47) The working principle of the new sensor (Fig. 48) is based on the well known diffusion phenomenon of a beam of light from condensed water. The beam is generated by an emitter, reflected directly by the glass surface itself without using other disturbing surfaces, and then collected by a receiver. When condensation takes place 121 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 122 (a) (b) Fig. 48 a-b. Dew Sensor. The new device (a) and a picture of its installation in the Lascaux caves. (2008) on the glass surface, the emitted beam is diffused, so a weaker beam is collected by the receiver. The signal is semi-quantitative, that means that the amplitude of the peak that indicates the presence of condensation is proportional to the intensity of the event itself. In conclusion the study proved that the direct survey of condensation with the dew sensor is more accurate and reliable, that some microclimatic sensors have instrumental limits, in particular at high relative humidity values, and that other devices on the market are not reliable. e) Infrared detectors for remote temperature measurements A theoretically valid method, though difficult to interpret, is that of measuring the radiometric temperature of the surface by means of a “radiometer”. Each surface emits energy, in relation to its temperature. According to the Stefan-Boltzman law, the energy irradiated by a blackbody* may be expressed as: R = σ • T4 (Watt/m2) where T is the absolute temperature (K) and σ = 5.7 * 10-8 W/m2K4. From the measurement of the energy emitted by a body it is possible to obtain the temperature of that body. This operation, however, is not very simple, because in actual fact no body is a perfect “blackbody*”. The Stefan-Bolzman law for any type of body becomes: R = ε • σ • T4 (Watt/m2) where ε is the emissivity of a body defined as the ratio of the radiation emitted by the surface to the radiation emitted by a blackbody* at the same temperature (ε =1 for a perfect blackbody*, between 0.75 and 1 for the others materials). The infrared detectors, as we have stated, measure the energy emitted by the body under examination. From this value, if the emissivity (ε) of the surface of that body 122 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 123 is known, it is possible to obtain its temperature. What is problematic is precisely the emissivity value: it is not very easy to know the ε value of every material. There are several charts for this in the literature (for instance, at ambient temperature, in nonmetallic materials the ε value always exceeds 0.8; in rough, oxidised surfaces it is between 0.6 and 0.9), but each material has its own specific emissivity. Within the sphere of a microclimatic analysis, these instruments are used chiefly for measuring the temperature of surfaces that cannot be reached easily and require an indirect reading. The usefulness of the available instruments is still limited, because of the measuring precision* required by the operating method that has just been described. A more complex technique, based on the same physical principle and used for measuring surface temperatures, is thermography [92]. This technique is useful because it makes it possible to make comparisons or to gain further information about some phenomena that have been studied through a microclimatic analysis. In this case too, if the emissivity of the surfaces is known, it is possible to obtain maps of the distribution of the temperature on the surfaces. The thermograph is an instrument that can be very useful in many applications, but it is quite expensive, and the images it supplies are hard to interpret unless one is an expert in this field. In the market a particular kind of radiometer exists to detect the temperature of the surface without touching the surface itself. They can be called “almost contact radiometer” because they place the detector very near the surface but without touching it. The terminal part of this device is a parabolic cavity where the radiation emitted by the surface is totally captured, so this law is valid Re + Rr = 1 where Re is the radiation emitted and Rr is the radiation reflected. In this way the error coming from the determination of the ε is eliminated. f) Anemometers In indoor environments (museums, churches, etc.), ventilation is not measured with the usual cup anemometers that are used in classical meteorology. The limits of these last instruments are so severe that they practically do not detect anything, because the values present in the circulation within an indoor environment are below their threshold. The natural ventilation in an indoor environment, the inflow of air from the outside, the air circulation in a confined environment such as a showcase, the ventilation near the surfaces, etc., are measured by means of an instrument called “hot-wire anemometer”. 123 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 124 This instrument is based on the principle that the loss of heat of a body is in relation to the air flow that crosses it. The heat lost by the sensitive element depends on the characteristics of the sensor (temperature, geometric shape, size) and also on those of the air (speed, temperature, pressure, density, thermodynamic properties). Since the only unknown variable is air speed, the loss of heat measures the change in speed: the sensitive element is formed of an extremely fine filament (depending on the manufacturer, its diameter is a few μm and its length a few mm) overheated by the Joule effect. The quantity of heat that the moving air carries away depends on the ventilation speed and on the difference in temperature. The measuring limits are practically fixed, above all by the recording instrument. The hot-wire anemometer, strongly developed in the last years, has a quick response (time constant* less than 1 s), an accuracy* of at least 1-2 cm/sec and a resolution* up to some mm/s, so it is very sensitive to low air speeds like those of a confined environment. These instruments can measure the intensity and direction of the air flow. These devices are very delicate, fragile, very sensible to the pollution, so they are subject to drift*, so they need calibration with a time interval in function of the sensible element; they are energy demanding for the functioning and expensive. g) Instruments for analysis of particle distribution in the air Many instruments for measuring the atmospheric particulate are available and we need to use different kind of instruments in order to obtain a complete picture. Most of these instruments are based on the suction of an air flow or on natural deposition [73, 83, 122]. In the latter case, the material is usually collected on special supports (the most common are filters with different characteristics and degrees of porosity), then analysed by means of various techniques, such as optical or electron microscopy, X-ray diffractometry, infrared spectrophotometry, ionic chromatography, differential and gravimetric thermal analyses, etc. [66, 109, 133]. These are not the topic of this book and for this we refer to specialized bibliography, similarly to the chemical aspects of this argument. Different instruments are used for the detection of particles and gases in the atmosphere, like bulk systems, simple particle collectors or particle counters. From a chemical point of view a specialised bibliography is suggested [66, 72, 73, 83, 87, 88, 109, 123]. From the physical point of view, in order to do a correlation between microclimatic conditions and particle deposition, the most important parameter is the diameter as explained in the Chapter 4. This distribution permits to define the physical and behavioural characteristics of particles in the analysis of deposition processes. Measuring the concentration of particles in the atmosphere in relation to their diameters requires more com124 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 125 plex instruments. The most precise detection techniques use, above all, particle counters, whose operating principle is based on the scattering of light by the particles. These systems perform a computer-aided analysis of the optical scattering of a laser beam that hits an air flow that has been sucked in and contains the particles, counting them in relation to their diameters: according to the theory of light scattering (Mie’s theory), it is possible to deduct exact information about the size of the particles present in the atmosphere. The particle counters supply a measurement of the number of suspended particles in relation to their diameters; the most common ones detect a range of diameters from 0.1 µm to more than 10 µm; the most sophisticated ones may even reach a few hundredths of a µm. They are always very delicate and expensive instruments, and need frequent calibrations. 