Characterisation of mosaic glazes from the S.Giusto Basilica (Puglia, Italy)

Several glass mosaic tesserae (small square pieces) found during archaeological excavations in south-east Italy were analysed. The tesserae probably belonged to wall mosaic decorations (now completely lost) of an early Christian basilica (Basilica of S. Giusto, second half of the 5th century). They have chemical compositions not very dissimilar from those of other Byzantine mosaic glasses found in different areas, making it possible to group all the tesserae from S. Giusto, except for one, in a single category: soda-lime silicate glasses. The chemical stability of the materials was evaluated by applying criteria used for ceramic glazes. It was shown that all the examined glasses are easily subject to deterioration. This gives an explanation of the observed surface deterioration observed in the glazes of S. Giusto. 4 refs.