A XANES study of the structural role of lead in glazes from decorated tiles, XVI to XVIII century manufacture

Aged lead-rich, tin-opacified glazes from polychrome tiles manufactured in the 16th–18th century were studied to ascertain the structural role of lead. Glaze fragments with white, blue, yellow, brown and green colouring were analysed using non-destructive X-ray techniques, both laboratorial – X-ray diffraction to identify crystalline components – and synchrotron-based. Elemental analyses by synchrotron radiation X-ray fluorescence were performed at the former LURE photon microprobe (line D15A at DCI, in Orsay). The instrumental set-up of beamline BM29 at the ESRF, in Grenoble, was applied to collect X-ray absorption spectra at the Pb L3-edge. Natural minerals and synthetics with known crystal structure were used as model oxy-compounds to configure different formal valences and coordinations of lead ions by oxygen anions, and to interpret the effects upon details of X-ray absorption near-edge spectroscopy (XANES) spectra. Experimental evidence supports the general conclusion that lead is hosted by the glassy matrix, irrespective of the glaze colour. Furthermore, it was concluded that lead ions assume coordinations higher than usual for silica glasses, acting as network modifiers in the silica-lime-alkali glasses of ancient tile glazes.