Portoro, the black and gold Italian “marble”

Rend. Fis. Acc. Lincei (2015) 26:415–423 DOI 10.1007/s12210-015-0420-7 GEOSCIENCES AND CULTURAL HERITAGE Portoro, the black and gold Italian ‘‘marble’’ Fabio Fratini1 • Elena Pecchioni2 • Emma Cantisani1 • Fabrizio Antonelli3 Marco Giamello4 • Marco Lezzerini5 • Roberta Canova6 • Received: 22 December 2014 / Accepted: 31 March 2015 / Published online: 21 April 2015  Accademia Nazionale dei Lincei 2015 Abstract Portoro is one of the most famous Italian black limestones due to its characteristic golden-yellow veins on a black background. It was used since Roman times, mainly in the city of Luni. Since the Middle Ages, its use is widespread in Genoa, and from the XVII century, it became one of the most common stones in religious buildings throughout Italy. At the end of the XIX century, its use has spread abroad, particularly in Europe and USA. It was extracted in several quarrying areas located near La Spezia, but at present, only five quarries are active. This stone, exposed to weathering, tends to bleach losing the appearance of its golden streaks that determine its aesthetic appeal. This research deals with the petrographic and This contribution is the extended, peer reviewed version of a paper presented at the session ‘‘Archaeometry and Cultural Heritage: the contribution of Geosciences’’ held during the conference ‘‘The future of the Italian Geosciences, the Italian Geosciences of the future’’, organized by the Società Geologica Italiana and the Società Italiana di Mineralogia e Petrologia, Milano, 10–12 September 2014. & Elena Pecchioni [email protected] 1 CNR, Institute for Conservation and Valorization of Cultural Heritage, Via Madonna del Piano 10, Sesto Fiorentino, 50019 Florence, Italy 2 Earth Sciences Department, University of Florence, Via G. La Pira, 4, 50121 Florence, Italy 3 Analysis Laboratory Ancient Materials, DACC, University IUAV of Venice, San Polo 2468, 30125 Venice, Italy 4 Physical Sciences, Earth and Environmental Department, University of Siena, Via Laterina, 8, 53100 Siena, Italy 5 Earth Sciences Department, University of Pisa, Via Santa Maria 53, 56126 Pisa, Italy 6 Viareggio, Lucca, Italy chemical characterization of the Portoro macchia larga variety as well as the study of its chromatic alteration in order to define guidelines for the most suitable use of this stone and for restoration works. Keywords Portoro
Limestone
Characterization
Chromatic alteration
Patina 1 Introduction Black limestones, traditionally called Neri Antichi or Bigi Morati by modern day Roman stonemasons (Brilli et al. 2010), were commonly used in antiquity, often in conjunction with coloured stones/marbles, for architectonic and sculptural elements such as capitals, columns, bases, opera sectilia and statues. Numerous quarries were exploited for this material in various parts of the Mediterranean basin (Brilli et al. 2010); most were used exclusively on a local scale because of the poor quality of the stone, whereas better quality stones from a few quarries spread throughout the Mediterranean itself. Exploitation of many of these quarries started in very early periods, generally reaching its maximum extraction phase during the Roman Empire (Brilli et al. 2010 and references therein). However, some of them had a ‘‘second commercial life’’ in modern times, particularly during the Renaissance and the Baroque periods, due to the appeal of their particular textures and colour patterns (i.e. black limestones with more or less coloured veins). The Portoro limestone, also called Mischio giallo e nero, Portovenere marble, Black and Gold, Giallo e Nero di Portovenere (Caselli 1924), is one of the most famous and valuable Italian black limestones whose success in modern times is mainly due to its unique texture 123 416 Rend. Fis. Acc. Lincei (2015) 26:415–423 characterized by prevailing golden-yellow veins on a black background. Quarried both on the promontory and on islands in front of Portovenere (Liguria), it was already used in Roman times, particularly in the city of Luni as slabs for the cardo and decumanus (II century sec. BC), in little blocks for the amphitheatre (I century BC) and also for the realization of columns (one of them is in the Archaeological Museum of La Spezia) (Del Soldato and Pintus 1985). Its use widely increased since the XII century when the Republic of Genoa exploited it mainly