Analytical characterization of glass tesserae from mosaics of early Christian basilicas in Albania

Abstract

The present paper constitutes the first archaeometric study of the glass mosaic tesserae recovered from the archaeological excavation of the mosaics at the Early Christian basilicas in Bylis, Lin and Elbasan, belonging to fifth to sixth century AD. The main objective of the study is the characterization of the materials, glass matrix, colourants and opacifiers used for their production. A multi-analytical approach, which includes optical microscopy, scanning electron microscopy equipped with energy-dispersive spectrometer, micro-X-ray fluorescence and Raman spectroscopy, is used during the investigation of 72 tesserae of different colours collected from the three sites. All the tesserae are opaque and have heterogeneous surfaces with several bubbles and crystalline inclusions. Most of the glass tesserae have the characteristic soda-lime-silica composition, and low concentrations of potash and magnesium indicate natron as soda source, which is typical for the Roman glass. Compounds containing Mn, Fe, Cu, Co, Pb and Sn are used as colourants in the tesserae of different colours. Mineral inclusions are mostly composed of undissolved raw materials, i.e., silicon-, calcium- and tin-rich particles. It is interesting to notice that SnO2 has been used as opacifier in all the tesserae. Thin layers of Au foil placed between two transparent glasses are identified in the gold-coloured tesserae.