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KSbOSiO 4 microcrystallites as a source of corrosion of blue-green lead-potassium glass beads of the 19th century Presently, deterioration of glass beads is a significant problem in conservation and restoration of beaded exhibits in museums. Glass corrosion affects nearly all kinds of beads but cloudy blue-green ones are more than others subjected to disastrous destruction. However, physical and chemical mechanisms of this phenomenon have not been understood thus far. This article presents results of a study of elemental and phase composition of glass of the blue-green beads of the 19th century obtained from exhibits kept in Russian museums. Using scanning electron microscopy, X-ray microanalysis and X-ray powder analysis we have detected and investigated Sb-rich microinclusions in the glass matrix of these beads and found them to be micro crystallites of KSbSiO 5 . These crystallites were not detected in other kinds of beads which are much less subjected to corrosion than the blue-green ones and deteriorate in a different way. We believe that individual precipitates of KSbSiO 5 and especially their clusters play a major role in the blue-green bead deterioration giving rise to slow internal corrosion of the bead glass.

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 thermalfluctuation 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.

Analysis of the IR spectra of samples from 230 Russian oil paintings of the 20th century is used to propose a procedure for the threshold estimation of the age of paintings based on measured parameters (intensity ratios of spectral bands). The bands of compounds that are formed upon interaction of pigment (zinc white) with oil are used for dating.

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