for building defensive structures (Pandolfi 1971; Cimmino and Robbiano 2005) but also for the construction and decoration of monuments, cathedrals and villas. Nevertheless, it is in the Renaissance and in the Baroque times that the use of Portoro, which was frequently paired with the less famous Portargento, a white-veined black limestone quarried in the same region (Beltrame et al. 2012), spreads all over Italy as ornamental material (Bonci 2007): for instance, in the Baptistery of the Church of Saint Mary in La Spezia, in the Palace of the Marquis of Castagnola in Genoa, in Portovenere in Jesus Church, Sts. Ambrogio and Andrea Church, St. Siro’s Church and in the font of the St. Peter’s Church (Fig. 1); in the St. Mark’s Basilica of Venice, where post-antique squared and triangular tiles are visible in the floor of the western arm of the narthex; in Rome, in the St. Peter’s Basilica (a mask carved by Giacomo della Porta in the deposition of Paul III), in the churches of San Pietro in Vincoli (St. Peter in Chains), San Silvestro in Capite (St. Sylvester the First), San Paolo fuori le Mura (St. Paul outside the Walls), San Giovanni in Laterano (St. John Lateran), San Lorenzo fuori le Mura (St. Lawrence outside the Walls), Santa Maria Maddalena in Campo Marzio (St. Mary Magdalene in Marzio Field), Sts. John and Paul and San Luigi dei Francesi (St. Louis of the French); in Palermo, in Jesus Church also known as Casa Professa, etc. At the end of the XIX century, its use has spread abroad, especially in England, North America (Paramount Home Theatre) and secondarily Belgium and France (Versailles, Marly, Compiegne), where it was utilized for fireplaces, coverings, plinths and panels for furniture (Pandolfi 1971; Bonci 2007). The dam crossing the gulf of La Spezia was realized partially with the blocks of Portoro coming from Palmaria Island. Similar but poorly known historical stones were the Portoro quarried in medieval times in the Monti Pisani, which was suggested to correspond to the Nero della Duchessa stone (Cataldi et al. 2013) and the brecciate varieties quarried in the Apuan Alps (Bartelletti and Amorfini 2003) both used only locally. Historically, other materials of similar aspect, but frequently brecciate, were extracted in Piedmont (Portoro di Nava) (Fiora and Alciati 2003; Fiora et al. 2003) and in France (Portor de Saint Maximin in Var, Portor des Pyrénées and Portor des Pyrénées Bramevaque in the Pyrénées region) (Fiora and Alciati 2007). This material, as well as other carbonaceous black ornamental stones (Canova 2002) exposed to the weathering, tends to bleach with time losing the look of its golden streaks and spots on the black background that determine the particular aesthetic appeal (Fig. 2). The aim of this study was to perform a mineralogical, chemical and petrographic characterization of the black portion of the Portoro macchia larga variety and to define its chromatic alteration which develops with the formation of a whitish patina, in order to define guidelines for the most suitable use of this stone and for restoration works. Fig. 1 Font in Portoro (Baroque age), inside the St. Peter Church in Portovenere Fig. 2 Hand specimen of Portoro displaying the colour contrast between the internal black stone and the bleached surface 123 Rend. Fis. Acc. Lincei (2015) 26:415–423 2 Geological setting and varieties of Portoro Portoro crops out in the mountain chain that closes to the west the gulf of La Spezia, from the village of Biassa to the north, to Palmaria, Tino and Tinetto Islands to the south. This lithotype is related to the stratigraphy and structure of the area, constituted by a reversed NW–SE anticline with Tyrrhenian vergence. Such anticline is followed to the east by the La Spezia graben and the Lerici-Montemarcello promontory (Ciarrapica and Passeri 1980). Such structures are the consequence of the deformative phases that affected the Tuscan Nappe from Late Miocene (Federici and Raggi 1975). Moreover, systems of direct and transcurrent faults with direction NW–SE, NE–SW and ENE–WSW successive to the Late Miocene deformative processes are also present (Carter 1991). Particularly, the Portoro formation is delimited at the bottom by the ‘‘La Spezia Formation’’ (which is in turn divided into two members, ‘‘limestones and marls of Monte Santa Croce’’ and ‘‘Portovenere limestones’’) and at the top by the ‘‘Monte Castellana