The Rotunda in Thessaloniki, Greece, preserves in its interior a magnificent wall mosaic assemblage of unique inspiration and beauty. Thirty-six (36) glass tesserae, blue, green, yellow, brown, black, gold and silver in color, were examined for the first time via UV-Vis reflectance spectroscopy, scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD) analysis, in order to investigate the base glass composition, and their technological and morphological features. Despite the heterogeneity observed in the glass composition, the results indicated similarities with other Early Christian and Byzantine wall mosaics in the use of colorants, opacifiers and decolorizers. Cobalt, copper, iron and manganese along with lead and tin compounds are responsible for the blue, green, yellow, brown and black colors. Tin-based opacifiers and bone ash contribute to the glass opacity. The use of different glass recipes and opacifiers in the Rotunda’s assemblage reflects the transition from the Roman glass tradition to the Byzantine glass production of the fourth and the fifth century in the eastern Mediterranean.
Kyranoudi et al. (2025) studied this question.