Glass in imitation of exotic marbles: An analytical investigation of 2nd century AD Roman sectilia from the Gorga collection

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Original article

Glass in imitation of exotic marbles: An analytical investigation of 2nd century AD Roman sectilia from the Gorga collection E. Tesser a , M. Verità a , L. Lazzarini a , R. Falcone b , L. Saguì c , F. Antonelli a,∗ a b c

LAMA–Laboratory for Analysing Materials of Ancient origin–Iuav University of Venice, Venice, Italy Stazione Sperimentale del Vetro, Murano (VE), Italy Sapienza, Università di Roma, Rome, Italy

a r t i c l e

i n f o

Article history: Received 16 January 2019 Accepted 12 July 2019 Available online xxx

a b s t r a c t The materials uncovered in excavations in several provinces of the Roman empire provide important evidence to the frequent use of glass in most precious opera sectilia. Glass pieces imitating a dozen of ancient marbles and exotic stones have been identified in the collection belonged to Evangelista Gorga, samples of which were selected for analysis in order to obtain valuable information on the Roman glassmaking technology of the 2nd century AD, especially on coloration techniques. The monochrome and polychrome glass tesserae replicating precious marbles and rare stones were: porfido verde antico, litomarga verde, turquoise, pavonazzetto, gabbro eufotide, diaspro nero e giallo, semesanto, alabastro tartarugato, rosso antico, cipollino rosso, giallo antico, and onyx. XRF quantitative chemical composition was determined allowing the types of glass used and the colouring techniques to be identified. Results have been compared with similar data obtained by studies on coloured Roman glass and mosaic tesserae. Similarities and discrepancies are discussed and hypotheses are suggested on the technology of this extraordinary Roman production. © 2019 Elsevier Masson SAS. All rights reserved.

1. Introduction and research aims The use of glass in thin slabs of different shapes and colours to create wall and floor decorative motives was a kind of artwork widely spread from the 1st century AD to the early Middle Ages [1,2]. The glass was used solely, sometimes depicting exotic stones and ancient marbles, or combined with real marbles and stones (opus sectile [3]). The “Gorga collection” includes 160,000 glass items excavated in Rome end 19th–early 20th c. and bought by the famous Italian tenor Evan Gorga (1865–1957). Among them, a huge amount of Roman opus sectile glass pieces (about 15,000 monochrome and 11,000 polychrome pieces) is preserved in the deposits of the Soprintendenza Archeologica in Rome. Taking into account that the origin of these glass pieces was attributed to the villa of emperor Lucius Verus (161–169 AD), located on the via Cassia in Rome [4], they offer a very useful opportunity for investigating the glassmaking technology of the 2nd century AD. Generally, Roman glass was a soda-lime-silica glass with little variations in its major and minor elemental composition. Potassium

∗ Corresponding author. E-mail address: [email protected] (F. Antonelli).

and magnesium oxides, each below about 1.5%, and the low content of phosphorous are usually attributed to the use of an evaporite mineral soda (Natron) as a flux [5]. Natron glass was mainly used, but a small proportion of plant ash glass was also utilized for some specific colours as opaque red and orange and transparent green [6]. The natron glass batch was made of natron and a silica-lime sand, where quartz and lime were naturally mixed in the right ratio to make glass. The natron glass was melted on a very large scale in a limited number of primary tank furnaces, especially in Egypt [7] and Levant [8], conforming to a centralised model. The raw glass in form of chunks was distributed to secondary workshops throughout the Mediterranean, central and northern Europe. In the secondary workshops the glass was remelted, coloured and/or opacified before being worked into artefacts. Variations in the composition of the raw materials allow identifying different provenances and periods of production of natron glass [9]. The main groups are Levantine I (4th to 7th c. AD), Levantine II (7th to 8th c. AD) [7,10], Egypt I (from a yet unknown start date up to the 6th c. AD) and Egypt II (8th–9th c. AD) (Wadi Natrum and Ashmunein) [11], Roman 2nd c. and HIMT (probably Egypt, from the late 4th to the 6th c. AD) [12,13]. All these groups were further divided into sub-types on grounds of their contents of minor and trace elements [14,15].

https://doi.org/10.1016/j.culher.2019.07.009 1296-2074/© 2019 Elsevier Masson SAS. All rights reserved.

