The palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy): archaeometric characterisation of tesserae with copper- or tin-based opacifiers
Introduction
The polychrome tesserae of the glass mosaic which decorated the votive chapel of St. Prosdocimus in the Basilica of St. Justine (Padova) represent one of the only two existing palaeo-Christian glass mosaics known in the Veneto region (Italy). Their characterisation was thus a unique occasion for extending knowledge on the production technologies of these materials dated to a precise chronological range, the 6th century AD, perceived to be a period of technological transition. While the “Gold” and coloured tesserae with antimony- and phosphorus-based opacifiers have been studied in two previous papers (Silvestri et al., 2011b, Silvestri et al., 2012b), the present manuscript, which closes the set of papers on the St. Prosdocimus tesserae, focuses on samples with copper- or tin-based opacifiers and/or deliberately coloured.
Glass, opacified by means of copper-based phases, has long held a particular fascination for specialists in early glass technology, and the literature on the subject is substantial (e.g., Weyl, 1959, Brill, 1976, Ahmed and Ashour, 1981, Cable and Smedley, 1987, Freestone, 1987, Brill and Cahill, 1988, Brun et al., 1991, Nakai et al., 1999, Arletti et al., 2006, Figueiredo et al., 2006, Padovani et al., 2006, Santagostino Barbone et al., 2008, Barber et al., 2009). Many works focus specifically on glass tesserae (e.g., Freestone et al., 1990, Shugar, 2000, Fiori et al., 2004, van der Werf et al., 2009, Arletti et al., 2008, Arletti et al., 2010, Arletti et al., 2011a, Arletti et al., 2011b, Fiori, 2011, Gliozzo et al., 2012, Schibille et al., 2012). All researchers agree that copper is responsible for the orange/red/brown colour and opacity of the glass, and that rigorous reducing conditions must have been established and maintained throughout the glass melting and working, but there is no clear consensus on the nature of the copper-rich phase responsible. Some authors believe it to be crystals of cuprous oxide, Cu2O, mineralogically known as cuprite (e.g., Brill, 1976, Cable and Smedley, 1987, Brill and Cahill, 1988, Freestone et al., 1990); others that they are particles of metallic copper (e.g., Weyl, 1959, Nakai et al., 1999, Padovani et al., 2006). In general, the mechanism of colour generation is complex, and it has been demonstrated that lead and copper contents in glass play an important role in influencing the precipitation of the copper-rich phase (Barber et al., 2009). High copper–high lead (Cu2O > 5 wt%; PbO > 15 wt%) glass is characterised by well-developed cuprite dendrites, whereas low copper glass, with or without lead, shows precipitation of nanoparticles of metallic copper, although the co-existence of both phases has also been observed (e.g., Freestone, 1987, Brill and Cahill, 1988, Brun et al., 1991, Barber et al., 2009).
In the case of cuprite-based glass, colour is considered to be related to the number and size of the crystals, which in turn depend on the total amount of copper and lead, on crystal texture, and on possible co-precipitation of other metals or metalloids, such as minute globules of metallic copper, lead or copper alloyed with other metals (Freestone, 1987, Brill and Cahill, 1988, Barber et al., 2009). According to the above authors, higher copper and lead contents (generally CuO > 5 wt% and PbO > 15 wt%) lead to larger crystals and slightly redder glass; a higher number of particles, favoured by lower copper and lead (generally CuO < 5 wt% and 10 < PbO < 1 wt%) should improve opacity and impart an intense orange or brownish-red colour. The importance of crystal size, crystal texture and duration of heat treatment is also stressed: the larger the crystals, the better defined the dendritic structure of the cuprite; the longer the heat treatment, the darker the colour of the glass (Freestone, 1987, Brill and Cahill, 1988).
