Mosaic Tesserae from the Basilica of San Severo and Glass Production in Classe, Ravenna, Italy

The study of glass tesserae found in the excavations of the S.Severo basilica, Classe (Ravenna), sixth century A.D., has stimulated a comparison between these materials and those used for the decorations of the other churches of Ravenna built in the same epoch. The comparison has been carried out also with the materials utilized or produced in the glassworks operating in the same period in Classe to verify if it was used a technology shared with the production of the coeval mosaic glass. The typology of the glass tesserae from S.Severo fully corresponds to that of the other fifth-sixth century church of Ravenna, all classified as soda-lime silicatic glasses produced using natron as flux. On the contrary, it has been verified that, on the whole, there is no correspondence between the production of Classe and that of the mosaic glasses. It has been discussed, in particular, the production technology of the red tesserae, which colour is due to the presence of particle of metallic copper and/or cuprite (Cu2O). For these glasses it has been suggested the use of lead and lead-tin alloy scraps, besides additions of ash of the vegetal fuel, to attain the suitable reducing conditions during the melting process.