Wall mosaics of Thessaloniki: A comparative study of the brown tesserae

The present study focuses on the archaeometric investigation of 10 brown tesserae belonging to Early Christian/Byzantine wall mosaics of three monuments of Thessaloniki, inscribed on the World Heritage List of UNESCO: Rotunda, St. Sophia, and St. Demetrios. The tesserae were analyzed via optical microscopy (OM), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and UV-Vis reflectance spectroscopy to define their composition and technological characteristics. Nine of the tesserae are made of silica glass while one tessera is made of a siliceous rock. The majority of the glass tesserae, despite the chronological distance of the monuments they were collected from, present similar technological features, base glass composition, colorants, and opacifiers. Metallic copper is the main element responsible for the brown color in relation to iron which serves as a reducing agent. Opacification is attributed to tin compounds.