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On the basis of archaeological evidence, in Roman times primary glass was transparent colourless or “naturally” coloured glass. Quite little evidence exists of the circulation of opaque and/or intentionally coloured primary glass [16]. Glass opus sectile is underrepresented in archaeometric literature and little attention is attributed to this working practice [17,18]. In this context, the considerable extent of Gorga’s collection of glass sectilia makes it a unique and precious source for the study of the glassmaking and glassworking techniques used in 2nd c. AD Roman glass. In this paper a selection of twenty-six glass sectilia fragments of this collection were analysed, representing all the glass colours and decorative patterns used for imitating ancient stones and marbles present in the collection. The aims of this research are: (i) to improve knowledge about raw materials and glassmaking technologies employed during the 2nd c. AD; (ii) to determine the type of glass used as well as the colouring and opacifying techniques adopted for the production of sectilia; (iii) to identify the coulored exotic marbles and stones imitated by Gorga glass sectilia. 2. Materials and methods 2.1. Samples Twenty-six sectilia representative of the monochrome and polychrome glass pieces imitating marbles, rare stones or with fanciful patterns from the Gorga collection were selected and studied through non-destructive and non-invasive analytical methods. The samples represent 12 lithotypes, as clarified in detail in table 1, Fig. 1. The selected glass pieces show several shapes, varying in size between 14–69 × 11–39 mm, and thickness from 2 to 12 mm. A number of hot working techniques have been observed: monochrome slabs, canes, millefiori glass [19]; this topic will be discussed in the next future. 2.2. Analytical techniques Considering the artistic relevance of the items, the glass tesserae were exclusively studied by means of non-invasive and nondestructive analytical methodologies. The glass pieces were observed with a stereomicroscope (Leica WILD M3Z) in order to describe their morphological characteristics. Quantitative chemical analysis was performed by energy dispersive X-ray micro-fluorescence spectrometry (␮EDXRF) using a Bruker M4 Tornado spectrometer equipped with a rhodium Xray tube (maximum power 30 W) with polycapillary lens optics (spot size ∼ 30 ␮m) and a Peltier cooled energy dispersive Silicon Drift Detector (SDD) with a 30-mm2 sensitive area and an energy resolution ∼ 140 eV for Mn K␣. When possible the analyses were performed on a fracture surface to exclude corroded areas. Otherwise, small areas were slightly abraded with a 1000 mesh silicon carbide paper to remove corroded layers. Analyses were performed at 40 kV–700 mA (28 W power). X-rays were collected for 200 s. In polychrome samples each phase was analysed separately in at least five different points and the average compositions was calculated. The quantitative chemical compositions were determined by correcting the raw data with a software supplied by Bruker. Under the selected experimental conditions the analytical precision, accuracy and lower detection limits of the analytical method were verified on a set of more than 30 glass samples including international glass standards (NIRST, BGIRA, Corning, etc.), glasses whose composition was tested in interlab tests in laboratories involved in glass research and industrial production, and glasses expressly melted at the Stazione Sperimentale del Vetro of Murano (Italy) and anal-

ysed with several methods (atomic absorption, WDS-XRF, ICP). An analytical precision (determined as the relative standard deviation) between 0.5 and 1% for major elements and between 1 and 10% for minor and trace elements, an accuracy within 1% for SiO2 , Na2 O and CaO and within 5% for the remaining oxides were estimated. Low detection limits in the range 0.005–0.01% for most of the oxides were calculated. For As2 O3 and P2 O5 detection limits of 0.12% were estimated. Chlorine was not measured in samples with PbO > 10% because of peaks overlap. Qualitative identification of opacifiers and pigment particles was made, when possible, by focusing the X-ray beam on particles emerging on the surface of the glass pieces. In the next future, Raman spectroscopy, XRD and SEM-EDS analyses will be used to confirm the mineralogical composition of opacifiers and pigments hypothesized on the basis of the chemical results obtained here. To better address the identification of compositional groups, hierarchical cluster analysis (HCA) was applied to all chemical data. STATISTICA 7.1 software and Ward’s method [20] were used for elaborating and diagramming the results. The analysis was performed on the major and minor base glass elements (SiO2 , Na2 O, CaO, Al2 O3 , TiO2 , MgO, K2 O, P2 O5 , SO3 , Cl, SrO, and BaO) representing geological (minero-petrographic composition of sands) and anthropogenic factors (i.e. recipes). Fe2 O3 and MnO were not considered because they are colouring agents. The “not detected” elements were treated for the statistical analysis as zero values. 3. Results and discussion 3.1. Identification of the imitated stone materials By investigating the glass sectilia of the Gorga collection a dozen of antique marbles and exotic stones imitations [21], mostly distributed in the province of the Roman empire during the 2nd century AD, were identified as follows (Figs. 1-3). 3.1.1. Monochrome glass sectilia The green opus sectile pieces Gorga7.12-5 and 7.12-3 are well comparable with the litomarga verde (Fig. 2a), an Italian marly limestone from the central Apennine with a celadon green colour, which was mainly used in late Republican scutulata pavimenta and opera sectilia [22]. The yellow glass sectilia Gorga7.12-14, Gorga7.12-15 and Gorga7.12-16 (Fig. 1) show different chromatic variants imitating several facies of giallo antico (or marmor numidicum; Fig. 2b). This limestone–quarried in Numidia (Tunisia), at Simitthus [23] (now Chemtou)–was highly appreciated since the 1st c. BC by Romans for opus sectile and the production of columns and small sculptures; its price was very high (200 denarii per cubic foot in Diocletian’s edict) [21]. Two red glass pieces Gorga7.12-7 and 7.12-11 are totally comparable with rosso antico (or marmor taenarium rubrum; Fig. 2d), a Greek impure marble coloured by hematite quarried near Cape Tainaron (Matapan) and other localities of the Mani Peninsula (Greece). Due to its colour quite similar to those of the Tyrian purple, in Roman times it was widely exploited from the late Republican age up to the Imperial period, thus becoming one of the most precious and highly sought-after stone for cornices, mouldings and opera sectilia, small capitals, sculptures and rarely columns [24]. The light blue Gorga7.12-6 may be related to turquoise (Fig. 2c), a blue-to-green mineral found in Turkey prized in antiquity as gemstone for its unique hue [25]. 3.1.2. Polychrome glass sectilia Gorga7.12-13 is a clear imitation of a calcite alabaster; in particular, its macroscopic aspect (Fig. 3a) is comparable with alabastro tartarugato. This Italian calcareous alabaster quarried at Iano di

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ARTICLE IN PRESS 3 Fig. 1. Chemical composition of 2nd century AD Gorga samples; wt% oxides are given as average value of five measures by ␮XRF and they are coupled with standard deviations; n.d.: under the detection limit; TR: translucent; OP: opaque. Searched for and not detected: NiO, ZnO, As2 O3 , ZrO2 , Cr2 O3 .