Dolomites’’ (upper Rhaetian–Hettangian). The main layer of Portoro suffered stretching that reduced, in many areas, its thickness that is at maximum 6–7 m. It consists of a dark grey to black nodular limestone with white- and yellow-coloured dolomitic vein patterns (Abbate et al. 2005) (Fig. 3). A more detailed observation of the Portoro bed (which crops out always overturned) makes it possible to evidence the following levels, named, from top to bottom (overturned sequence): Scalino marmorizzato, Scalino, Banco, Sottobanco or Zoccolo, Sottozoccolo (Pandolfi 1971; Chelli et al. 2005; Ciarrapica 1985) (Fig. 4): • • • • • Scalino marmorizzato is a fine-grained black-grey limestone that, when very rich in dolomite, is called Tarso and it is not commercialized; Scalino is constituted by at least five golden-yellow veins on an absolute black background. The thickness of this level is between 0.8 and 1 m; in every six veins, there is a quite straight vein (sometimes, it can be found in the Banco); Banco shows a black background with at least seven golden-yellow veins, some of them wider than the previous ones. The thickness of this level is 1.20–1.50 m; Sottobanco or Zoccolo is characterized by narrower yellow veins and diffused white veins. The thickness of this level is 1.20–1.50 m; Sottozoccolo is thicker than the previous one and with more diffused white veins; The described sequence is that typical of Portoro macchia larga (large spotted), the most famous and prized, 417 which is separated from the underlying Portoro macchia fine (narrow spotted), by the Nero e Bianco di La Spezia (also named Portargento), a level that shows completely dominant white veins. The Portoro macchia fine is constituted by very fine straight or irregular veins and is less valuable. Therefore, within the Portoro macchia larga, the first quality that can be found is the Scalino, characterized by an absolute black background with golden-yellow veins, while a gradual fading is observed going towards the bottom of the Portoro bed (Sottozzoccolo) where less valuable varieties are present (Pieri 1966). From the commercial point of view, both the Portoro macchia larga and Portoro macchia fine can be distinguished in four qualities for a total of eight typologies according to the intensity of the colour of the background and of the veins (Cimmino et al. 2003), today not all of them available on the market. According to Ciarrapica and Passeri (1980), the Portoro limestone has been classified as a mudstone (Dunham 1962), and the black colour is due to the dispersion of very minute particles of carbonaceous matter, sometimes concentrated in veins and stylolites. Locally, some veins of secondary microsparitic calcite, often isoparallel, are present. Authigenic grains of quartz are sometimes observed dispersed in the carbonatic mass or concentrated in thin layers. The pigmentation is particularly evident in the Scalino facies. The veins of different colours show the following composition: – – the golden-yellow veins are characterized by limonite and sulphides present among dolomite crystals; the white veins are formed by coarse-grained dolomite crystals. Sometimes are present also purplish veins constituted by mosaics of dolomite crystals coloured by haematitic pigment dispersed or concentrated into red stylolites. 3 The quarrying areas Portoro was extracted in several quarries located near La Spezia (Liguria region, NW Italy), precisely on the promontory of Portovenere and in Palmaria and Tino Islands, located in front of Portovenere, where the extraction began in the open air and continued underground with the technique of the abandoned pillars (Fornaro 1999; Cimmino et al. 2006). In the XIX century, Cappellini (1864) reports the location of thirty quarries existing in the area of La Spezia, one of which on Tino Island and five on Palmaria Island 123 418 Fig. 3 Geological map of the area where Portoro outcrops are present (modified after Abbate et al. 2005) 123 Rend. Fis. Acc. Lincei (2015) 26:415–423 Rend. Fis. Acc. Lincei (2015) 26:415–423 419 Fig. 4 Lithostratigraphic sequence of the Portovenere limestones (modified after Chelli et al. 2005) Fig. 5 Abandoned quarry of Portoro in Palmaria Island (Fig. 5). In the XX century, the quarrying activities on the islands continued with ups and downs. Indeed, at the beginning of the century, the activity in Palmaria was remarkable (at