Study of deteriorating semiopaque turquoise lead-potassium glass beads at different stages of corrosion using micro-FTIR spectroscopy

Nowadays, a problem of historical beadworks conservation in museum collections is actual more than ever because of fatal corrosion of the 19th century glass beads. Vibrational spectroscopy is a powerful method for investigation of glass, namely, of correlation of the structure-chemical composition. Therefore, Fourier-transform infrared spectroscopy was used for examination of degradation processes in cloudy turquoise glass beads, which in contrast to other color ones deteriorate especially strongly. Micro-X-ray fluorescence spectrometry of samples has shown that lead-potassium glass PbO-K$_2$O-SiO$_2$ with small amount of Cu and Sb was used for manufacture of cloudy turquoise beads. Fourier-transform infrared spectroscopy study of the beads at different stages of glass corrosion was carried out in the range from 200 to 4000 cm$^{-1}$ in the attenuated total reflection mode. In all the spectra, we have observed shifts of two major absorption bands to low-frequency range (~1000 and ~775 cm$^{-1}$) compared to ones typical for amorphous SiO2 (~1100 and 800 cm$^{-1}$, respectively). Such an effect is connected with Pb$^{2+}$ and K$^+$ appending to the glass network. The presence of a weak band at ~1630 cm$^{-1}$ in all the spectra is attributed to the adsorption of H$_2$O. After annealing of the beads, the band disappeared completely in less deteriorated samples and became significantly weaker in more destroyed ones. Based on that we conclude that there is adsorbed molecular water on the beads. However, products of corrosion (e.g., alkali in the form of white crystals or droplets of liquid alkali) were not observed on the glass surface. We have also observed glass depolymerisation in the strongly degraded beads, which is exhibited in domination of the band peaked at ~1000 cm$^{-1}$.