5.3 STANDARDS In meteorological measurements there have existed for a long time several types of standards, which are laid down in each country where there are institutions that carry out this work. The primary purpose of these institutions is to determine technical standards, which are correctly defined, tested and checked independently, or to define protocols with clear action procedures, purposes and limits. In this context we will not undertake a detailed discussion of existing standards, which would require a long treatise, but only offer a few remarks on them. There are standards for: - calibration; - performance: they supply the definition of terms and methods, e.g. time constant*, response time, etc.; - exposure: they indicate how the instrument must be exposed, depending on the sensor and on the measurement to be performed. Every country has its official authority. For example in Italy the Ente Nazionale Italiano di Unificazione (UNI) is the authority that is officially in charge of unifying all the national standards. During the last few years, an agreement between the Ministry for the Cultural Heritage and UNI has made it possible, also for cultural heritage, for experts to lay down Italian standards, in order to give people who work in this area instruments that are precise, correct and common to everybody. In Europe the CEN is the authority officially in charge of uniting all the European standards. In the last years different groups are created to establish the European standards in Cultural Heritage uniting and harmonising the national ones. These standards, however, must not be an adaptation of other standards created for other purposes in the civil or industrial sphere, but must be conceived and 125 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 126 tested for the protection of works of art, whose needs are quite different from those of other sectors and are very specific. As in the civil sphere, also in the sphere of works of art situated in buildings, there is a tendency to give the people in charge standards with precise numerical limits that must not be exceeded. The variables are usually temperature, humidity and lighting, and the acceptable ranges are regarded as “optimum” conditions for the works of art. These conditions, however, usually do not allow for the complexity that exists in the area of cultural heritage and has been pointed out in our previous discussion. For instance, these “optimum” ranges don’t take into account the presence of different materials in the same room, the type of material of which a work of art is made, its origin, its original climate and the new climate in which it is moved. For instance, Padfield [90] describes some interesting cases of the application of standards relevant to relative humidity and lighting. He remarks that only a posteriori it was noticed that, despite the literal application of certain standards, the overall effects turned out to be unacceptable, because many other parameters had been neglected. As Padfield writes, “standards should be a property linked to a single object, not to an empty room that is on the point of being used for an exhibition and about which no other detail is known”. Thermohygrometric standards, as a matter of fact, cannot be generally valid: for instance, the ones that have been laid down for a humid climate like that of London should be different from the ones laid down for a warmer, drier climate like that of Luxor in Egypt. This is true also for lighting standards. As a rule these standards are formulated in terms of the intensity of the light that falls on the object. For instance, the fading of colours is a cumulative effect that does not depend only on the intensity of the light (lux*) that falls on the object, so this is not the only parameter that should be considered. In order to avoid gross evaluation mistakes, therefore, during the last few years the experts in the field of cultural heritage have been making a considerable effort to start defining a usable, clear set of standards. National standards, in particular, have already been approved in the area of microclimatology (e.g. in Italy UNI 10969 [125]), where also the author was directly involved. As European standards a series of CEN norms are being defined It is necessary, in any case, to follow with great attention everything that happens in this area (where both technical and scientific progress is moving quickly), and to be always attentive and above all critical. The field in which we are acting is extremely complex, sensitive and not easy to standardise. Moreover the standardisation of the basic principles and the methodologies that have to be followed are surely necessary and very useful for all the work in the field of conservation. 126 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 127 6. PROFESSIONALS INVOLVED IN CONSERVATION To allow for all the requirements of a good conservation is difficult even for the experts in the area. In any case, they always have a partial vision of a problem, whereas an overall, interdisciplinary one is needed: a physicist will be chiefly interested in dealing with the causes of physical alteration, without possessing an equal expertise on the chemical, biologic, historic or aesthetic aspects, while with an art historian or architect the respective spheres of competence will take priority. Nobody, however, can ask a physicist or a chemist to turn suddenly into an architect or historian, and on the other hand it is not correct to saddle a single person with responsibilities that are not within his or her specific province. What is important is to be aware of these limits, and therefore to establish a multidisciplinary collaboration and take any decision on this basis. As the reader has certainly understood from our previous discussion of individual issues, to correctly manage a work of art and prevent its deterioration is not a simple problem, because of the unhomogeneity of the materials of which it is formed, or of the external events that have acted on it and made it become particularly fragile and sensitive. It is a really great responsibility for those who are in charge and must decide, e.g., whether a work of art can be kept outdoor or must be brought indoors; whether a certain type of microclimate is suitable for its conservation and what to do if it is not; which works of art can coexist in the same exhibition environment and how many visitors can be admitted to see them; whether a certain building is suitable for containing certain materials or what to do to make it become suitable; whether the lighting or heating systems are the optimum ones for conservation; etc. For these complicated reasons, the person who takes these decisions must possess a new type of professionalism, and must be able to co-ordinate and take into account all the specific pieces of expertise that are supplied to him/her, assimilating them and applying them in an overall manner; moreover this person must be able to follow the work of each professional, harmonising it with that of the others. It is a very difficult, delicate role, and involves an extremely high degree of competence and responsibility; but it is necessary for achieving a correct policy for the management of environments that contain works of art. These requirements do not refer only to the top-level managing professionals: it is also necessary for the rest of the staff that works in environments containing works of art to be suitably instructed and made aware of the consequences of all the actions performed in day-by-day activities - down to the simple opening of a window. This is the only way in which it becomes thinkable to achieve a correct environmental management: in most cases the latter is formed of simple, daily actions that must be carried out advisedly by those who work in this area, at all levels. If this type of training which involve all the levels does not underlie all the measures taken at management level, their effect may easily be nullified. 127 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 128 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 129 CONCLUSIONS Our descriptions in this text have highlighted the fact that, during the last few decades, new branches of science, or rather new applications of known scientific principles, have become an important help in the area of the conservation of works of art. In the past, this area was chiefly a province of humanists, chemists and restorers. The physics of the atmosphere, and microclimatology in particular, are a new instrument for the study of the preventive conservation of works of art. This means that it is important to understand - also from a physical and meteorological point of view, and not only from the standpoint of chemists, biologists, historians, etc. - the processes that lead to the deterioration of the precious cultural heritage. During the last few years, mass tourism has become a great cultural breakthrough, but also an enormous source of problems to be solved in terms of conservation. In the past, attention was always focussed on restoration problems. Nowadays, fortunately, it has been understood that this is the last remedy to which one should resort, because restoration, no matter how masterfully it is carried out, is always an alteration of the original work of art. The knowledge and solution of some physical and microclimatological problems that underlie other problems may lengthen the life of our artefacts. After more than 20 years of analysis in conservation environments, the study of the microclimate has by now yielded many basic rules that are essential for making an environment suitable for conservation. An environment that is open to the public has some needs (lighting, heating, changes of air, etc.) that considerably complicate the situation to be kept under control. The presence of visitors (with the introduction of heat, vapour, pollutants, etc.) results in a constant change in the environmental conditions, and its management becomes difficult because it is “dynamic”: an incorrect management causes frequent thermhygrometrical cycles, with dangerous effects on the works of art. The problem of the management of conservation environments - whether they be museums, libraries or churches - is, as we have seen, far from simple. Obtaining the microclimatic conditions suitable for the conservation of works of art requires the knowledge of the environmental microclimate in all its spatial and temporal conditions (day-night, seasons, etc.) and also in the effects due to external forcing factors connected to the use of the rooms (conditioning and lighting systems, opening of doors and windows, cleaning operations, inflow of visitors, etc.). Through the studies that have been carried out, we have seen that certain buildings that are intrinsically optimum for the conservation of works of art are caused 129 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 130 to become unsuitable, in many cases, as a result of the forcing factors introduced in order to meet the needs connected to the growth of mass tourism. All the problems described here must therefore be considered simultaneously when a museum or any sort of exhibition environment must be set up, or when the existing conservation conditions must be improved. Mistaken decisions, not always preferred because of their simplicity, but often dictated by financial interests or image-making considerations, have led to considerable, irreversible damages in our precious cultural heritage. It is our hope that this simple book may help in the difficult task of achieving an optimum conservation of our artistic heritage, which it is our duty to preserve for the future generations. 