In glass production, tin-based opacifiers (lead stannate, yellow, and cassiterite, white) started to be used systematically during the 4th century AD, to replace antimony-based ones (Tite et al., 2008), although tin oxide was first introduced as early as the 2nd century BC (Henderson, 2000). Their use spread from the eastern Mediterranean to Northern Europe (Turner and Rooksby, 1959) and appears in Italian products from the 5th century AD onwards, although antimony-based opacifiers continued to be used, or reused, and only ceased around the 13th century (Uboldi and Verità, 2003, Fiori et al., 2004, Arletti et al., 2008). Tin-based opacifiers were also used from the 5th to the 9th centuries to produce yellow and white beads in Anglo-Saxon England (Bayley and Wilthew, 1986), Ireland (Henderson, 1988) and Germany (Heck and Hoffman, 2000), Islamic white and yellow enamels in the 12th century (Mason and Tite, 1997, Mason, 2004) and Venetian glass in the 13th century (Freestone and Bimson, 1995). Lastly, in Italy, the same kind of opacifiers were used to make glass tesserae, dated to the 13th–14th centuries in Florence (Arletti et al., 2011a) and to the 16th century in Venice (Verità, 2000) and Rome (Arletti et al., 2011b). In the Mediterranean region, explanations for the change from antimony- to tin-based opacifiers during the 4th century are either a breakdown in the supply of antimony, due to changes in the production of silver in which antimony ores are frequently associated, or a consequence of the close links between the Roman Empire and India (Mass et al., 1998, Tite et al., 2008). The latter explanation may also clarify why tin-based opacifiers tend to be concentrated in the eastern Mediterranean basin. It should be stressed that the introduction of tin-based opacifiers in the northernmost regions and beyond the limits of the Roman Empire probably occurred independently of their introduction to the Mediterranean region, again reflecting the choice to use the readily available tin, thanks to its use in metal production (e.g., bronze, pewter, soft solders) (Tite et al., 2008).
Reassessment of how these opacifiers were produced is reported by Tite et al. (2008). Tin-based opacifiers were mostly used to produce yellow glass as a cubic PbSnO3 phase (yellow) and not in the orthorhombic form Pb2SnO4 (white), which does not impart the desired colour. The latter phase (produced by melting lead and tin oxides) is converted into the cubic phase on heating to 850 °C in the presence of silica (Rooksby, 1964). Interestingly, in an early 15th-century recipe (Kuhn, 1968) – comparable to the recipes found for Venetian manufacturing of the 18th and 19th centuries (Moretti and Hreglich, 2005) – a similar two-step procedure was proposed to produce yellow lead-tin opacifiers. The first step consisted of the production of the so-called ‘lead-tin calx’, by mixing lead and tin metals. This product was then mixed with lead oxide and silica to produce what was called “lead-tin yellow anime”. The ‘‘anime’’ was then powdered and added to a molten colourless translucent glass at a relatively low temperature, in order to prevent the yellow pigment from dissolving within the glass (Moretti and Hreglich, 2005, Tite et al., 2008). The PbO/SnO2 ratios for the lead-tin calx of Venetian recipes vary from 0.4 to 2.3, matching those of the ancient white and blue glasses and enamels reported in the literature (average PbO/SnO2 ratios from 0.3 to 2.2; data from Tite et al., 2008). However, as reported by Tite et al. (2008), since lead in yellow anime was produced by adding both lead oxide and silica to calx, its PbO/SnO2 ratio depends on the relative amounts of lead oxide added. Therefore, if sufficient lead oxide is added, the ratios for anime produced using the calx described in the Venetian recipes may vary and match those of ancient yellow glasses and enamels (average PbO/SnO2 ratios from 2.8 to 12.6; data from Tite et al., 2008).
