Glass depolymerization in the process of long-term corrosion: A study of deteriorating semiopaque turquoise glass beads 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. Study of the beads at different stages of glass corrosion using FTIR was carried out in the attenuated total reflection mode in the range from 200 to 4000 cm ^−1 . We have observed glass depolymerization in the degraded beads, which is exhibited in domination of the band peaked at ∼1000 cm ^−1 . We conclude that the simplification of the glass structure during its long-term degradation at room temperature may be explained within the thermal-fluctuation theory of materials fracture. We consider glass depolymerization, caused by the internal stress and decreasing the glass strength, as an essential corrosion mechanism of strongly stressed glass. We have also revealed shifts of two major absorption bands to low-frequency range (∼1000 and ∼775 cm ^−1 ) compared to ones typical for amorphous SiO _2 (∼1100 and 800 cm ^−1 , respectively) 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 latter band disappeared completely in less deteriorated samples and significantly weakened in more degraded ones. Based on that we conclude that there is molecular water adsorbed 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 surface.

Keywords

• glass corrosion
• glass depolimerization
• historical glass
• ATR FTIR spectroscopy
• stress-induced corrosion