130 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 131 REFERENCES 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) AIDI, 1996: Guida per l’illuminazione delle opere d’arte negli interni. Associazione Italiana d’Illuminazione, ottobre 1996, 45 pp. ARNOLD A. and ZEHNDER K.,1990: Salt Weathering on Monument. In F. Zezza (ed.). The Conservation of Monument in the Mediterranean Basin. Grafo, Bari, 31-58. ASHRAE STANDARDS: Thermal Environmental Conditions for Human Occupancy, 55-81. ASHRAE HANDBOOK, 2001: Fundamentals. ASHRAE, 2003: HVAC Applications Handbook, Chapter 21 - Museums, Libraries, and Archives SI Units Edition. BAER N.S. and BANK P.N., 1985: Indoor Air Pollution: Effects on Cultural and Historic Materials, Museum Management and Curatorship 4, 9-20. BERNARDI A., 1982: Un importante fattore di degrado dei beni artistici: la condensazione nei materiali porosi. In Atti XI Sep Pollution, Padova, 16-22/4/1982, 149-155. BERNARDI A., CAMUFFO D., CARBONI G. e PIANO A., 1981: Propagazione dell’onda termica e del flusso di calore nel suolo. XX Convegno Internazionale ANIAI. BERNARDI A., CAMUFFO D., DEL MONTE M. and SABBIONI C., 1985: Microclimate and Weathering of an Historical Building: the Ducal Palace in Urbino. The Science of the Total Environment, 46, 243-261. BERNARDI A., CAMUFFO D. and VINCENZI S., 1988a: Modelling the Thermal Pattern of a Statue or Column Exposed to Periodic Lighting Conditions. Il Nuovo Cimento, Vol. 11C, N.5-6, 453-466. BERNARDI A., CAMUFFO D., DEL MONTE M. e SABBIONI C., 1988b: L’inquinamento atmosferico e il degrado dei monumenti. Difesa Ambientale, 12, 4, 62-68. BERNARDI A., 1990a: Microclimate in the British Museum, London. Museum Management and Curatorship, 9, 169-182. BERNARDI A., 1990b: Effects of Lighting: Problems and Modelling. European Cultural Heritage Newsletter on Research, 4, 2, 11-12. BERNARDI A. and VINCENZI S., 1993: Modelling Daily Thermal Cycles in the Trajan Column. The Science of the Total Environment, 128, 257-267. BERNARDI A. and VINCENZI S., 1994: Diurnal Variation of Solar Radiation on Differently Oriented Surfaces of Monuments. Il Nuovo Cimento, 17C, 4, 431-442. BERNARDI A. and CAMUFFO D., 1995a: Microclimate in Chiericati Palace Municipal Museum, Vicenza. Museum Management and Curatorship, 14,1, 5-18. BERNARDI A. and CAMUFFO D., 1995b: Uffizi Galleries in Florence: a Comparison between Two Different Air Conditioning Systems. Science and Technology for Cultural Heritage, 4, 2, 11-22. BERNARDI A., CAMUFFO D., STURARO G. e VALENTINO A., 1998: Indagini microclimatiche per la conservazione ed il restauro in ambienti interni ed esterni. Technology Highlights Information, 22-37. BERNARDI A., BOSSI F. and VILLANTI S., 1999: Contribution towards a “Preservative” Architecture. Analyses of the microclimate of the National Library “Marciana”- Venice. International Conference on Conservation and Restoration of Archive and Library Materials. Erice 22-29 April, Palumbo ed., 111-124. BERNARDI A., TODOROV V. and HIRISTOVA J., 2000: Microclimatic Analysis in the St. Stephan Church, Nessebar - Bulgaria after Interventions for the Conservation of the Frescoes. Science and Technology for Cultural Heritage, 1, 281-286. BERNARDI A., 2002: Metodologia per un’analisi microclimatica per la conservazione delle opere d’arte negli ambienti interni. In Elementi di archeometria: Metodi fisici per i beni culturali, Egea, 343-368. 131 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 132 22) BERNARDI A. e BECHERINI F., 2002a: Un nuovo approccio allo studio del degrado delle vetrate antiche. Rivista della Stazione Sperimentale del Vetro, Vol. 4, 28-37. 23) BERNARDI A. e BECHERINI F., 2002b: Controllo microclimatico: gestione dell’ambiente e conseguenze. Proceedings of International Conference “Scelte e strategie per la conservazione della memoria”. Dobbiaco (BL), 143-149. 24) BERNARDI A., BECHERINI F., BASSATO G. and BELLIO M, 2005: Do Protective Glazing Systems Enhance the Thermo-hygrometric Stresses of Stained Glass Windows? Special Issue on EU VIDRIO project of Rivista della Stazione Sperimentale del Vetro, 3, 25-31. 25) BERNARDI A., BECHERINI F., BASSATO G. and BELLIO M, 2006: Condensation on Ancient Stained Glass Windows and Efficiency of Protective Glazing Systems: Two French Case Studies, Sainte Chapelle (Paris) and Saint Urbain Basilica (Troyes). Journal of Cultural Heritage, 7, 1, 71-78. 26) BERNARDI A. and BECHERINI F., 2005: Management and Consequences in the Control of the Microclimate. In Museums, Libraries and Archives. Arkhiyyon, Reader in Archive Studies and Documentation, No 13, Israel Archives Association, Unigraph Jerusalem, 9-21. 27) BERNARDI A. and BECHERINI F., 2006: Evaluation of the Microclimatic Conditions in the Vasari Room Aimed at its Future Exploitation by the Public. In The frescoes of Casa Vasari in Florence: an Interdisciplinary Approach to Understanding, Conserving, Exploiting and Promoting. U. Baldini e P.A. Vigato, Ed. Polistampa, Firenze, 65-79. 28) BERNARDI A., BECHERINI F., KONTONZOVA V., GODOI R., VAN GRIEKEN R. and F. DEUTSCH, 2004: Effects of Air Pollution and Microclimate on Stained Glass Windows: Preliminary Results in the Sainte Chapelle (Paris). In Air Pollution and Cultural Heritage. C. Saiz-Jimenez Ed.,133-140. 29) BLADES N., KRUPPA D. and CASSAR M., 2002: Development of a Web-based Software Tool for Predicting the Occurrence and Effect of Air Pollutants Inside Museum Buildings. In ICOM Conservation Committee 13th Triennial Meeting, Rio de Janeiro. London, James & James, vol. 1, 9-14. 30) BLADES N.: http://www.ucl.ac.uk/sustainableheritage 31) BRIMBLECOMBE P. and RAMER B., 1983: Museum Display Cases and the Exchange of Water Vapour. Studies in Conservation 28, 179-188. 32) BRIMBLECOMBE P., 1990: The composition of Museum Atmosphere, Atmospheric Environment 24B, 1-18. 33) BRIMBLECOMBE P., 1994: Balance of Environmental Factors Attacking Artefacts. In Durability and Change. Wiley, 67-80. 34) BETTEMBOURG J.M., 1976: Composition et altération des verres de vitraux anciens. Verres et réfractaires, vol. 30, No 1, 36-42. 35) BETTEMBOURG J.M., 1991: Les verres des vitraux du moyen âge: composition et corrosion. Technique et science: les arts du verre, actes du Colloque de Namur, 111-117. 36) BONACINA C., CAVALLINI A and MATTAROLO L., 1985: Trasmissione del calore. Cleup Ed., 473 pp. 37) BRITISH MUSEUM LIBRARY, 1983: Air Quality Criteria for Storage of Paper-Based Archivial Records. NBSIR 83-2795 (National Bureau of Standards), Washington DC. 38) BROOK F. B. and RICHARDSON S.J., 2001: Meteorological Measurement Systems. Oxford University Press, 290 pp. 39) BROWN J.P. and ROSE W.B., 1996: Humidity and moisture in historic buildings: the origin of building and object conservation, APT Bulletin, 27/3, 12-24. 40) BROWN T., 1998: Show-case Induced Cadmium Corrosion. Abstract from UK Conservation Science Group Meeting. 41) CAMUFFO D. and BERNARDI A., 1982: The Diurnal Trend in Surface Mixing Ratio at Padova, Italy. Boundary Layer Meteorol. 22, 273-282. 132 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 133 42) CAMUFFO D., 1984: Condensation-Evaporation Cycles in Pore and Capillary Systems According to the Kelvin Model. Water, Air and Soil Pollution, 21, 151-159. 43) CAMUFFO D and VINCENZI S., 1985: Computing the Energy Balance of a Statue of Bronze: the San Marco’s Horses as a Case Study. The Science of the Total Environment, 44, 147-158. 44) CAMUFFO D. e BERNARDI A., 1986: Dinamica del microclima e scambi termoigrometrici tra pareti e atmosfera interna nella Cappella Sistina. Bollettino Monumenti Musei e Gallerie Pontificie, IV, 211-257. 45) CAMUFFO D., DEL MONTE M. e SABBIONI C., 1987: Influenza delle precipitazioni e della condensazione sul degrado superficiale dei monumenti in marmo e calcare. In Materiali Lapidei. Istituto Poligrafico dello Stato, Roma, 15-36. 46) CAMUFFO D. and BERNARDI A., 1988: Microclimate and Interactions between Atmosphere and the Orvieto Cathedral. The Science of the Total Environment, 68, 1-10. 47) CAMUFFO D., BERNARDI A. and ZANETTI M., 1988: Analysis of the Real-Time Measurement of the pH of Rainfall at Padova, Italy: Seasonal Variation and Meteorological Aspects. The Science of the Total Environment, 71, 187-200. 48) CAMUFFO D., 1991: Wall Temperature and Soiling of Mural. Museum Management and Curatorship, 10, 373-383. 49) CAMUFFO D. and BERNARDI A., 1991a: The Microclimate of Leonardo’s “Last Supper”. Joint Edition Bollettino Geofisico and European Cultural Heritage Newsletter on Research, Spec. Vol., XIV, 3, 1-123. 