least 10 quarries); at the end of the 1930s, the extractive industry faced a crisis which partially recovered after the Second World War. Subsequently, because of operational difficulties due to both environmental constraints and depletion of stone veins of good quality, the quarrying activity on the islands slowly declined up to cease at the beginning of the 1980s when the quarry of ‘‘Caletta’’, located in front of Tino Island, was closed. Since 1997, Portovenere together with Palmaria and Tino Islands are part of the UNESCO World Heritage sites and since 2001 constitute the Natural Regional Park of Portovenere. Concerning the amount of production, 1959 was the year of greater production, with 18,024 tons extracted; in the following years, the production stabilized around 6000 tons per year (Giordano 1969) and towards the beginning of the seventies raised to about 10,000 tons per year (Pandolfi 1971). Currently, only five quarries are active: the quarries of ‘‘Cavetta’’ and ‘‘Anime’’ in the municipality of Portovenere, the quarries ‘‘Castellana I’’ by Falconi Domenico, ‘‘Castellana’’ by Portoro BCC and ‘‘Santa Croce’’, in the locality of Santa Croce, all in the municipality of La Spezia. The consequent fall in productivity exposed this ‘‘marble’’ to the competition of similar commercial materials from abroad, cheaper and with huge production like Portoro Leonardo from Namibia, Portoro Santo Domingo and a variety from China (Fiora and Alciati 2007). Only the knowledge and awareness of the original Italian Portoro (namely the kind and disposition of the veins, their composition and the background colour) may help in 123 420 Rend. Fis. Acc. Lincei (2015) 26:415–423 powder after heating at 600 C and on extracts in different solvents (chloroform, hexane/methylene chloride 70/30, benzene/ethanol 2/1). recognizing its value with respect to other lithotypes commercialized with the same trading name. 4 Materials and analytical methods The research has been carried out on ten samples with a whitish chromatic alteration taken from the ‘‘Castellana I’’ quarry, located in the municipality of La Spezia belonging to the Portoro macchia larga variety and particularly from the Scalino level. On the black portion of the internal unweathered part of the stone (below the patina), the following analyses have been carried out: – – – – – mineralogical composition was carried out on powdered samples using a PANalytical X’Pert PRO diffractometer with monochromatic CuK a1 radiation, operating at 40 kV, 30 mA, investigated 2h range = 3–70, equipped with X’ Celerator multirevelatory and High Score data acquisition and interpretation software. Analyses were carried out on the bulk rock and also on the residue after acid attack with hydrochloric acid (2 % w/v) and on the fraction \4 lm extracted by sedimentation according to the Stokes’law; major elements composition was carried out by X-ray fluorescence (XRF) on pressed powder pellets, using a Philips PW 1480 wavelength dispersive XRF spectrometer with Rh anode. The procedure for the correction of the matrix effect and the calculation of the percentages (all elements are expressed as a percentage rounded to the second decimal) according to the method of Franzini et al. (1975) was followed; total volatile components (H2O? and CO2) were determined as loss on ignition (LOI) at 950 C on powder dried at 105 ± 5 C; determination of the CaCO3 content was carried out with a Dietrich-Fruhling calcimeter; quantitative analysis of total carbon and organic carbon was carried out using a GC-CHN Elementar Analyzer FISONS NA 2000 gas chromatograph with a thermal conductivity detector at the Laboratory of CHN—Mass spectrometer of the Institute for Marine Geology of CNR—Bologna: the total carbon was determined from the untreated sample powder, while the organic carbon was determined from the analysis of the powder treated with hydrochloric acid; in order to investigate the composition of the organic substances, FT-IR (Fourier Transform Infrared Spectroscopy) analyses through the Golden Gate were carried out on untreated powders, on insoluble residues after attack with hydrochloric acid, on the residual 123 On the bulk sample inclusive of the patina, the following analyses were performed: – – petrographic analyses was carried out on thin sections with a ZEISS Axio Scope.A1 polarized microscope equipped with a 5 megapixel camera resolution