KSbOSiO₄ 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 belive 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.