50) CAMUFFO D. and BERNARDI A., 1991b; Transboundary Transport of Atmospheric Pollutants through the Eastern Alps. Atmospheric Environment, 25A, 12, 2863-2871. 51) CAMUFFO D. and BERNARDI A., 1993: Microclimatic Factors Affecting the Trajan Column. The Science of the Total Environment. 128, 227-255. 52) CAMUFFO D. and BERNARDI A., 1995a: The Microclimate in the Sistine Chapel. Joint Edition Bollettino Geofisico and European Cultural Heritage Newsletter on Research, Volume Spec., 9, 1-32 53) CAMUFFO D. and BERNARDI A., 1995b: Study of the Microclimate of the Giant Hall of the Da Carrara’s Royal Palace, Padova. Studies in Conservation., 40, 237-249. 54) CAMUFFO D. e BERNARDI A., 1996a: Il condizionamento estivo nei musei: il Museo Correr (Venezia), la Galleria degli Uffizi (Firenze) e il Museo del Louvre (Parigi). 1st Intern. Congress on Science and Technology for the Safeguard of Cultural Heritage in the Mediterranean Basin. Luxograph, Palermo, 1679-1685. 55) CAMUFFO D. and BERNARDI A., 1996b: Deposition of Urban Pollution on the Ara Pacis, Rome. The Science of the Total Environment, 189/190, 235-245. 56) CAMUFFO D. and BERNARDI A., 1997a: Controlling the Microclimate and the Particulate Matter inside the Historic Anatomic Theatre, Padova. Museum Management and Curatorship, 15, 3, 285298 57) CAMUFFO D., 1998: Microclimate for Cultural Heritage. Elsevier, Amsterdam, 415 pp. 58) CAMUFFO D., BRIMBLECOMBE P., BUSSE H.J., VAN GRIEKEN R., STURARO G., VALENTINO A., BERNARDI A., BLADES N., SHOOTER D., DE BOCK L., GYSELS K., WIESER M. and KIM O., 1999: Environmental Risk to Cultural Heritage related to Unsound Use of Technology and Mass Tourism at the Correr Museum, Venice. The Science of the Total Environment, 236, 135-152. 59) CAMUFFO D., STURARO G. and VALENTINO A., 2000: Showcases: a Really Effective Mean for Protecting Artworks? Thermochimica Acta, 365, 65-77. 60) CAMUFFO D., VAN GRIEKEN, R., BUSSE H.J., STURARO, G., VALENTINO, A., BERNARDI, A., BLADES, N., SHOOTER, D., GYSELS, K., DEUTSCH, F., WIESER, M., KIM, O. and ULRYCH, U., 133 Pleiadi1_16102008:Pleiadi_eng_16102008 61) 62) 63) 64) 65) 66) 67) 68) 69) 70) 71) 72) 73) 74) 75) 76) 77) 78) 79) 134 16-10-2008 13:27 Pagina 134 2001: Environmental Monitoring in Four European Museums. Atmospheric Environment, 35,1, S127-S140 CAMUFFO D., BERNARDI A., STURARO G. and VALENTINO A., 2002: Microclimatic Conditions Inside the Pollaiolo and Botticelli Rooms in the Uffizi Gallery, Florence. Science and Technology for Cultural Heritage, 2, 155-162. CAMUFFO D., BERNARDI A., PAGAN E., BECHERINI F, 2004: The Impact of Heating, Lighting and People in Re-using Historical Buildings: a Case Study. Journal of Cultural Heritage, 5, 409-416. CAMUFFO D., BERNARDI A., BECHERINI F., PAGAN E. and R.A. LEFÈVRE, 2005: Microclimate measurements in the Cour Marly, Louvre Museum. In Cultural Heritage Conservation and Environmental Impact Assessment by Non Destructive Testing and Micro-Analysis. V. Grieken & Janssens (Ed.), A.A. Balkema Publishers, London, 127-133. CASSAR M. and MARTIN G., 1994: The Environmental Performance of Museum Display Cases. In Preventive Conservation: Practice, Theory and Research. International Institute for Conservation of Historic and Artistic Works. 171-173. CASSAR M., 1995: Environmental Management. Routledge, 165 pp. CASTELLANO A., MARTINI M. e SIBILLA E. (ed.), 2002: Elementi di archeometria: Metodi fisici per i beni culturali. Egea, 371 pp. COPPEDÉ M., 1991: La carta e il suo degrado. Nardini Ed., 165 pp. COPPI M., 1990: Clima artificiale e benessere termoigrometrico. ESA, 79 pp. COULSON J.M. and RICHARDSON E.G., 1985: Chemical Engineering. Pergamon Press, Oxford, Vol. 2, 814 pp. DANTI C., BODDI R., CASTELLI C. e CRESCIOLI A., 1990: Esame del comportamento delle tavole lignee dipinte in relazione alle escursioni termoigrometriche. In Il restauro del legno. Nardini Ed.,Vol. 2, 135-139. DE FELICE T.P., 1998: An Introduction to Meteorological Instrumentation and Measurement. Prentice Hall Inc. Upper Saddle River, 229 pp. DE SANTIS, F., MONTAGNOLI, M., PASELLA D. and ALLEGRINI, I., 1999. The Measurement and Model Prediction of Photochemical Pollutants in the Uffizi Gallery, Florence. Annali di Chimica, 89. DE SANTIS F., ALLEGRINI I, DI FILIPPO P., M. and PASELLA D., 1996. Simultaneous Determination of Nitrogen Dioxide and Peroxyacetyl Nitrate in Ambient Atmosphere by Carbon-coated Annular Diffusion Denuder, Atmospheric Environment, 30, 2637-2646. EBRAHIMZALEH P.R, 1997: Study of Mechanical Properties of Wood and Paper During Humidity Change. Int. Conference Wood Water Relations. Copenhagen, 397-408. ERHARDT D. and MECKLEMBURG M.1994: Relative humidity Re-examined. In Preventive Conservation: Practice, Theory and Research. Roy and Smith Ed. International Institute for Conservation of Historic and Artistic Works. London. ETCHEVERRY M.P., TROCELLIER P., DJANARTHANY S., BECK L., MAGASSOUBA B., 2005: Characterization of the Surface Composition of Exposed Grisaille Sensors Using Highly Sensitive Chemical Analysis Techniques. Potentiality and Limit Regarding Research in Conservation. Rivista della Stazione Sperimentale del Vetro, 3, 63-74. FIORAVANTI M. e UZIELLI L, 1992: Il supporto ligneo. In La Madonna d’Ognissanti di Giotto restaurata, Gli Uffizi Studi e Ricerche, 8, 105-117. RIEDLANDER S.K., 1977: Smoke, Dust and Haze. Wiley, N.Y., 317 pp. GEOTTI-BIANCHINI F., NICOLA C., PREO M., VALLOTTO M., VERITÀ M.: MicroIRRS and EPMA Study of the Weathering of Potash-lime-silicate Glasses. Special Issue on EU VIDRIO project of Rivista della Stazione Sperimentale del Vetro, 3, 49-61. Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 135 80) GINELL W.S., 1994: The Nature of Changes Caused by Physical Factors. In Durability and Change. Wiley, 81-94. 81) GIORDANO G., 1993: Tecnica delle Costruzioni in legno. Hoepli, 826 pp. 82) GIORDANO G., 1981, 1983, 1986,1988: Tecnologia del legno. UTET, VOL I, II, III. 83) HARRISON R.M. and VAN GRIEKEN R.E. (Ed.), 1998: Atmospheric particles. IUPAC Series on Analytical and Physical Chemistry of Environmental Systems. John Wiley & Sons, vol. 5, 609 pp. 84) HENDERSON S.M., 1973: Predicting Equilibrium Moisture Content of Wood by Mathematical Models. Wood and Fiber, Vol. 5, 41- 49. 85) HICKS B.B., 1982: Wet and Dry Surface Deposition of Air Pollutants and Their Modelling. In National Material Advisory Board (Ed.): Conservation of Historic Stone Buildings and Monuments. National Academy Press, Washington D.C., 183-196. 86) KONGRATYEV Y.A., 1969: Radiation in the Atmosphere. Academic Press, 912 pp. 87) KONTOZOVA V., GODOI R.H.M., SPOLNIK. Z., WOROBIEC. A., DEUTSCH. F. and VAN GRIEKEN R., 2005: Characterization of Air Pollutants Observed in Three European Cathedrals. Is the Double Glazing Really Protective? Special Issue on EU VIDRIO project of Rivista della Stazione Sperimentale del Vetro, 3, 33-40. 88) KONTOZOVA V., DEUTSCH F., GODOI R., GODOI A. F., JOOS P. And VAN GRIEKEN R., 2002: Characterization of Air Pollutants in Museum Showcases, Proceedings of the art 2002 - the 7th International Conference on Non-destructive Testing and Microanalysis for the Diagnostics and Conservation of the Cultural and Environmental Heritage, Antwerp. 89) KRUMBEIN W. E., URZI C. E. and GEHRMANN C., 1991: Biocorrosion of antique and medieval glass. Geomicrobiology J, 9, 139-160. 90) KRUMBEIN W.E, BRIMBLECOMBE P., COSGROVE D.E. and STANIFORTH S. (Ed.), 1992: In Durability and Change. Wiley, 307 pp. 91) LEFÈVRE R.A., IONESCU A, AUSSET P., CACHIER H., CHABAS A., FAVEZ O., LOMBARDO T., OIKONOMOU K., 2005: In the Field Experimental Simulation Using Glass Soiling Sensors and Modelling of the Atmospheric Particles Deposition on Stained-glass Windows With and Without Protective Double-glazing. Special Issue on EU VIDRIO project of Rivista della Stazione Sperimentale del Vetro, 3, 41-48. 92) LUDWIG N., 2001: Tecniche termografiche per la diagnostica sull’edilizia storica. In Elementi di archeometria: Metodi fisici per i beni culturali. Egea, 288-313. 93) JAKIELA S., BRATASZ L. and KOSLOWSKI R., 2007: Acoustic Emission for Tracing Fracture Intensity. In Lime Wood Due to Climatic Variations. Wood Science and Technology, in press 94) JAKIELA S., BRATASZ L. and KOSLOWSKI R., 2007: Numerical Modelling of Moisture Movement and Related Stress Field in Lime Wood Subjected to Changing Climate Conditions. Wood Science and Technology, in press 95) MARABELLI M., SANTOPADRE P. and VERITÀ M., 1993: Influence of the External Protective Glazing Upon Conservation of the Medieval Stained Glass Window in Orvieto Cathedral. Rivista della Stazione Sperimentale del Vetro, 23, 137-144. 96) MARTIN G., 2000: Air Exchange Rates in Display Cases - A Standards? In Proceedings of the Third Indoor Air Pollution Meeting, Indoor Air Quality Group. <iaq.dk/iap/iaq2000/2000_04.htm> 97) MATÉ D. and RESIDORI L., 2002: The conservation of photographs. In Memoria e Futuro dei documenti su carta, Forum, 289-297. 98) MECKLENBURG M.F. and TUMOSA C.S., 1999: Temperature and Relative Humidity Effects on the Mechanical and Chemical Stability of Collections. ASHRAE Journal, 77-82. 135 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 136 99) MICHALSKI S., 2000: Guidelines for Humidity and Temperature for Canadian Archives. CCI Technical Bulletin, n. 23, Canadian Conservation Institute, Ottawa. 100) MONCADA LO GIUDICE G. e COPPI M., 1997: Benessere termico e qualità dell’aria interna. Masso, 133 pp. 101) MUELLER W. (1992) Corrosion Phenomena of Medieval Stained Glasses, Bol. Soc. Esp. Ceram. Vid. 31-C, 219-239. 102) NAZAROFF W.W. and CASS G.R., 1991: Protecting Museum Collections from Soiling due to the Deposition of Airborne Particles. Atmospheric Environment 25, 841-852. 