The palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy): archaeometric characterisation of tesserae with copper- or tin-based opacifiers

This paper reports the results of archaeometric characterisation of the tesserae, intentionally coloured with or without copper- or tin-based opacifiers, in the palaeo-Christian glass mosaic of St. Prosdocimus in Padova. In particular, 54 tesserae belonging to colour types Orange, Red, Brown, Green, Blue and Yellow are examined here. The multi-methodological approach (SEM-EDS, EMPA, XRPD, imaging spectroscopy in some cases coupled with XAS) gave valuable insights into the complex technologies behind palaeo-Christian glass mosaic production, with identification of various glassy matrixes typical of both Roman and Late Roman periods, and of opacifiers, both crystallised in situ (e.g., metallic copper and cuprite) and ex situ (e.g., cassiterite and lead stannate) and colourants (mainly iron, manganese and copper), all variously mixed in order to obtain the desired shades. In addition, the combination of all textural, chemical, diffractometric and spectroscopic data allows us to hypothesise that all the tesserae, used to decorate the Paduan chapel, were produced in the 6th century AD. This is because each chromatic group examined here reveals at least one technical feature typical of the 6th century and, in this context, the type of glassy matrix and/or opacifier used turns out to be particularly discriminatory. In particular, tesserae with copper-based opacifiers (cuprite and metallic copper), although the latter had been used from the Bronze Age onwards, are all characterised by the matrixes typical of the Late Roman period; tesserae with tin-based opacifiers are characterised by matrixes typical of both Roman and Late Roman periods and by opacifiers, which was not systematically used before the 4th century. However, the close compositional, textural and technological similarities of the tesserae from each chromatic group, particularly evident into those with tin-based opacifiers, is consistent with a small number of specialised workshops and skilled workers. Peculiar relationships among the oxidation states of colouring elements, their contents in the matrix, the type of opacifiers used, and the final colour of the tesserae were all identified. The correlations between copper and other associated elements (e.g., tin, zinc, antimony, iron), together with micro-textural observations, allow inferences regarding possible sources. Technological connections between Padova and Ravenna, the capital of Byzantine mosaic production in Italy, were documented, although in the case of orange tesserae good chemical correspondence was also identified with the Near East. Lastly, soda ash identified in Opaque Red and Orange tesserae are indicative of Medieval restoration.