Study of color and structural changes in silver painted medieval glasses

Silver was introduced into medieval glass by an ancient painting process using different clay minerals (ochre, illitic, montmorillonitic, and kaolinitic clays). The colorimetric properties, studied by UV–Vis spectroscopy, were dependent on the clay mineral as a result of different concentration of Ag ions diffused into the glass surface. TEM results showed the well known formation of silver nanoclusters which give the yellow coloration of the glass. The obtained results showed that clay properties such as specific surface area, pore volume and iron concentration (Fe2O3), are important factors that affect the yellow coloration. It is also observed that Fe2O3 acts as an oxidant agent for silver atoms providing the Ag2O formation. This oxide cannot diffuse into the glass structure and avoid the ion-exchanged process. After Ag ion diffusion some structural changes occur in the glass as it has been shown by Raman spectroscopy. It is observed that the diffusion process leads to depolymerization of the glass network as it is determined by analyzing the Qn components of Raman spectra. Two Raman bands at 148 and 244 cm−1 assigned to Ag–O bonds can be associated to the presence of Ag2O on the glass painted surface.