Glass and metal analyses of gold leaf tesserae from 1st to 9th century mosaics. A contribution to technological and chronological knowledge

An archaeometric study was carried out on 40 gold leaf tesserae from mosaics in Italy dated 1st to the 9th century AD. Glass layers and gold leaf were both analysed by X-ray microanalysis. The main aim was the identification of the composition of the glass and of the gold leaf, in order to assess the variations in composition and nature of the tesserae in the examined period. The analytical results show that the tesserae were obtained with natron type glass; three compositional groups were identified, each characterized by a specific colour. The coexistence of colourless (antimony-manganese as decolourizer) and naturally coloured (manganese as decolourizer) glass in each mosaic suggests the simultaneous use of the two materials, probably related to aesthetic choices. The analyses of the gold leaves indicate the use of pure gold or of gold-silver alloys. The good matches between the compositions of the gold leaves and the analyses of contemporary gold coins in the late antique and byzantine samples (3rde9th c.) suggest that the leaves were made by beating circulating gold coins. This, in some cases, can help the dating of the tesserae and a more precise information on the type of glass used in this period.