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Fig. 2. Analysed monochrome opus sectile glass pieces of the Gorga collection in imitation of (a) litomarga verde–sample Gorga7.12-5 (b) giallo antico–Gorga7.12-15 (c) turquoise–Gorga 7.12-6 (d) rosso antico–Gorga7.12-11.

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Fig. 3. Polychrome opus sectile glass pieces of Gorga collection in imitation of (a) alabastro tartarugato–Gorga7.12-13 (b) onyx–Gorga7.12-10 (c) gabbro eufotide Gorga 7.12-8 (d) cipollino rosso Gorga7.12-4. Pictures for onyx is from Wikipedia.org. Polychrome opus sectile glass pieces of Gorga collection in imitation of (e) diaspro nero e giallo–Gorga7.12-9 (f) pavonazzetto–Gorga7.12-1 (g) semesanto–Gorga7.12-12 (h) porfido verde antico–Gorga2-SP4.

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Fig. 3. (Continued)

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Montaione (Florence) was used during the Roman age mainly for the production of crustae and slabs for opera sectilia [26]. The use of opaque and transparent glasses together gives the illusion of depth. Gorga7.12-10 is a glass replica of onyx (Fig. 3b), namely a banded variety of chalcedony (micro-crystalline quartz with fibrous texture). Most likely, the high hardness and consequent difficult (and expensive) workability of this hardstone determined its imitation with glass. As for alabasters, the glassy provided depth illusion and several possible chromatic patterns. Gorga7.12-8 well imitates the gabbro eufotide, a mafic intrusive granitoid from the Eastern Egyptian Desert [27], with variable grain-size and dark green-to-pale green colour with greyish spots (Fig. 3c). It was used in the I c. AD for opus sectile, cornices, and rare small columns [21]. The main reason for its imitation could be its rarity and availability in small pieces. Gorga7.12-4 evokes the brecciated facies of cipollino rosso (marmor iassense or marmor carium; Fig. 3d), a true red hematitic marble from the surroundings of Iasos (Caria, now Kiykislacik–Milas Province, Turkey [29]), widely exploited by Romans, especially from the Severian age onwards, for the production of columns, slabs, bases and sculptures [21]. The glass imitation most likely solved the problem of the rarity of this beautiful brecciated facies, offering also the possibility to obtain patterns with some chromatic variants. Gorga7.12-9 imitates diaspro nero e giallo (Fig. 3e), a very rare stone with a yellowish ground and dark green-to-grayish/black spots, of unknown origin [21,28], possibly from Egypt. The renowned pavonazzetto is well replicated by the veined glass fragment Gorga7.12-1 (Fig. 3f). Extracted by Romans since the Augustan age in the district of Synnada, central Phrygia near the town of Docimium (modern Iscehisar, Turkey), and then commercialized with the names of marmor docimium, phrygium, synnadicum. The quarries (still active) provided both a white finegrained marble and a yellowish-to-purplish more or less brecciated coloured facies. The white one was widely used for statuary and sarcophagi, while the rarest purplish meta-breccia (the classical pavonazzetto, in Diocletian’s time one the most valuable and expensive marble with the same price of giallo antico) for architectural elements and crustae [30]. Gorga7.12-12 realistically imitates the semesanto variety (Fig. 3g) of the Greek breccia di settebasi (marmor scyreticum) quarried by Romans in the island of Skyros (Sporades, Greece) since the middle of the 1st c. BC. Due to its limited geological occurrence and low availability on the market, this variety of calcareous breccia was very rare and precious and used as crustae and opera sectilia in private domus and public buildings, occasionally for table tops and small columns [21,31]. The glass reproduction of semesanto probably allowed the use of larger slabs having probably a lower cost. A huge amount (almost 10,000 pieces) of polychrome beautiful glass slabs with dark green translucent glass background and multiple embedded canes in light opaque green glass excellently imitate the famous porfido verde antico, a Greek porphyritic lava, quarried not far from Lacedaemon (Sparta), in Laconia, which is also known as lapis lacedaemonius or serpentino (Fig. 1 and Fig. 3h). After a first limited use during the Minoan and Mycenaean periods it was rediscovered by the Romans in the late Republican age and soon became the most sought-after and expensive stone (together with the Egyptian red porphyry, at 250 denarii per cubic foot in Diocletian’s edict) of antiquity. Due to its geological irregular outcropping and consequent impossibility to obtain large and medium-sized blocks, it was mainly used for crustae and opera sectilia (small columns and capitals are very rare) [21,24]. Furthermore, the exceptionally high hardness of porfido verde antico makes it very difficult to cut and work. The glassy copies probably solved this problem, provided larger availability of ornamental material and the production of creative patterns.

