Chemical characterisation of glass mosaic tesserae from sixth-century Sagalassos (south-west Turkey): chronology and production techniques

Little is known about the origin, supply pattern and production technology of Byzantine glass mosaic tesserae. In this study, we have analysed forty-eight glass tesserae from Sagalassos (Asia Minor) of different colours and from two archaeological contexts that were stratigraphically dated to the sixth century CE. The main aim was to identify the raw materials, colourants and opacifiers as well as secondary working practices that are reflected in the composition (EPMA, LA-ICP-MS analyses) and the microstructure (XRD, SEM-BSE) of the tesserae. The set of samples retrieved from the Roman Baths complex at Sagalassos is compositionally very homogeneous, representing possibly a single commission, and can be tentatively dated to the late Roman period. In contrast, the assemblage associated with the construction of a Byzantine church around the turn of the sixth century CE is more diverse, suggesting that these tesserae were produced from more than one silica source. This highlights a diversification in the supply and manufacture of glass tesserae during the Byzantine period. Although all analysed samples are natron-type soda-lime-silica glasses, a strong correlation between MgO, K2O, P2O5 and CaO was observed, pointing to the contamination of the glass batch through fuel ash either intentionally in case of the red tesserae or accidentally as regards the green, peacock and white samples. The degree of contamination reflects differences in the duration of the secondary production stage according to the colour, which is also evidenced by the shapes and sizes of the opacifying crystals present in the opaque tesserae. X-ray diffraction measurements identified calcium antimonate (in the blue, peacock and white tesserae) and lead antimonate (green and yellow) as the main opacifying agents. The morphological analysis suggests that the calcium antimonate particles crystallised in situ, whereas a lead antimonate pigment was synthesised prior to its addition to the glass melt. Taken together, the compositional and microstructural characteristics clearly demonstrate different production processes that are related to the colours of the tesserae, thus indicating colour-specific secondary workshop practices