103) NAIXIN X., LINGYUAN Z., CUIHONG D., CHENGDIAN Z., RUNSHEN L. and QIGDONG Z., 2002: Laboratory Observation of Dew Formation at an Early Stage of Atmospheric Corrosion of Metals. Corrosion Science 44, 163-170. 104) NEWTON R. G., 1975: Protective Glazing. Results From the Externally Ventilated Windows at York Minster. CVMA News Letters, 16, 4-10. 105) NEWTON G. and DAVISON S., 1989: Conservation of glass. Butterworths, London, 322 pp. 106) OITMANN S., LEISSNER J. and RÖMICH H., 2000: Schutzverglasungen. In: Restaurierung und Konservierung historischer Glasmalereien. Wolff A. (Ed.), Verlag Philipp von Zabern, Mainz. 107) PALLOT-FROSSARD I. and PHILIPPON J., 1999: Monuments historiques et environnement: Recherches franco-allemandes sur la conservation de la pierre et du vitral. Exé productions, France. 108) PALLOT-FROSSARD I, BERNARDI A., VAN GRIEKEN R., ROELLEKE S. and VERITÀ M., 2005: Main Conclusions from VIDRIO EU Research Programme and Recommendations for End-users and Practitioners. Special Issue on EU VIDRIO project of Rivista della Stazione Sperimentale del Vetro, 3, 75-83. 109) PANTANI M., SABBIONI C., BRUZZI L. and MANCO D., 1998: Air Pollution on Stone of Artistic Interest. VI International Conference on Air Pollution. WIT Press, 675-684. 110) PEREZ Y JORBA M., DALLAS J.P., BAUER C., BAHEZRE C. and MARTIN J.C., 1980: Deterioration of Stained Glass by Atmospheric Corrosion and Micro-organisms. Journal of Materials Science, 15, 1640-1647. 111) PRODI F. and TAMPIERI F., 1982: The Removal of Particulate Matter from the Atmosphere: the Physical Mechanisms. Pageoph, 120, 286, 325. 112) PRUPPACHER H.R. and KLETT J.D., 1980: Microphysics of Clouds and Precipitation. Reidel Pu. Co., Holland, 714 pp. 113) RESIDORI L., 2002: Il deterioramento dei materiali fotografici: aspetti chimico-fisici. In Chimica e Biologia applicate alla conservazione degli archivi. Pubblicazione degli Archivi di Stato. Saggi 74, 557575. 114) ROEMICH H., 1999: Historic Glass and Its Interaction with the Environment, in The Conservation of Glass and Ceramics. James & James, London, 5-14. 115) ROEMICH H., FUCHS D. R. and LEISSNER J., 1999: Glass Sensor Studies for the Evaluation of Protective Glazings on Stained Glass Windows – a Survey of Results After 10 Years Experience. Proceedings of the CVMA-Colloquium, Krakow, 223-231. 116) SANTACHIARA G., PRODI F. and VIVARELLI F., 1991: Dry Deposition Measurements of Aerosol Particles in a Corn Field. Journal of Aerosol Science, 22, S577. 117) SCLOCCHI M.C., 2002: La disinfezione e la disinfestazione dei supporti archivistici. In Chimica e Biologia applicate alla conservazione degli archivi. Pubblicazione degli Archivi di Stato. Saggi 74, 557575 118) SEINFELD J.H., 1986: Atmospheric Chemistry and Physics of Air Pollution. Wiley, N.Y., 738 pp. 119) SHAVER C.L., CASS G.R. and DRUZIK J.R., 1983: Ozone and the deterioration of works of art. Environmental Science & Technology 17, 748-752. 136 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 137 120) TANASI M.T., 2002: La pergamena: caratteristiche fisiche e tecnologiche. In Chimica e Biologia applicate alla conservazione degli archivi. Pubblicazione degli Archivi di Stato. Saggi 74, 557-575. 121) THOMSON G., 1986: The Museum Environment. Butterworths, 293 pp. 122) TÉTREAULT, J. and STAMATOPOULOU, E., 1997: Determination of Concentrations of Acetic Acid Emitted from Wood Coatings in Enclosures. Studies in Conservation, 42, 141-156. 123) TÉTREAULT J., 2003: Airborne Pollutants in Museums, Galleries and Archives: Risk assessment, Control Strategies, and Preservation Management. Canadian Conservation Institute, Ottawa, Canada, 168 pp. 124) TORATTI T. and SVENSSON S., 1997: Experimental Study on Mechano-Sorptive Strains and Internal Stresses under Fluctuations of Humidity. Int. Conference Wood Water relations. 16-17 June, Copenhagen, Denmark, 385-394. 125) UNI 10969, 2001: Principi generali per la scelta e il controllo del microclima per la conservazione dei beni culturali in ambienti interni. 126) URZÌ C. and KRUMBEIN W.E., 1994: Microbiological Impacts on the Cultural Heritage. In Durability and Change. Wiley, 107-135. 127) UZIELLI L. e FIORAVANTI M., 2000: Il comportamento fisico-meccanico del legno nei dipinti su tavola. In Dipinti su tavola - La tecnica e la conservazione dei supporti. M. Ciatti, C. Castelli e A. Santacesaria (Ed.), Edifir, 37-58. 128) DIONISI VICI P., MAZZANTI P., UZIELLI L., 2006: Mechanical Response of Wooden Boards Subjected to Humidity Step Variations: Climatic Chamber Measurements and Fitted Mathematical Models. Journal of Cultural Heritage, Vol 7 (1), 37-48. 129) VALENTIN N., LIDSTROM M. and PREUSSER F., 1990: Microbial Control by Low Oxigen and Low Relative Humidity Environment. Studies in Conservation, 35, 223-230. 130) VALENTIN N., GARCIA R., DE LUIS O. and MAECHAWA S., 1998: Microbial Control in Archives, library and museums by ventilation systems. Restaurator 19, 85-107. 131) VAN BOMMEL M., VAN ELST B. and BROEKENS F., 2001: Emission of Organic Acids from Wooden Construction Materials in a Small Test Chamber; Preliminary Results of Optimisation of the Solid Phase Micro Extraction technique. Abstracts of the 4th meeting of the Indoor Air Pollution Working Group, National Museum of Denmark, Copenhagen, November 8th-9th 2001. 132) VERITÀ M., 1996: Composition, structure et mécanisme de détérioration des grisailles, Proceedings of the Forum pour la Conservation et la Restauration des Vitraux, Liège, juin 1996, in: Dossier de la Commission Royale des monuments, Sites et Fouilles, 3, 61-67. 133) ZAPPIA G., SABBIONI C and GOBBI G., 1993: Noncarbonate Carbon Content on Black and White Areas of Damaged Stone Monuments. Atmospheric Environment, 27A, 117-1121. 134) ZUFALL M.J. and DAVIDSON C.I., 1998: Dry deposition of Particles. In Atmospheric Particles. Harrison R.M. and van Grieken R.E. (Ed). IUPAC, 425-475. 135) Thesis of Ms. Elisa Frassoldati, promoter Prof. Adriana Bernardi Title: Analisi microclimatiche alla tomba del Petrarca, University of Ferrara. Degree in Technologies for Cultural Heritage, 2003-2004. 136) Thesis of Ms. Eleonora Cannella promoter Prof. Adriana Bernardi Title: Monitoraggio microclimatico di una sala della pinacoteca nazionale del palazzo dei Diamanti, Ferrara. University of Ferrara. Degree in Technologies for Cultural Heritage, 2005-2006. 137) Thesis of Ms. Elena Caniato, promoter Prof. Adriana Bernardi. Title: Monitoraggio microclimatico nella stagione autunnale di una sala della pinacoteca nazionale del palazzo dei Diamanti, Ferrara. University of Ferrara. Degree in Technologies for Cultural Heritage, 2005-2006. 137 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 138 138) Thesis of Ms. Minarelli Francesca promoter Prof. Adriana Bernardi Title: Monitoraggio microclimatico durante la stagione invernale di una sala della pinacoteca nazionale del palazzo dei Diamanti, Ferrara. University of Ferrara. Degree in Technologies for Cultural Heritage, 2005-2006. 139) Thesis of Ms. Isabella Sartori, promoter Prof. Adriana Bernardi Title: Un anno di monitoraggio microclimatico in una sala presso la pinacoteca di Ferrara: problematiche di gestione ambientale e conclusioni, University of Ferrara. Degree in Technologies for Cultural Heritage, 2005-2006. 138 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 139 APPENDIX 1 DEFINITION OF MAIN THERMOHYGROMETRICAL PARAMETERS AND OF THEIR FUNCTIONAL MATHEMATICAL RELATIONSHIP The analysis of the microclimate is based on the study of the spatial and temporal evolution of the thermohygrometrical parameters that have been described qualitatively in our previous discussion. This Appendix contains a more rigorous physical and mathematical treatment of the subject. First of all we must briefly define the quantities used in the study of the dynamics of gases. The thermodynamic state of a gas is defined by the values of three variables: pressure, volume and temperature. Pressure is measured in Pascals (Pa), most commonly in the multiple hectopascal* (hPa), which corresponds to 1 millibar* (mbar) and to approximately 1 thousandth of an atmosphere, traditional measurement unit that has now been declared obsolete by the Bureau International des Poids et Measures. Volume is measured by means of the units of the International System (SI), in m3. Temperature is measured in Kelvins (K), in the so-called absolute scale, which begins with the absolute zero and assigns the value 273.16 K to the melting point of ice and 373.16 K to the boiling point of water. The centigrade scale, which is used more commonly, assigns the values 0°C and 100°C, respectively, to these two temperatures. The equation of state of ideal gas interconnects these three quantities, expressed in the respective measurement units. In the same expression: p•V=n•R •T or 139 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 140 there also appear: - m, mass of the gas expressed in g; - M, atomic or molecular weight of the gas under consideration, expressed in g; - n, number of moles; - R, universal constant of gases, equal, in the International System (SI), to 8.31 J/mol K; - T, temperature. We will now describe the quantities that are most frequently used in the study of the microclimate. a) Water vapour partial pressure (ev) Water vapour partial pressure (ev) is the pressure exerted by the vapour molecules present in the air mass, and is physically defined as: but R = Rv • Mv, so the equation becomes ev • V = mv • Rv • T where Rv is the universal constant of vapour, equal, in the SI, to 461.525 J/kg K; ev is measured in hectopascal* (hPa) or in mbar. This formula expresses the contribution of the water vapour to the atmospheric pressure. b) Water vapour saturation pressure (esat) The saturation pressure, esat, is the pressure exerted by the vapour molecules at the maximum concentration allowed to them by the temperature of the air mass. Physically, the saturation pressure is defined by stating that when humid air, at atmospheric pressure p and ambient temperature T, coexists in dynamically neutral equilibrium with a flat water surface at the same temperature and pressure, ev reaches the saturation value, esat, called also saturated vapour pressure. 