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3.2. Glass analysis The composition of the forty-six (transparent and opaque) coloured glasses composing the twenty-six analysed samples is reported in weight percent of the oxides (wt%) in Fig. 1. The results are presented and discussed in terms of base glass, colouring and opacification materials and manufacturing techniques. The reduced chemical compositions of the glass analysed can be found in Appendix A, in the online version, as supplementary data. 3.2.1. Base glass The composition of the base glass of the samples was obtained by subtracting the content of the elements related to colouring and opacifying from the bulk composition of Fig. 1 and then normalizing to 100 wt%. All of the analysed samples are soda-lime silica glass with a SiO2 content generally between 64 and 71%, Na2 O mainly in the range 14–21% and CaO from 5 to 12%. The analyses of samples Gorga4-SP8 (glass 1 and 2) and GorgaSP1–(glass 2, green-yellow opaque) showing a very low amount of Na2 O (average value 8.5 ± 1.8%) and a quite large content of SiO2 (average value 72.9 ± 5.8%) clearly refer to weathered glass and are excluded from discussion. MgO and K2 O contents, both lower than 1.5% for the majority of the samples, and a content of phosphorous under the limit of detection (P2 O5 0.12%) suggest mainly the use of a natron type glass. The greater amounts of these oxides in samples Gorga5-SP5-2, SP11, 7.12-8 glass1 and 7.12-4 glass2, suggest the use of plant ash glass for their production [32]. Other authors suggest for these differences the addition of small amounts of potash plant ash to natron glass [33]. Gorga7.12-14-glass3 shows high magnesium amount (MgO 4.80%) and low phosphorus (P2 O5 under the detection limit value) and potassium contents (K2 O 0.46%). In the literature, this composition was found for a group of natron-high magnesium white opaque glasses discovered in some mosaics in Rome [34]. The comparison of alumina and calcium oxides in the analysed samples with primary natron glass groups from the literature shows a positive match with published results for Roman glass type of 2nd c. AD [35–38] (Fig. 4). The HCA results split the collected data into two main groups, 1 and 2 in Fig. 5, which correspond to natron and plant ash glasses respectively (Fig. 6a). This HCA separation and the consequent potential subdivisions of the samples have been investigated and confirmed using elemental diagrams. For a distance of about 15, group 1 may be further divided into subgroups 1a and 1b (Fig. 5). The average composition and the corresponding standard deviation show that the silica content is higher in group 1a (69.9 ± 1.1%) than in group1b (67.8 ± 0.7%), which is richer in calcium (CaO mean value is 6.7 ± 1.1% in 1a and 7.9 ± 0.8% in 1b). These results indicate that similar silica-lime sands were used for the samples belonging to groups 1a and 1b; but with different silica to lime ratios (SiO2 : CaO is 10.4 in 1a and 8.6 in 1b). For a distance of about 10, the groups may be split again (Fig. 5). In particular, in subgroup 1a, 1a-1 spreads out from 1a-2 for a higher amount of sodium (18.3 ± 1.3% and 16.2 ± 0.5%, respectively), due probably to a different recipe used. In subgroup 1b, 1b-1 shows a higher amount of titania (TiO2 0.21 ± 0.07%), iron and manganese (the last two not used as variables in HCA) than 1b-2 (TiO2 mean value 0.12 ± 0.03%) (Fig. 6b) suggesting the use of a less contaminated sand in the first case. It is interesting to observe that mainly green and purple glasses belong to these groups (more details in this regard are given in paragraphs 3.2.2.1 and 3.2.2.3). For a distance of about 10, also group 2 is divided in 2a and 2b subgroups (Fig. 5), probably due to a different sand-flux ratio used for the production of the primary glass (SiO2 : NaO is 4.40 in 2a and 3.71 in 2b).

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Fig. 4. CaO (wt%) vs Al2 O3 (wt%) diagram demonstrating the correspondence of the Gorga samples to Roman 2nd c. AD glass production. Comparative data from the literature [35–38].

Fig. 5. Cluster analysis (Ward’s method) showing the principal groupings (corroded Gorga4-SP8 and Gorga-SP1–glass 2 were removed).

The medium value of strontium is 830 ppm, which could refer to the origin of calcium carbonate from shells. However, some samples belonging both to natron (i.e. Gorga 7.12-9) and plant ash (i.e. Gorga5-sp5-2) glasses show values greater than 1000 ppm. These seem quite high values of strontium for this sort of glass. Therefore,

an in-deep test was performed to check the strontium content in Corning standards. The SrO reference values for A, B and C standards (SrO 0.10%, 0.019%, and 0.29%, respectively) were quite well matched by XRF analyses (SrO 0.12%, 0.020% and 0.29%, respectively), confirming the reliability of strontium measurement in the

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Fig. 6. Diagrams (a) potassium oxide (wt%) vs magnesium oxide (wt%); (b) titania (wt%) versus alumina (wt%) vs alumina (wt%) versus silica (wt%).

