The coloured tesserae decorating the vaults of the Faragola balneum (Ascoli Satriano, Foggia, Southern Italy)

Twenty-six tesserae (red, orange, yellow, light amber, green, blue and white) from the balneum of the villa at Faragola (Ascoli Satriano, Foggia) have been examined by colorimetry, ICP–MS, ICP–OES and SEM–EDS. Different types of calcareous sands have been used as the source of silica (network former), also providing the stabilizing agent. A natron-type soda source served as the network modifier; however, the use of a sodium-rich plant ash and the recycling process have been hypothesized for the production of two tesserae (FT 1 red and FT 3 orange). The colouring and opacifying agents were Cu oxide (cuprite, orange), metallic copper (red), Pb antimonates (yellow), Ca antimonates (white), a mixture of copper (Cu2+) and Pb antimonates (green), a mixture of cobalt (Co2+) or copper (Cu2+) and Ca antimonates (blue). The light amber tesserae should owe their colour to iron (Fe3+) alone or associated with sulphide (S2-) and Ca antimonates. It is likely that the Faragola tesserae were locally produced in a secondary glass workshop.