Mosaic glass from St. Peter's, Rome: manufacturing techniques and raw materials employed in late 16th century Italian opaque glass

A recent restoration of late 16th-century mosaics in one of the vaults beneath the dome of St.Peter’s Basilica in Rome allowed sampling and analysis of a group of glass tesserae. The aim of this work is the characterization of opaque coloured glasses possibly produced in Rome. The characteristics of the glass from St Peter’s were compared with those of Venetian and Tuscan production, in order to assess possible common origins. Chemical analysis of 30 samples was carried out by electron microprobe, while the nature and morphology of opacifiers were determined by X-ray diffraction and scanning electron microscopy. Almost all the opaque samples were characterized by the presence of SnO2 crystals. In addition, depending on the colour of the glass, other crystalline phases were identified: lead-tin oxide (PbSnO3) in yellow glass, cuprite (Cu2O) in orange glass and two calcium-tin silicates with different stoichiometry (CaSnSiO5 and Ca3SnSi2O9) in the green-yellow variety. A frame of reference for identifying raw materials and glass-making techniques is provided by textual sources, here examined in comparison with the compositional characteristics of the tesserae from St Peter’s.