investigated samples. From the literature [39], concentration of strontium higher than 1000 ppm in plant ash glass could be related to the addition of potash plant ash to natron glass. However, in our case, this seems unlikely because the values of barium, that is an element present in significant amounts in potash plant ash, are totally comparable with those of natron glass. The average compositions and standard deviations of each sample with respect to the compositional groups here defined can be found in Appendix B, as supplementary data of the online version. 3.2.2. Coloured translucent glass 3.2.2.1. Green glass. Green samples can be divided into two macrogroups: one imitating litomarga verde (2 samples) and one imitating porfido verde antico (10 green and 9 yellow-green samples). In general, the green colour was obtained by the addition of copper and iron to the base glass, the latter largely present as a minor element in the silica-lime sand. The two samples imitating litomarga show macroscopically two different hues. Gorga7.12-5 is a deep green tessera containing a higher concentration of copper (CuO 2.20% instead of 1.20%), whereas Gorga7.12-3, green-yellow in colour, shows a higher content of antimony (Sb2 O3 0.57%, n.d. in Gorga7.12-5) and lead (PbO 3.0% instead of 2.0%). Comparing the chemical composition of these samples, it can be supposed that the same metallurgical scrap in which copper and lead were linked, was used as source of copper. However, part of the lead content of sample Gorga7.12-3 may be also related to the addition of particles of lead antimonate used as a yellow pigment for obtaining a lighter colour hue. Small amounts of tin (SnO2 0.14–0.18%) have been detected in both samples. Microscopically, sample Gorga7.12-5 reveals the presence of yellow and black microparticles and fragments of terracotta (Si, Al, Fe rich particles) with sharp edges, embedded in the glass (Fig. 7). In order to achieve particular shades of green, these particles were probably introduced into the melted at the last moment, mixing and rapidly working the pieces to avoid the dissolution of the terracotta particles. Samples imitating porfido verde antico are made in general of a translucent green phase (divided into 4 green-blue and 6 green hues, as described in Fig. 1) and an opaque green-yellow one. The translucent greens were coloured by addition of copper and iron. Copper content is quite similar in all the samples (average content CuO 1.91 ± 0.21%), except in sample Gorga2 SP7-2 (CuO 1.05%). Differences in iron are probably due to the presence of this element in different amounts in the primary glass (Fe2 O3 average content 1.23 ± 0.31%). In particular, samples Gorga3 SP5, Gorga2 SP7-2 and Gorga SP6 show a higher content of titania than the other samples, which suggests iron was already

Fig. 7. Optical micrograph of sample Gorga7.12-5 showing a red terracotta, and black (a) and yellow (b) micro-fragments. Long size of the image is 3.8 mm.

present in the primary glass. In general, all the green translucent phases contain a variable amount of lead and tin (average values: PbO 1.42 ± 1.16%, SnO2 0.33 ± 0.25%). Probably, they were dissolved by the opaque yellow-green glass during the shaping of the pieces. Finally, the presence of cobalt (CoO 0.018%) in sample Gorga3-SP5 was detected, which confers a blue hue to the translucent glass. The green-yellow opaque phases of these sectilia were probably obtained by adding lead, tin and antimony yellow pigments as colouring and opacifying agents to the corresponding translucent green glass. Possibly, different yellow pigments were used as indicated by chemical analyses (average values: SnO2 0.46 ± 0.36%, Sb2 O3 0.36 ± 0.20%, PbO 3.52 ± 1.17%). In particular, Gorga3-SP5, 2-SP7-2 and SP6 show similar low concentrations of antimony and high concentrations of tin as compared to the other sectilia. GorgaSP10c is an outside of the group. In the yellow-green opaque phase, in fact no copper was detected indicating that to obtain a different hue lead-tin-antimony yellow pigments were added to colourless transparent glass instead of the related green transparent glass. 3.2.2.2. Amber glass. The amber glasses of samples Gorga7.1213 (alabastro tartarugato), 7.12-14 (giallo antico) and Gorga7.12-8 (gabbro eufotide) were produced without any intentional addition of colourants. The amber colour is probably due to the iron-sulphur chromophore, a complex that forms in the glass under strongly reducing conditions [40]. In samples Gorga7.12-13 and 7.12-14 the

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low iron content (Fe2 O3 0.35%, 0.71%) attests that it was already present as a contaminant of the raw materials. 3.2.2.3. Purple glass. The purple transparent glasses Gorga7.12-1, 7.12-9, 7.12-10 and 7.12-12 have high manganese contents (MnO 1.97–3.82%) responsible of the glass colour. In sample Gorga7.129 and Gorga 7.12-10 a particular high quantity of manganese was used (MnO 3.57% and 3.82%, respectively) in order to confer a black aspect to the glass and to better imitate the black portions of the diaspro nero e giallo and of onyx. The barium content of these glasses is higher than in the other samples, and probably linked to the presence of manganese. Small amounts of antimony (Sb2 O3 0.11–0.84%) are probably due to the contamination by the neighbouring white or yellow opaque glasses during the forming process. The purple glasses show high concentrations of iron, probably already present as a contaminant in the sands used for the glass melting. Gorga7.12-1 and 7.12-12 contain some cobalt (CoO ca. 0.01%) probably intentionally added to influence their purple colour. In fact, also today, the addition of cobalt to purple glass is a common practice in order to obtain a particular hue named ametista [41]. 3.2.2.4. Turquoise glass. The turquoise glass Gorga7.12-6 was coloured with copper (CuO 1.8%). The significant content of antimony (Sb2 O3 0.63%) is probably related to the presence in the sample of very thin white opaque canes observed under the optical microscope and too thin to be analysed separately. 3.2.3. Opaque glass: pigments and opacifiers 3.2.3.1. White glass. Taking into account the antimony concentrations (Sb2 O3 0.85–3.5%) in the white opaque glass samples (Gorga7.12-4, 7.12-1, 7.12-8, 7.12-10, 7.12-12, 7.12-13, 7.12-14), the opacity and colour of these glasses could be due, in general, to the presence of calcium antimonate crystals dispersed in the colourless glass matrix [42]. Sample Gorga7.12-12 is particularly rich in lead oxide, PbO 4.7%. In the literature, lead has been attributed to a mineral gangue (contamination) in an antimony source in use until the 1st c. AD [43]. However, in our case the high amount of lead is probably related to the presence of yellow coloured particles in the examined area. In fact, microscopically, the sample is the result of a complicated working technique obtained by juxtaposing, fusing, pressing and stretching together whitepurple, yellow and green canes. 3.2.3.2. Yellow glass. The colouring technique of yellow glass sectilia of the Gorga collection has been already discussed in a previous contribution [44]. The chemical analyses of the yellow phases (Sb2 O3 1.0–1.3%, PbO 5.6–8%, SnO2 about 0.1%) used for the production of sectilia imitating giallo antico (Gorga7.12-14 and 7.12-15), and diaspro nero e giallo (Gorga 7.12-9) show that they were probably coloured and opacified by semi-finished yellow pigments (lead antimonate or lead-stannate-antimonate) added to the glass melted at the last moment to avoid their dissolution. 3.2.3.3. Red glass. For the production of the three examined red samples, colour was most likely obtained by two different agents: cuprite crystals (Cu2 O–sealing wax) and metallic copper nanoparticles (CuO–red brown). Samples Gorga7.12-7 and Gorga 7.12-11, both imitating rosso antico, correspond to sealing wax type, in which dendritic red crystals are embedded in a colourless transparent glass [45: 455, fig. 16]. These samples are rich in copper (CuO 7.90–8.30%) and lead oxide (PbO 32.0–36.0%). Sample Gorga7.124, which was produced in imitation of cipollino rosso, owes its colour to the metallic copper nanoparticles (red-brown type). The amounts of copper (CuO 2.12%) and lead (PbO 0.79%) in this sample are much lower than in the previous case, but the addition of iron is

