Title: Study of deteriorating semiopaque turquoise lead-potassium glass beads at different stages of corrosion using micro-FTIR spectroscopy,
Abstract: 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$^{-1}$.