Glass depolymerization in the process of long-term corrosion: A study of deteriorating semiopaque turquoise glass beads using micro-FTIR spectroscopy

摘要

Nowadays, problem of historic beadworks conservation in museum collections isactual because of fatal corrosion of the 19th century glass beads. Vibrationalspectroscopy is a powerful method for investigation of glass, namely, ofcorrelation of the structure-chemical composition. Therefore, FTIR spectroscopywas used for examination of degradation processes in cloudy turquoise glassbeads, which in contrast to other color ones deteriorate especially strongly.Micro-X-ray fluorescence (XRF) spectrometry of samples has shown thatlead-potassium glass PbO-K$_2$O-SiO$_2$ with of small amount of Cu and Sb wasused for turquoise beads manufacture. Fourier Transform Infrared (FTIR)spectroscopy study of the beads at three different stages of glass corrosion(intact, severely cracked with color alteration, partially fragmented) wascarried out in 600 to 4000 cm$^{-1}$ range in ATR mode. In all spectra we haveobserved shifts of two major absorption bands to low-frequency range($sim,$1000 and $sim,$775 cm$^{-1}$) compared to ones typical for amorphousSiO$_2$ ($sim,$1100 and 800 cm$^{-1}$, respectively). Such effect isconnected with Pb$^{2+}$ and K$^+$ appending to the glass network. Presence ofweak band at $sim,$1630 cm$^{-1}$ in all spectra is attributed absorption ofH$_2$O. After heating of the beads, the band disappeared completely in lessdeteriorated samples and became significantly weaker in more destroyed ones.Based on that we conclude that there is adsorbed molecular water in the beads.Products of glass corrosion (for example, alkali in the form of white crystalsor droplets of liquid alkali) were not observed on the glass surface.