The palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy): archaeometric characterisation of "gold" tesserae

A systematic and extensive archaeometric study was carried out on the loose polychrome tesserae of the palaeo-Christian glass mosaic which decorated the votive chapel of St. Prosdocimus in Padova until its replacement by the current frescoes of Renaissance age and which is one of the only two known in the Veneto region (Italy). In particular, the present paper focuses on the ‘gold’ tesserae, i.e., tesserae with a metal foil, usually composed of gold, and characterised by transparent glass not deliberately coloured or decolourised. The main research aims were the identification of the ‘base composition’ of the glass used for mosaics and the reconstruction of production techniques. In addition, comparisons with major compositional groups identified in the literature allowed us to contextualise Paduan ‘gold’ tesserae in the wider context of glass production in the 6th century AD. The compositional characterisation of Paduan ‘gold’ tesserae, carried out by means of EMPA and LA-ICP-MS analyses, showed that most of the glass samples were obtained with natron as flux, although the coexistence of various production technologies and the extent of recycling, which confirm the 6th century AD as a period of technological transition, are documented. Identification of some soda ash tesserae also allowed us to attest Medieval restoration operations. The good match between compositional groups identified in the literature for glass vessels and compositional groups identified in the ‘gold’ tesserae suggests technological links between these two production types. The chemical similarity between ‘gold’ tesserae from Ravenna and some from Padova also links production technologies in these two towns during the 6th century; coupled with the results of the historical-artistic study, these factors provide further evidence to the hypothesis that the Paduan production was mainly influenced by the nearby city of Ravenna, the capital of Byzantine mosaics in Italy.