140 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 141 esat, as we have said, depends on the temperature and can be expressed by Magnus’s empirical equation: esat (T) = esat (0) • 10aT/(b+T) where esat(0) = 6.112 mbar; a=7.65; b=243.12 °C. If T<0°C, esat(T) must be calculated with respect to ice, and in this case a=9.5, b=265.5 °C. Please notice that esat (T) is independent of volume: if the volume in which the humid air is contained increases - for instance because the air mass is transferred to a larger environment - evaporation takes place, and the quantity of vapour in the air increases, keeping esat(T) constant. Likewise if the volume decreases, a part of the vapour condenses, with the same result. c) Specific humidity (SH) The specific humidity is the ratio of the vapour mass, mv, to the overall mass of the vapour and air (mv+ma) in which the vapour is dispersed, i.e.: sh = [mv / (mv + ma)] (kg/kg) This value is often multiplied by 1 000 in order to express it in g/kg and is indicated as SH: SH = 1000 • [mv / (mv + ma)] (g/kg) As the reader can notice from the formula, SH is a parameter that is independent both of temperature and volume, so it remains unchanged in any process that does not directly involve the production or condensation of vapour. We have seen that in conditions of saturation ev becomes esat(T). In these conditions, therefore, SH becomes the saturation specific humidity SHsat ; i.e. it represents the maximum amount of vapour that that air mass with those characteristics can contain. Any further vapour molecules added to that air mass will condense, because the SHsat value is the maximum vapour content of that air mass. Particularly in physics of the atmosphere, another parameter that is quite similar to sh and often used instead of it is the “mixing ratio”, mr, which is defined as mr = mv /ma and is measured in kg/kg (mr), or in g/kg if multiplied by 1000 (MR). 141 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 142 d) Absolute humidity (AH) The absolute humidity (or water vapour density) is the amount of water vapour, mv, contained in a volume unit, V. It is defined as: ah = mv / V (kg/m3) or AH = 1000 • ah (g/m3) As the formula shows, it depends both on the amount of water vapour contained in the atmosphere and on the volume of the air mass, which is a variable quantity connected to a possible compression or expansion of the air mass. For this reason it is better to use SH or mr to understand if the characteristics of the air masses are changing. e) Relative humidity (RH) The relative humidity is expressed as a ratio of the water vapour mass, mv, present in any atmosphere volume, to the vapour mass, msat, that would be present if at the same temperature the same atmosphere were in a state a saturation: rh = mv / msat From this definition, the reader can notice that rh depends directly on mv. This implies a dependence on vapour pressure ev, or even on specific humidity sh. Moreover, it is possible to notice that there is an inverse dependence on msat, or on shsat, which we know depends on T. The relative humidity, in fact, can also be approximately expressed, on the basis of the definition of ev and esat, as: rh = ev /esat = sh/shsat In practical terms, this explains why, if the temperature is kept constant, when the water vapour content of the air, mv, increases, the relative humidity rises. Vice versa, if the water vapour is constant, a rise in temperature results in a decrease in the relative humidity. Small changes in vapour content or in temperature may take place in the environment, but if the relative humidity is to be kept constant (as is desirable), the variations in both these parameters should compensate for each other. RH is a non-dimensional quantity and is usually multiplied by 100 in order to be more conveniently expressed as a percentage, i.e.: RH = 100 • rh 142 (%) Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 143 Mathematically speaking, if the measurements are performed by means of an electronic psychrometer (described in paragraph 5,2), that is by measuring the temperature of the surrounding air (T) e and the wet-bulb temperature (Tw), RH can be calculated by means of the following formula: or where p is the atmospheric pressure expressed in millibar*, T is expressed in °C, and a and b are the previously introduced constants (a=7.65, b=243.12 °C). Through other mathematical calculations, it is also possible to express the other psychrometric variables in relation to the surrounding air (T) and RH and p: ev = 0.06112 • 10 7.65T / (243.12 +T) • RH (hPa) Or in relation to the surrounding air (T), the wet-bulb temperature (Tw) and p by means of the following formula: ev = 0.006112 • 10 7.65T / (243.12 +T) - 0.0001068 • p • (T–Tw) (hPa) 143 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 144 f) Dew Point (DP) The dew point is the temperature to which the air must be cooled down for the saturation of water vapour to take place with respect to free water, without any change in p or SH. When the T and RH values are known, DP can be calculated as: When the surrounding air (T) e and the wet-bulb temperature (Tw) are known, DP can be calculated as: g) Frost temperature (FP) The definition of FP is similar to that of DP, but it refers to ice instead of water. FP ≤ T and as a rule FP < 0°C. h) Dew point spread (DPS) When the dew point has been defined, the dew point spread is expressed as: DPS = T – DP ( °C) and therefore depends both on the T and degree of saturation of the air. Mathematically it can be expressed as: 144 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 145 or i) Wet-bulb temperature (Tw) or psychrometric temperature From a physical point of view, the wet-bulb temperature (Tw) is defined as the constant-pressure equilibrium temperature indicated by a thermometer covered with gauze soaked in distilled water and subjected to an adiabatic* evaporation by means of forced ventilation at a speed of 3-5 m/s. The lowering of the temperature, T-Tw (also called wet-bulb depression), is due to the absorption* of heat by the evaporating water, and increases when the evaporation increases and therefore the water vapour content at that temperature decreases, in other terms as the RH decreases. When T and Tw are known, all the other psychrometric quantities can be obtained mathematically, by means of the previously indicated formulas. 145 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 146 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 147 APPENDIX 2 USE OF THE PSYCHROMETRIC CHART A psychrometric chart [4] (Fig. 49) is the graphic representation of the set of mathematical relations (described in Appendix 1) that interconnect the main thermohygrometrical parameters. It is one of the most useful and practical instruments, because it makes it possible, on the basis of the instrumental readings, speedily to achieve first-approach approximate evaluations. This psychrometric chart is valid only for the atmospheric pressure value at sea level, considered of 1013 hPa. Any reasoning based on a chart must therefore be made considering the pressure constant. In fact a variation of the pressure causes change of the thermohygrometric values. For example a variation of 1% of the atmospheric normal pressure causes a variation of some hundredth of the temperature that for some particular applications (for example near of the dew point so near the danger of condensation) can be important for the starting of the dangerous phenomena (for example condensation). In order to characterise an air mass from the thermohygrometrical point of view, it is sufficient to know two values, temperature T and a second one related to the humidity, for example the specific humidity SH, which are the coordinates of 1 point of the psychrometric chart, respectively its abscissa and its ordinate. Moving along the lines of this chart, we obtain the other thermodynamic information about the air mass. Any variations in the thermohygrometrical characteristics of the air mass at the beginning involve a shift along the lines of the psychrometric chart. The main parameters that can be read in a psychrometric chart starting from the knowledge of T and SH, (or sh, depending on whether the charts express it as g/kg or kg/kg) and moving along the lines of the chart, are: 1) relative humidity (RH) 2) dew point (DP) 3) wet-bulb temperature (Tw) 4) vapour pressure (ev) 5) saturation specific humidity (SHsat) 6) saturated vapour pressure (esat) 147 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 148 (From a chart it is also possible to extract other parameters that will not be mentioned here because they are less relevant so they have not been considered in this text.) To be exact, starting from a point A with temperature T and specific humidity SH, which we will indicate as A(T, SH), we can obtain the following. Relative humidity (RH): it is the reading in point A done on the curve that passes through A and belongs to the set of exponential curves that range from 0% al 100%, air saturation value. Dew point (DP): we move horizontally from point A (i.e. SH keeps constant, while T decreases) until we reach the saturation curve (RH=100%). The corresponding temperature at this point of intersection is the DP Wet-bulb temperature (Tw): we move diagonally along the straight lines (isenthalpic lines) that intersect the relative humidity curves, starting from point A (SH increases, while T decreases), until we reach the saturation curve (RH=100%). The corresponding temperature, at this point of intersection, is the wet-bulb temperature. Vapour pressure (ev) it is read directly on the second ordinate next to SH (a psychrometric chart usually contains both the scales). Saturation values of ev and SH: in order to obtain the saturation value of the specific humidity (SHsat) and the saturated vapour pressure (esat), it is sufficient to move from the point A vertically (T remains constant, while SH and ev increase) until we meet the saturation curve RH=100%. The corresponding values on the ordinate are SHsat and esat. Let us see a few examples. 