proven (Fe2 O3 1.15%). It is interesting to observe that for the manifacture of sample 7.12-4 natron glass (white) and plant ash glass (red) were fused together.

3.2.3.4. Orange glass. According to the literature [45], the orange colour of sample Gorga 7.12-16 may be due to the development of cuprite crystals (CuO 9.7%) with micrometric dimensions, as observed under the optical microscope, in a glass phase characterized by a high concentration of lead (PbO 28.0%). Red veins are observable macroscopically in the sample, which correspond microscopically to larger cuprite crystals [45: 457, fig. 20b].

4. Conclusions The glass sectilia of the Gorga collection are a “mine” of samples useful for investigating the Roman glassmaking technology of the 2nd century AD. A selection of glass pieces produced with the evident goal of replicating coloured ancient ornamental semiprecious stones and marbles permitted to identify twelve among the most important natural lithotypes/hardstones used in antiquity. The main features of the imitated stones have been discussed in order to discover the possible reasons that may have led to this imitation practice. In general, the use of glass allowed easier workability with lower costs and greater availability of materials. It made also possible to produce several colour variants, which in some cases did not occur naturally, a surface with a greater shininess and the illusion of depth. The study contributes to a substantial dataset of major and minor element analyses of Roman glass of 2nd c. AD. It shows the production of mainly natron glass but also the use of plant ash glass, sometimes worked together in order to obtain a unique polychrome glass piece. The sources of sand seem to be more than one; some differences have been discovered in the calcium-silica ratio and in titanium-iron contents. Despite these small differences, all the pieces show the use of similar silica-lime sands. In some cases, the sodium-silica ratio allowed to discriminate glass groups. The study of these well-contextualised glass sectilia allowed a series of significant techniques of coloured glass production to be elucidated. The glass pieces show a general homogeneity and comparability with published data on coloured glass mosaic tesserae coeval in age. Some differences in the colouring techniques used for the production of the sectilia suggest the existence of various secondary furnaces capable of making imitation stones. In fact, two different colour technologies were identified for the production of porfido verde antico: the crystals added as yellow colouring and opacifying agents to the transparent green glass in samples Gorga3 SP5, Gorga2 SP7-2 and Gorga SP6 are different from the other samples produced in imitation of serpentine, as well as the iron and titanium contents in the related green glass. Furthermore, the different macroscopic aspect of these sectilia suggest different hot-forming technique adopted for their production. Also this evidence seems supporting the hypothesis of the existence of various secondary furnaces. Details of the hot-forming techniques used will be soon enhanced and discussed in a forthcoming paper.

Acknowledgments The authors gratefully acknowledge Pentagram Stiftung Foundation for generously funding this research. Dr. Mario Bandiera contributed to the study of red samples, thanks to a specific research on red colouring techniques, which he is developing at VICARTE (Universidade Nova de Lisboa) in Lisbon (Portugal).

Please cite this article in press as: E. Tesser, et al., Glass in imitation of exotic marbles: An analytical investigation of 2nd century AD Roman sectilia from the Gorga collection, Journal of Cultural Heritage (2019), https://doi.org/10.1016/j.culher.2019.07.009

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.culher.2019.07.009.

[20] [21] [22]