and a dedicated AxioVision image analysis software, in order to study the texture of the stone and the aspect of the surface in cross thin sections; morphological and microchemical analyses on stratigraphic sections perpendicular to the surfaces of alteration were carried out with a scanning electron microscope Zeiss EVO MA 15, coupled to an analytical system (EDS OXFORD INCA 250). The following standards were used for the semi-quantitative analyses: albite, MgO, Al2O3, SiO2, wollastonite, MAD-10 feldspar, Ti and Fe. 5 Results and discussion The XRPD mineralogical analyses of the internal unweathered part of the stone (below the whitish patina) show the exclusive presence of calcite with traces of dolomite and quartz. The analysis of the insoluble residue after hydrochloric acid attack shows the presence of quartz and phyllosilicates (micas and clay minerals); the analysis of the fraction \4 lm makes it possible to recognize the types of clay minerals: illite, kaolinite and chlorite (Table 1). The results of the XRF chemical analyses of the major elements are reported in Table 2. The XRF data show mainly high values of CaO, in agreement with the CaCO3 data obtained through calcimetry and with the abundant presence of calcite (XRPD). The low amount of MgO (XRF) is referred to the presence of dolomite in traces (XRDP). Generally, the presence of MgO and dolomite is to be put in relation with Table 1 Mineralogical data obtained on ten samples of Portoro (internal unweathered part of the stone, insoluble residue after acid attack, fraction \4 microns) Samples Composition inner stone Insoluble residue Fraction B4 lm Portoro Calcite xxx Quartz xx Illite xxx Dolomite tr Phyllosilicates x Kaolinite x Quartz tr xxx very abundant, xx abundant, x present, tr traces Chlorite tr Rend. Fis. Acc. Lincei (2015) 26:415–423 421 Table 2 XRF data (wt%) and calcimetry (CaCO3 contents) of ten Portoro samples (internal unweathered part of the stone) Samples SiO2 TiO2 Al2O3 Fe2O3TOT MnO MgO CaO Na2O K2O P2O5 L.O.I. CaCO3g PT1 0.78 0.01 0.39 0.15 bdl 1.66 53.30 0.01 0.10 bdl 43.70 97.90 PT2 0.65 0.02 0.33 0.18 bdl 1.50 53.50 0.00 0.10 bdl 43.90 98.40 PT3 0.58 0.01 0.29 0.10 bdl 1.16 54.00 0.01 0.10 bdl 44.30 98.50 PT4 0.75 0.01 0.38 0.08 bdl 1.80 53.30 0.03 0.10 bdl 43.60 98.45 PT5 0.64 0.02 0.32 0.07 bdl 1.28 54.60 0.00 0.10 bdl 43.20 98.75 PT6 0.58 0.01 0.29 0.15 bdl 2.06 53.80 0.06 0.00 bdl 44.10 98.50 PT7 0.66 0.01 0.33 0.14 bdl 1.82 53.40 0.01 0.10 bdl 43.30 98.60 PT8 0.89 0.01 0.45 0.14 bdl 1.90 52.80 0.04 0.10 bdl 42.20 98.30 PT9 0.75 0.02 0.38 0.13 bdl 2.00 53.20 0.05 0.20 bdl 43.10 98.40 PT10 X 0.90 0.01 0.45 0.13 bdl 1.80 53.00 0.04 0.20 bdl 43.20 98.60 0.72 0.01 0.36 0.13 bdl 1.70 53.50 0.03 0.11 bdl 43.46 98.44 r 0.12 0.00 0.06 0.03 bdl 0.30 0.50 0.00 0.06 bdl 0.59 0.22 X average value, r standard deviation, g gas for calcimetry, bdl below detection limit, Fe2O3TOT total iron expressed as Fe2O3 Table 3 Comparison between LOIf and CO2g Campione LOIf CO2g LOIf–CO2g Portoro mean values 43.46 ± 0.59 43.29 ± 0.20 0.17 ± 0.05 f data from fluorescence, g data from gas calcimetry Fig. 7 Cross thin section of the weathered surface of Portoro observed with SEM (BS image) showing the presence of a more porous layer below the surface Fig. 6 Cross thin section of the weathered surface of Portoro observed in transmitted light (xpl): the surface shows a thin layer of recrystallized calcite (20 lm thick). Below this recrystallization layer, at a depth of about 100 lm, a 80-lm-thick layer rich in dark dots is present dolomitization phenomena that in the variety Scalino are very low, justifying the particular dark colour of the rock. Also, the XRF data of SiO2 are low and must be put in relation with the low content of quartz. Besides, the good correlation between the values of CO2 obtained by calcimetry compared with data of calcination obtained through loss on ignition which refers particularly to the loss of CO2 (Table 3) suggests only a possible presence of organic matter, certainly below the 1 %, analytical detection limit of both methods. The analyses performed through GC-CHN Elemental Analyzer (after attack in hydrochloric acid) show an average value of organic carbon of 0.081 ± 0.005 C %, therefore a low percentage, but sufficient to determine the dark colour when widespread in the fine carbonatic matrix. The FT-IR data determined on the bulk samples and on the