1st example Outdoor air with the thermohygrometrical characteristics of a temperature of 9°C and SH of 5 g/kg (i.e. sh = 0.005 kg/kg) is warmed up to 22°C. Using a psychrometric chart, we must determine: a) which variation in RH that air mass has undergone; b) which variation the DP has undergone after the warming up; c) which is the initial and final SH sat. 148 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 149 Solution a) At the beginning, the RH of the air mass with T =9°C and SH=5 g/kg is 70%. After the warming up, it becomes 30% (we move horizontally towards the right, keeping the SH constant). There has been a 40% decrease in the RH. b) Since there has been no change in the SH, the DP value is unchanged (we move horizontally on the same line corresponding to SH=5 g/kg until we reach the saturation curve, which corresponds to a DP of 4°C). c) At the beginning, the SHsat was 7 g/kg; after the warming up it is approximately 16.5 g/kg (we move vertically starting from the two temperatures, respectively 9 and 22 °C, until we meet the saturation curve RH=100%). Please notice that the amount of vapour that saturates the air depends only on the temperature of the air. The higher the temperature, the more vapour the air can contain before it reaches saturation. (This explains why during the summer the vapour content of the air required to reach saturation is considerably higher than in the winter.) 2nd example In a room where the air has the following characteristics: T=18.5 °C and RH=30%, a humidifier is installed; after it has been working for a certain time, it introduces 4 g/kg of vapour in the room. We must determine: a) which was the initial SH; b) which RH has been reached by the air mass; c) whether the risk of surface condensation has increased or decreased, and which is the new DP; d) which was the initial Tw and which is the present one. Solution a) The SH of the initial air mass is 4 g/kg (from the point of intersection between T=18.5 °C and the exponential curve that corresponds to the 30 % curve, we move horizontally and read the corresponding ordinate). b) After the addition of vapour due to the action of the humidifier, the RH of the air becomes approximately 60% (we move vertically until we reach the point that corresponds to a SH ordinate of 8 g/kg and a T of 18.5 °C: at that point we can read the value of the exponential curve that crosses it). c) At the beginning, the dew point was 1°C (we move horizontally starting from the SH of 4 g/kg ordinate, until we meet the 100% exponential curve; the DP is the temperature in the abscissa that corresponds to that point); then the DP became 11°C (same procedure as before, but starting from a SH of 8 g/kg). This shows that after humidification the risk of surface condensation has increased. d) At the beginning, Tw was 10 °C (we move from the point of intersection between T and RH along the oblique line that crosses it, until we reach the 100% expo149 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 150 nential curve; Tw is the temperature in the abscissa that corresponds to that point); after humidification, Tw becomes 14 °C (same procedure as before, starting from a temperature still of 18.5 °C, but a RH of 60%), moving nearer to the drybulb temperature, which means a greater saturation of the air. Fig. 49. Psychrometric chart. 150 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 151 GLOSSARY absorption/to absorb: process by which the energy of a photon* is taken up by another entity accuracy: the closeness of agreement between a test result and the accepted reference value. It indicates how close a measurement is to the accepted (real) value adiabatic: without any heat exchange with the surrounding environment adsorption/to adsorb: to fix on a surface some molecules in gaseous or liquid state air exchange rate: the rate at which in a room inner air is replaced by outside air anthropogenic: coming from human activity atmospheric residence time: the average period that a particle stays suspended in ambient air blackbody: surface which absorbs all radiation incident upon it having any wavelength and reradiates energy which is characteristic of this radiating system only, not dependent upon the type of radiation which is incident upon it. It is a perfect absorber and also emitter candela: the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian capacitor: system of conductors that are separated by insulating material called “dielectric* material” convective cell: vertical motion of an air mass caused by a thermal imbalance deadsorption: the opposite phenomenon of adsorption deposition velocity: the rate at which a particle reached the surface of the material (m/sec or m/h) dielectric: material that is a poor conductor of electricity drift: deviation from original value with time electrostatic capacity: ratio of the electric charge accumulated by one of the two faces of a capacitor* to the difference between the potential of the two faces The electrostatic capacity of a capacitor* indicates the amount of electric energy the capacitor* can charge gradient of a physical quantity A: ratio of the difference between the values that quantity A takes on in points 2 and 1 to the distance between those two points. It is the measure of how much a quantity changes in space. For example, the gradient of temperature T (or relative humidity, RH): the temperature (or RH) measured in point 2 and that measured in point 1, divided by the distance between the two points. 151 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 152 grisaille: fluid suspension made chiefly of glass that melts at a low temperature applied with a brush to a glass pane and fixed to it by firing. It is the precious part of an antique piece of glass because it achieves the painting on the coloured glass/ hectopascal (hPa): unit of pressure. It is equal to 100 Pascals (Pa) and to 1 millibar* hydrogen bond: weak bond between a hydrogen atom bound to a molecule and a pair of electrons of an adjacent molecule hysteresis: this word is used when the absorption* and desorption* curves (e.g. of water) do not coincide lapse rate (dry and wet): For the atmosphere, the drop in temperature of rising, unsaturated air is about 10 °C/1000 meters (dry); 6.5 °C/1000 meters (wet) latent heat: heat released or absorbed by a substance at a constant pressure during a state transition latent heat of condensation is energy released when water vapor condenses to form liquid droplets lumen: light flow emitted by a punctiform source whose intensity is 1 candela* within a 1-steradian solid angle lux: in the International System (SI) it is the unit of lighting, equal to 1 lumen* per m2 millibar (mbar): unit of atmospheric pressure. 1 mbar is equal to 1hPa photon: quantum of electromagnetic energy. Also called light quantum precision: it indicates how close together or how repeatable the results are. A precise measuring instrument will give very nearly the same result each time it is used. resolution: the smallest difference that can be read in a significant way at the output of an instrument steradian: unit of solid angle. Angle from which, at the centre of a sphere, it is possible to see a portion of spherical surface equal to the square of the radius of the sphere strain: result of stress. Fractional change in the size of an object: in terms of length, e.g., change in length in comparison with the original length of the object. It is a pure number. stress: force divided by an area. Tensile stress, e.g. is the force applied to a bar divided by the section of the bar to which the force is applied. thermal capacity: amount of heat required for raising the temperature of any mass by 1 °C, at constant pressure and volume values thermal conductivity (k): it indicates the ability of a material to conduct heat. It is defined as the quantity of heat transmitted in time through a thickness, in a direction normal to a surface 152 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 153 of area A, due to a temperature difference ΔT, under steady state conditions and when the heat transfer is dependent only on the temperature gradient*. thermistor: semiconductor whose elements undergo changes in electric resistivity in relation to temperature. They are very sensitive and have low time constants* thermocouples: two dissimilar metals are joined producing a predictable voltage that relates to the difference in temperature between the measuring junction and the reference junction. thermoresistance (platinum resistance thermometers): sensor formed of an electric platinum resistor that changes when the temperature changes; in trade it is called Pt 100 time constant: time required by a physical quantity or instrument to reach 63.2% of the real value or final reading, respectively uncertainty: parameter associated with the result of a measurement: it defines the scattering of the values that can be reasonably attributed to that measurement. It is a quantitative concept. van der Waals forces: weak mutual-attraction forces that arise when two atoms or molecules mutually distort their charge distribution water activity (aw): describes the (equilibrium) amount of water available for hydration of materials; a value of unity indicates pure water whereas zero indicates the total absence of ‘free’ water molecules SYMBOLS C C °C DP ev esat F Fv k K Kw KF hPa IR J m mv ma msat M Mv mr, MR = = = = = = = = = = = = = = = = = = = = = = Coulomb (unit of charge) Concentration of particles in atmosphere Celsius degree dew point (°C) partial vapour pressure saturated vapour pressure, or saturation pressure, or saturated vapour tension heat flow vapour flow Boltzmann’s constant (k=1.381*10-16 erg/K) Kelvin degrees coefficient of vapour diffusion along direction n coefficient of heat exchange hectopascal infrared radiation Joule mass of a gas mass of vapour mass of air mass of saturated vapour molar mass of a gas molar mass of the water mixing ratio (g/g, kg/kg or g/kg)) 153 Pleiadi1_16102008:Pleiadi_eng_16102008 mbar n = m/M N p R Re Rr Rv R T Tw ah, AH rh, RH sh, SH shsat, SHsat UV V vp vdep ε λ ρ σ 154 = = = = = = = = = = = = = = = = = = = = = = = 16-10-2008 13:27 Pagina 154 millibar number of moles Newton atmospheric pressure universal constant of gases = 0.06857 cal g-1 K-1 emitted radiation reflected radiation constant of vapour = 0.1102 cal g-1 K-1 energy irradiated by a blackbody * temperature (°C) wet-bulb or psychrometric temperature absolute humidity (g/m3) relative humidity (number between 0 and 1 or %) specific humidity (g/g, kg/kg or g/kg) saturation specific humidity ultraviolet radiation volume velocity of a particle in atmosphere velocity of deposition of a particle on a surface emissivity of a body wavelength density of air surface tension of water Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 155 ACKNOWLEDGEMENTS There are many people I would like to mention and thank for everything they have done for me, both from a professional and a human point of view. I can only refer here to a few of them, the ones whose help was more actual; in particular, I wish to thank my colleagues and friends Dr. Dario Camuffo, Dr. Sergio Vincenzi, Dr. Silvia Enzi and Prof. Ferruccio Petrucci, my invaluable collaborator and friend Dr. Francesca Becherini and finally, a special thought and heartfelt thanks to all the members of my family, who have supported me in every moment of my personal and professional life, and to whom I apologise for having deprived them of so much precious time. The studies described in the text have been carried out thanks to several projects, financially supported in particular by CNR - Progetto Strategico Beni Culturali and Progetto Finalizzato Beni Culturali, and by the European Commission since 1984 to today in particular the last ones: VIDRIO-contract no. EVK4-CT-2001-00045 and TeACH-FP7-ENV-2007-1 n.212458. 155 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 156 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 157 INDEX MOST IMPORTANT TERMS AIR CONDITIONING: 3, 9, 19-22, 29, 31-32, 131 DEW: 67, 75-76, 106, 112, 120-122, 136, 144, 146-148, 153 ANEMOMETER: 107, 123, 124 DIFFUSIOPHORESIS: 94, 96 ATMOSPHERIC STABILITY: 3, 66, 87-88,102, 117 ELECTROPHORESIS: 97-98 BIOLOGICAL GROWTH: 52, 54 BLACKENING: 89, 97, 99, 101-102 BUILDING: 3, 11, 14, 19, 21, 29,31, 45-46, 50-52, 54-56, 58-59, 65, 67, 76, 81-82, 99, 126-127, 131-135 CALIBRATION: 28, 115, 118, 121, 124-125 COMFORT: 3, 21, 55-56, 59, 62-67 CONDENSATION: 3, 42, 47, 50, 67, 75-79, 83, 94-96, 110, 112-113, 117, 121-122, 132-133, 141, 146, 148, 152 CONDITIONING SYSTEM: 15, 28, 55 CONDUCTION: 80, 81, 100 CONSERVATION: 3-5, 7-8, 14, 19, 21-22, 29, 31-33, 35-36, 39, 43-44, 4851, 55-56, 59-60, 62-67, 70-71, 73-75, 82, 104-105, 109, 121, 126-127, 129-138 CONVECTION: 28, 38, 44, 64, 66, 8081, 96, 100 CRYSTALLISATION: 21 ENERGY: 14, 21, 27, 29, 34-36, 38, 4043, 55, 58, 60, 65, 80, 108, 117, 122, 124, 133 151-152, 154 ENVIRONMENT: 3, 5-7, 9, 11-12, 1516, 19-22, 28-29, 3134, 38, 43-50, 52-56, 59-67, 72-73, 75-76, 79, 82-83, 87-89, 91-93, 98, 100-101, 103-109, 111-112, 114-119, 121, 123, 127, 129-135, 137138, 141-142, 151 EQUILIBRIUM MOISTURE CONTENT (EMC): 84-87 EXHIBITION: 5, 15, 31, 45, 47, 50, 52, 54, 56, 59-62, 65, 71-74, 79, 89, 91, 103, 117, 126, 127, 130 EVAPORATION: 13-14, 28, 42, 54, 77-79, 84, 96, 110, 112-113, 133, 141, 145 GRISAILLE: 36, 51-52, 90, 135, 137, 152 GRAVITATIONAL DEPOSITION: 92, 97, 99 HEATING SYSTEMS: 12, 19, 26-27, 29-31, 56, 60, 72, 88, 105, 111-112, 127 DEPOSITION: 3, 8, 15, 28, 47, 59, 71, 82, 87-89, 91-94, 96-102, 104, 107-108, 117, 124, 133, 135-138, 151, 154 HUMIDIFIER: 15-16, 22-25, 30, 59, 82, 111, 148 DETERIORATION: 8, 19-20, 22, 41, 49, 51-53, 61-63, 75-76, 82, 91, 111, 127, 129, 137 INDOOR: 5, 7, 11-12, 14, 16, 31-32, 50, 52, 55-56, 60, 66-67, 88, 91, 93, 100, 102, 104-106, 108, 114-115, 117, 123, KELVIN EQUATION: 78 157 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 158 127, 131, 136-137 POROSITY: 77-78, 124 INERTIAL DEPOSITION: 92, 96, 100-102 PRESSURE: 74, 79, 95-96, 110, 113, 124, 139-140, 142-143, 145-147, 152154 INFRARED: 34-35, 40, 67-68, 122, 124, 153 INSULATION: 29, 58, 66-67 IRRADIATION: 80-81 LIGHTING: 3, 5, 9, 13-15, 21, 31-32, 34-35, 38-44, 55-56, 61, 67-68, 96, 100, 105, 108, 126-127, 129, 131, 152 MANAGEMENT: 5, 9, 16, 19-21, 2425, 29, 33, 38, 43, 52-53, 56, 58-59, 62, 65-66, 71, 74, 89, 104, 127, 129, 131134, 137 MEASURE: 3-4, 7-9, 12, 14, 24-25, 2729, 39, 46, 49, 54, 58-59, 61, 65-66, 87, 91, 103, 105-107, 109-110, 112-114, 116-125, 127, 133-135, 137, 139-141, 143, 151, 153 MICROPORE: 3, 75, 77-78, 83, 94 MIXING RATIO; 11, 22, 82, 110-111, 133, 141, 153 OZONE: 103-104, 137 PAINTING: 8, 13, 15, 24-27, 30, 36, 38-39, 42, 51,52, 58-60, 72-73, 86-87, 89, 98, 152 PAPER: 19, 38-39, 41, 43, 49-50, 52-54, 62, 64, 83, 133, 135 PARTICLE: 47-48, 76, 88-89, 91-103, 107, 124-125, 135-138, 151, 153-154 PASSIVE MEASURE: 3, 65-66 PHOTOPHORESIS: 98 POLLUTION/POLLUTANT: 3, 6-8, 14-15, 28, 36, 44, 46-47, 59, 66-67, 71, 76, 82, 87-96, 98-99, 103-105, 108, 124, 129, 131-137 158 PSYCHROMETER: 106, 112, 118-119, 143 PUBLIC: 3, 6, 12-15, 20, 31-32, 46-47, 54, 56, 61,62, 71, 89, 102-104, 129, 132 RADIATION: 3, 8, 29, 34-36, 38-43, 51, 67-68, 76, 80-82, 92, 98, 108, 122123, 131, 135, 151, 153-154 SILICA GEL: 68-70 SHOWCASE: 3, 52, 67-68, 71, 123, 134-135 STABILITY (ATMOSPHERIC): 3, 66, 87-88, 102, 117 STANDARD: 4, 64, 71, 75, 103, 110, 116, 125-126,131, 136 TEMPERATURE: 11-13, 15-17, 19-24, 26, 30, 32-35, 43, 46, 48-60, 62-64, 6668, 72-73, 75-83, 85-88, 91-94, 96, 100, 102, 105-107, 110-113, 116-124, 126, 133, 136, 139-149, 151-154 THERMOMETER: 110, 112, 116, 145, 153 THERMOPHORESIS: 93, 98, 102 TOURISM: 6-7, 46, 129-130, 134 TURBULENCE: 3, 48, 87, 89, 93, 96, 100, 102, 108 UV: 25-27, 34, 38-43, 62, 98, 133-134, 154 VENTILATION: 3, 59, 71, 79, 87, 99, 118-119, 123-124, 137, 145 WOOD: 19-20, 29-30, 36, 38-39, 41, 49-50, 52, 60-61, 66, 68, 73, 83, 85, 86, 135-137 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 159 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 160 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 161 Color tables Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 162 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 163 Sistine Chapel - Rome (Italy) Fig. 1. See page 12. Sistine Chapel - Rome (Italy) 31 July 1984, h 10:30 Fig. 2. See page 13. 163 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 164 Koninklijk Museum voor Schone Kunsten - Antwerpen (Belgium) Van Dyck's room - 17 July 1999, h 14:00 Fig. 4. See page 15. Koninklijk Museum voor Schone Kunsten - Antwerpen (Belgium) Room I, Rubens - 10 February 1999 Fig. 6. See page 17. 164 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 165 Correr Museum Venice (Italy) Bellini room 15 March 1993 h 12:30 Fig. 7. See page 23. 165 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 166 Koninklijk Museum voor Schone Kunsten - Antwerpen (Belgium) T Room - 13 February 1999, h 10:30 Fig. 12. See page 27. Uffizi Galleries - Florence (Italy) Botticelli room - 18 February 1997, h 17:00 Fig. 13. See page 30. 166 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 167 Fig. 19. See page. 40 Sainte Chapelle - Paris (France) 20 August 2002, h 16:45 Fig. 22. See page 46. 167 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 168 British Museum, Egyptian Rooms - London (UK) 25 February 1988, h 18:00 Fig. 25. See page 57. British Museum, Egyptian Rooms - London (UK) 24 February 1988, h 10:40 Fig. 26. See page 57. 168 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 169 Chiericati Palace - Vicenza (Italy) Fig. 27. See page 58. Fig. 32. See page 73. 169 Pleiadi1_16102008:Pleiadi_eng_16102008 13:27 Pagina 170 b) a) Fig. 39a-b. See page 90. c) Fig. 39c. See page 90. 170 16-10-2008 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 171 Fig. 46. See page 119. (a) (b) Fig. 48 a-b. See page 122. 171 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 172 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 173 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 174 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 175 Pleiadi1_16102008:Pleiadi_eng_16102008 16-10-2008 13:27 Pagina 176 © il prato casa editrice via Lombardia 41/43 35020 Saonara (Pd) tel. 049 640105 www.ilprato.com [email protected] impaginazione: Scriptorium, Vicenza Finito di stampare nel mese di ottobre 2008 presso le Arti Grafiche Padovane di Saonara (Pd) per conto della casa editrice il prato