Analysis of glass tesserae from the mosaics of the 'Villa del Casale' near Piazza Armerina (Enna, Italy). Chemical composition, state of preservation and production technology

This paper represents a contribution to the safeguard project of The 'Villa del Casale' near Piazza Armerina (3rd―early 4th century AD), among the most important examples of Sicilian mosaic art, glass materials used for the mosaics execution have been investigated by several techniques. Optical Microscopy and Scanning Electron Microscopy (SEM) were used to give information about the porosity (microstructure) of glass pastes and the presence of crystalline areas or alteration layers at the surface inside the glass paste itself. SEM-EDX, X-ray Photoelectron Spectroscopy (XPS), X-ray Fluorescence (XRF) and UV-VIS-NIR reflectance analysis have provided information to obtain the chemical composition of the tesserae and on the chromophore groups. X-ray diffraction (XRD) was employed to identify opacifiers. The analytical results indicate that all tesserae are silica-soda-lime and silica-soda-lime-lead glasses. Chromophore ions consist of Cu(II) (green and pale blue tesserae), Cu(0) (red sample), Fe(II) (pale blue and the green glasses), Co(II) (deep blue samples) and Mn as a decolourant, while opacifiers found out are PbxSb2―z(O,OH,H2O)6 (bindheimite) in the yellow and green tesserae, CaSb2O6 (calcium antimony oxide) in the pale blue samples, and Ca2Sb2O7 (calcium antimony oxide) in the blue tesserae. The glasses that show the greatest signs of degradation processes are those of a pale blue hue.