Experiments were also carried out by Tite et al. (2008) to investigate the phase transformations occurring when mixtures of lead oxide, tin oxide and silica are fired up to 1100 °C. The above authors demonstrate that, in a mixture of SiO2, PbO and SnO2, the persistence of PbSnO3 is not greatly affected by an increase in the PbO/SnO2 ratio of lead-tin anime, although it is favoured by low PbO/SnO2 ratios and low levels of silica. However, a significant reduction in the temperature (from about 900 to 750 °C) at which the transformation from cubic lead stannate, responsible for the yellow colour, into cassiterite, white in colour, occurs when the silica content of anime increases, and this prevents the formation of the desired shade. Therefore, in order to produce a yellow glass (in which the SiO2 content is high, generally 60 wt% or more), it is essential to mix the anime and molten glass at low temperatures and as rapidly as possible, to prevent lead stannate from dissolving and cassiterite from forming (Tite et al., 2008).
Taking into account the above studies, carried out on glass with copper- or tin-based opacifiers, and the experimental data from the glass tesserae from St. Prosdocimus examined here, the present paper characterised the glass composition. Particular attention was paid to identifying opacifiers and colourants and correlating them with the chromatic and optic appearance of the samples. The raw materials and their source of supply were identified, together with reconstruction of production techniques, within clearly defined geographical and chronological contexts.
Section snippets
Materials and methods
The aim of the present paper was to characterise the opaque tesserae with copper- or tin-based opacifiers and deliberately coloured, selected among 110 opaque and coloured samples, previously subdivided into seven colour types and 20 chromatic groups (see Table 1, Silvestri et al., 2011b). In particular, colour types Orange (chromatic group: Opaque Orange – AV), Red (chromatic group: Opaque Red – RO), Brown (chromatic groups: Opaque Brown (M), and Light Brown/Amber (NC)), Blue (chromatic group:
Glassy matrix
The composition of the glassy matrix, obtained by EMPA, is shown in Table 1. It should be noted that two Opaque Orange tesserae, samples AV1 and AV5, have some layers composed not only of orange but also red glass. The chemical compositions of the two layers are reported as “separate”, for a total of 56 samples: in order to distinguish the two parts of the same sample, the letter “a” was added to the sample label for orange glass and “r” for the red one in Table 1.
The glass types were
Conclusions
The characterisation of Paduan tesserae with copper- and tin-based opacifiers by a multi-methodological approach gave valuable insights into the complex production technology of palaeo-Christian glass mosaics. Various glassy matrixes, comparable with compositional reference groups and typical of both Roman and Late Roman times, were identified; in situ and ex situ crytallisation was documented for the copper- and tin-based opacifiers, respectively, together with skilful use of colourants
Acknowledgements
The authors would like to thank the “Soprintendenza per il patrimonio storico-artistico ed etnoantropologico per le province di Belluno, Padova, Rovigo e Treviso”, and in particular Dr. A. Spiazzi, for authorising the present study, and the Abbot of the Basilica of St. Justine, Dom. I. Negrato, for providing glass samples. They are also grateful to R. Carampin (CNR-IGG, Padova, Italy), F. Zorzi (Department of Geosciences, University of Padova, Italy), G.M. Cortelazzo, F. Ratti and A. Paviotti
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2020, Journal of Cultural HeritageCitation Excerpt :Interestingly, phosphorus-based inclusions were also found in some white and turquoise tesserae from the Neonian Baptistery [6]. The addition of powdered bone ash acting as an opacifying agent has been, to date, detected in several assemblages of mosaic glass tesserae: late antique church at Kilise Tepe, Turkey [29]; the Baptistery of Tyana, Turkey [73]; Polis Chrysochous, Ayioi Pente, the Acropolis Basilica, Kalavasos-Kopetra and the Kourion, all sites located in Cyprus, Greece [74]; the Petra Church, Jordan [75,76]; the Chapel of St. Prosdocimus, Padova, Italy [50]; the Baths of Qusayr ‘Amra, Jordan [77]; the qasr of Khirbat al-Mafjar, Palestine [78], the Lower City Church at Amorium, Turkey [79]. According to the literature, the use of bone ash as opacifier seems to be not attested before the 5th century CE; moreover, the majority of the aforementioned study cases concern mosaic glass tesserae coming from archaeological sites mainly located in the eastern Mediterranean basin.