residue after acid attack on the extracts in hexane/methylene chloride and benzene/ethanol have not identified any organic substance. The petrographic analysis on cross thin section shows that the stone has the typical appearance of a mudstone, with a micritic texture and a partially heterogeneous aspect due to the presence of dispersed little dark dots with 123 422 Rend. Fis. Acc. Lincei (2015) 26:415–423 Table 4 Microchemical analyses (SEM–EDS) realized on the internal unweathered part and surface of Portoro Samples MgO Al2O3 SiO2 CaO FeO K2O ‘‘Portoro’’ internal 0.94 ± 0.16 1.17 ± 0.62 6.87 ± 1.69 88.78 ± 2.37 1.16 ± 0.45 0.67 ± 0.20 ‘‘Portoro’’ surface 0.90 ± 0.15 1.10 ± 0.60 7.00 ± 1.50 89.00 ± 2.00 1.18 ± 0.40 0.65 ± 0.20 dimensions of 20–30 lm whose rim is not well defined (organic substance). The surface, where the whitish chromatic alteration is present, shows a thin layer of recrystallized calcite (20 lm thick). Below this recrystallization layer, at a depth of about 100 lm, a layer 80 lm thick, rich in dark dots, is present (Fig. 6). The presence of this layer could be generated by imbibition and evaporation cycles able to mobilize the organic substance under the surface. According to these data, the chromatic alteration seems to be due partly to the presence of a layer of tiny calcite crystals which increase roughness, therefore favouring the scattering of light (bleaching effect) and in part to the depletion of pigmenting substances. The SEM analyses performed on the same cross thin section show the presence of a more porous layer below the surface with interconnected porosity decreasing without discontinuity inward (Fig. 7); the thickness of this layer is approximately 100 lm. The microchemical analysis (EDS) shows a similar composition between the surface and the internal unweathered part of the stone (Table 4). As a matter of fact, from the chemical point of view, it is impossible to recognize secondary calcite precipitation from a support of the same composition. 6 Conclusions Portoro, exposed to weathering, tends to bleach with time losing the appearance of its golden streaks and spots that determine its aesthetic appeal. The analyses carried out in order to characterize the Portoro and to investigate the chromatic alteration showed that the stone is constituted by a calcitic matrix dark in colour due to the widespread presence of a pigmenting substance. The literature data indicate the organic pigment as the most frequent dark dye of limestone rocks. Nevertheless, the analyses carried out in order to recognize the presence of the organic substance have only partially confirmed these data; as a matter of fact, the GC-CHN Elementar Analyzer revealed only small quantities (\1 %) of insoluble organic carbon (but evidently sufficient to give an homogeneous dark colour when dispersed in the fine carbonatic matrix), and the FTIR analysis was not able to detect soluble organic materials, particularly as bituminous substances. With respect to the whitish chromatic alteration of the Portoro after exposure to weathering, we evidenced the 123 development of the following stratigraphic sequence (from the surface): – – – a thin layer of fine-grained recrystallized calcite (20 lm thick); a 100-lm-thick porous level depleted in organic substance; a 80-lm-thick layer enriched in dark dots, probably made of organic substance. These data allow us to explain the chromatic alteration which is due to a double effect: in part to the presence of a layer of tiny calcite crystals whose roughness favours the scattering of light with a consequent decrease in the colour saturation (bleaching effect) and in part to the depletion of pigmenting substances just below this layer. These data allow us to confirm that the use of this lithotype as decorative material is not suitable in outdoor conditions because of its chromatic alteration. As a matter of fact, this stone material can be used preferably indoor or outdoor but in zones not directly exposed to the action of atmospheric agents, in order to prevent marked bleaching effects. As concluding remarks, the knowledge of the characteristics of the Italian Portoro is fundamental to help in recognizing and maintaining its value with respect to commercial materials from abroad, named Portoro but far from the aesthetic aspect of this historical Italian one. 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