References [23] [1] S.D. Fontaine, Un lot de verre ornemental pompéien, Bulletin dell’Association Franc¸aise pour l’Archéologie du Verre, 2006, pp. 45–46. [2] G. Volpe, G. De Felice, M. Turchiano, Musiva e sectilia in una lussuosa residenza rurale dell’Apulia tardoantica: la villa di Faragola (Ascoli Satriano, Foggia), Musiva e Sectilia 1 (2004) 127–158. [3] M. Verità, L. Lazzarini, E. Tesser, F. Antonelli, Villa del Casale (Piazza Armerina, Sicily): stone and glass tesserae in the baths floor mosaics, Archaeol. Anthropol. Sci. 11 (1) (2019) 373–385. [4] B. Bacchelli, M. Barbera, R. Pasqualucci, L. Saguì, “Nuove scoperte sulla provenienza dei pannelli in opus sectile vitreo della collezione Gorga.”, in: Atti del II colloquio dell’Associazione Italiana per lo Studio e la Conservazione del Mosaico, Roma 5–7 dicembre 1994. (Istituto Internazionale di Studi Liguri, Bordighera), 1995. [5] A. Shortland, L. Schachner, I. Freestone, M. Tite, Natron as a flux in the early vitreous materials industry: sources, beginnings and reasons for decline, J. Archeol. Sci. 33 (2006) 521–530. [6] J. Henderson, Tradition and experiment in 1st millennium AD glass production: the emerge of Early Islamic glass technology in late antiquity, Acc. Chem. Res. 35 (2002) 594–602. [7] M.-D. Nenna, Primary glass workshops in Graeco-Roman Egypt: preliminary report on the excavations of the site of Beni Salama, Wadi Natrun (2003, 20059), in: J. Bayley, I.C. Freestone, C.M. Jackson (Eds.), Glass of the Roman World, Oxbow Books, Oxford, 2015, pp. 1–22. [8] O. Tal, R.E. Jackson-Tal, I.C. Freestone, New evidence of the production of raw glass at Late Byzantine Apollonia-Arsuf, Istrael, J. Glass Stud. (2004) 51–66. [9] I.C. Freestone, Y. Gorin-Rosen, M.J. Hughes, Primary glass from Israel and the production of glass in Late Antiquity and the Early Islamic period, in: M.-D. Nenna (Ed.), La Route du Verre: ateliers primires et secondaires de verries du second millénaire av. J-C au Moyen Âge, Travaux de la Maison de l’Orient, 33, 2000, pp. 65–82. [10] Y. Gorin-Rosen, Hadera, Bet Eli’ezer, Excavations and Surveys in Israel 13 (1995) 42–43. [11] M.-D. Nenna, Egyptian glass abroad. HIMT glass and its markets, in: J. Bayley, C. Jackson, D. Keller, J. Price (Eds.), Neighbours and successors of Rome. Traditions of glass production in use in Europe and the Middle East in the later first millennium AD, Oxbow Books, Oxford, 2014, pp. 178–193. [12] D. Foy, M. Picon, M. Vichy, V. Thirion-Merle, Caractérisation des verres de la fin de l’Antiquité en Méditerranée occidentale : l’émergence de nouveaux courants commerciaux, in: D. Foy, M.-D. Nenna (Eds.), Échanges et commerce du verre dans le monde antique : actes du colloque de l’Association franc¸aise pour l’archéologie du verre, Aix-en-Provence et Marseille, 7–9 juin 2001, édition Monique Mergoil, Montagnac, 2003, pp. 41–85. [13] I.C. Freestone, S. Wolf, M. Thirlwall, The production of HIMT Glass: Elemental and Isotopic Evidence. Annales du 16e Congrès de l’Association Internationale pour l’Histories du Verre, London, 2005, pp. 153–157 (2003). [14] M. Phelps, I.C. Freeston, Y. Gorin-Rosen, B. Gratuze, Natron glass production and supply in the late antique and early medieval Near East: The effect of the Byzantine-Islamic transition, J. Archaeol. Sci. 75 (2016) 57–71. [15] N. Schibille, B. Gratuze, E. Ollivier, É. Blondeauc, Chronology of early Islamic glass compositions from Egypt, J. Archeol. Sci. 104 (2019) 10–18. [16] S. Paynter, T. Kearns, H. Cool, S. Chenery, Roman coloured glass in the Western provinces: the glass cakes and tesserae from West Clacton in England, J. Archaeol. Sci. (2015), http://dx.doi.org/10.1016/j.jas.2015.07.006. [17] A. Santagostino Barbone, E. Gliozzo, F. D’Acapito, I. Memmi Turbanti, M. Turchiano, G. Volpe, The Sectilia Panels of Faragola (Ascoli Satriano, Southern Italy): a multi-analytical study of the red, orange and yellow glass slabs, Archaeometry 50 (3) (2008) 451–473. [18] E. Gliozzo, A. Santagostino Barbone, F. D’Acapito, M. Turchiano, I. Memmi Turbanti, G. Volpe, The Sectilia panels of faragola (Ascoli Satriano, Southern Italy): a multi-analytical study of the green, marbled (Green and Yellow), blue and blackish glass slabs, Archaeometry 52 (3) (2010) 389–415. [19] L. Saguì, P. Santopadre, M. Verità, Technology, colours, forms, and shapes in the 2nd century glass opus sectile materials from the Villa of Lucius Verus in Rome,

[24]

[25] [26]

[27]

[28]

[29]

[30] [31]

[32] [33]

[34]

[35]

[36]

[37]

[38]

[39]

[40] [41] [42] [43]

[44]

[45]

