Searching for insights on production technologies in the Late-Antique/Byzantine period: glass tesserae from Tyana (Cappadocia, Turkey)

A selection of 46 loose glass tesserae, probably derived from a now-disrupted wall mosaic in Tyana (southeastern Cappadocia, Turkey) and dated to the 5th-6th century CE, was subjected to textural, chemical, and mineralogical analyses aimed at investigating the base glass compositions and colouring/opacifying techniques employed. Furthermore, they were compared with other coeval mosaics to discover insights on glass production technologies in the Late-Antique/Byzantine period.The Tyana tesserae were divided into six colour macrogroups (i.e., blue, green, yellow, red, black and gold), which were further grouped into various colours. The majority of the tesserae are opaque, with the exception of 16 that are transparent or translucent. Textural and chemical analyses were conducted by means of optical microscopy (OM), scanning electron microscopy (SEM-EDS), and electron microprobe (EPMA). X-ray powder diffraction (XRPD) was also carried out to determine the opacifiers involved.The results show that the majority of the tesserae are produced with base glass compositions and colouring/opacifying techniques, typical of the 4th-6th century CE, in accordance with the supposed dating of the Tyana assemblage. The majority of the coloured/opacified tesserae have base glass chemically comparable to the Levantine-Jalame group. Meanwhile the gold tesserae are produced with other Late-Antique compositions, such as HIMT/Foy 1, Foy 2.1 and/or Foy 3.2. The Tyana tesserae are mostly opacified and coloured by means of Ca-phosphate associated with copper and cobalt (turquoise and blue), Pb stannate (yellow), Pb-stannate coupled with copper (green), and metallic copper (red). A few tesserae show base glass similar to Roman compositions and opacifiers composed of Ca- or Pb-antimonates. These tesserae are, therefore, unequivocally interpreted as reemployed Roman tesserae. The use of local obsidian is also verified for the black tesserae.In conclusion, the present study suggests a sort of specialisation of production ateliers, as documented in Tyana and other Late-Antique/Byzantine mosaics, because the coloured tesserae are predominantly made of Levantine base glass, while the gold tesserae are produced with base glass of probable Egyptian origin. In addition, it testifies that in the 5th-6th century CE, Tyana was mostly supplied with fresh glass, with limited recycling/reuse of Roman glass. Despite its inland location, this city was very well connected to the main trade routes.