11

in: AIHV Annales du 18e Congrès, Thessaloniki, Editors Despina Ignatiadou, Anastassios Antonaras, 2009. J.H. Ward, Hierarchical grouping to optimize an objective function, J. Am. Stat. Assoc. 58 (1963) 236–244. R. Gnoli, Marmora Romana, Edizioni dell’Elefante, Roma, 1988. F. Antonelli, L. Lazzarini, White and coloured marbles of the roman town of Urbs Salvia (Urbisaglia, Macerata, Marche, Italy), Oxford J. Archaeol. 32 (3) (2013) 293–317. F. Rakob, Simitthus I. Die Steinbriiche und die antike Stadt, Mainz am Rhein, 1993, pp. 103. L. Lazzarini, Poikiloi lithoi, versiculores maculae: i marmi colorati della Grecia antica. Storia, uso, diffusione, cave, geologia, caratterizzazione scientifica, archeometria, Fabrizio Serra Editore, 2007, pp. 71–96. R. Webster, Gems: their sources, descriptions and identification, ButterworthHeinemann, 2000, pp. 254–263. L. Lazzarini, I.M. Villa, D. Visonà, Caratterizzazione e identificazione di alabastri usati in antico, in: G.M.G.M. Crisci, C. Gattuso (Eds.), “Archeometria del Costruito. L’edificato storico: strutture e rischio sismico”, Edipuglia, Bari, 2006, pp. 273–281. J.A. Harrell, V.M. Brown, L. Lazzarini, Two newly discovered Roman quarries in the Eastern Desert of Egypt, in: M. Schvoerer (Ed.), Archéomatériaux : marbres et autres roches, Presse Universitaire de Bordeaux, 1999, pp. 285–292. L. Lazzarini, I diaspri siciliani: genesi, composizione e storia di alcune tra le pietre più usate dall’Opificio fiorentino, in: Pietre colorate molto vaghe e belle. Arte senza tempo dal Museo del’Opificio delle Pietre Dure, A cura di S. Rossi, P. Assmann, A. Patera. Mantova, 2018, pp. 85–107. A. Andreoli, F. Berti, L. Lazzarini, R. Pierobon Benoit, New contributions on Marmor Iassense, in: L. Lazzarini (Ed.), ASMOSIA VI, Interdisciplinary Studies on Ancient Stone, Padova, 2002, pp. 13–18. D. Manna, P. Pensabene, Marmi dell’Asia Minore, Roma, 1977. L. Lazzarini, Considerazioni sul prezzo dei marmi bianchi e colorati in età imperiale, in “Arqueologìa de la Construccìon, II. Los procesos constructivos en el mundo romano: Italia y provincias orientales”(S. Camporeale, H. Dessales, A. Pizzo eds.), Madrid-Mèrida, 2010, pp. 485–490. C. Lilyquist, R. Brill, M. Wypyski, Studies in Early Egyptian Glass, Metropolitan Museum of Art, New York, 1993. N. Schibille, P. Degryse, M. Corremans, C.G. Specht, Chemical characterisation of glass mosaic tesserae from sixth-century Sagalassos (south-west Turkey): chronology and production techniques, J. Archaeol. Sci. 39 (2012) 1480–1492. M. Verità, Le tessere vitree dei mosaici medievali a Roma. Tecnologia e degrado. Mosaici medievali a Roma attraverso il restauro dell’ICR 1991–2004, cura di A. Andaloro e M., D’Angelo C. Gangemi editore, 2018, pp. 446. N. Schibille, A. Sterrett-Krause, I.C. Freestone, Glass groups, glass supply and recycling in late Roman Carthage, Archaeol. Anthropol. Sci. 9 (6) (2016) 1223–1241, http://dx.doi.org/10.1007/s12520-016-0316-1. S. Maltoni, A. Silvestri, A mosaic of colors: investigating production technologies of Roman Glass Tesserae from northeastern Italy, Minerals 8 (2018) 255, http://dx.doi.org/10.3390/min8060255. C. Boschetti, J. Henderson, J. Evans, C. Leonelli, Mosaic tesserae from Italy and the production of Mediterranean coloured glass (4rd c. BCE-4th c. CE). Part I: chemical composition and technology, J. Archeol. Sci. 7 (2016) 303–311. I.C. Freestone, Glass production in the first millennium CE: a compositional perspective, in: F. Klimscha, H.J. Karlsen, S. Hansen, J. Renn (Eds.), Glas und Glasproduktion in Ur- und Frühgeschichtlicher Zeit, Edition TOPOI, Berlin, Germany, 2016. I.C. Freestone, M.J. Hughes, C.P. Stapleton, The composition and production of Anglo-Saxon glass in the British museum. Catalog of Anglo-Saxon glass in the British museum, Vera I. Evison, Edited by Sonja Marzinzik, 2008, pp. 29–46. M.W.H. Schreurs, R.H. Brill, Iron and sulfur related colors in ancient glasses, Archaeometry 26 (1984) 199–209. M.B. Volf, Chemical approach to glass. Glass Science and Technology, 7, Elsevier, 1984. A.J. Shortland, The use and origin of antimonate colorants in early Egyptian glass, Archaeometry 44 (2002) 517–530. H. Foster, C.M. Jackson, A whiter shade of pale’? Chemical and experimental investigation of opaque white Roman glass gaming counters, Glass Technol. 46 (2005) 327–333. M. Verità, M. Maggetti, S. Saguì, P. Santopadre, Colors of Roman Glass: an investigation of the yellow sectilia in the Gorga Collection, J. Glass Stud. 55 (2013) 21–34. M. Verità, Le tessere vitree dei mosaici medievali a Roma, Tecnologia e degrado. In: Mosaici medievali a Roma attraverso il restauro dell’ICR 1991–2004. A cura di M. Andaloro e C. D’Angelo. Gangemi editore, 2018.

Please cite this article in press as: E. Tesser, et al., Glass in imitation of exotic marbles: An analytical investigation of 2nd century AD Roman sectilia from the Gorga collection, Journal of Cultural Heritage (2019), https://doi.org/10.1016/j.culher.2019.07.009