Crystallites in Color Glass Beads of the 19th Century and Their Influence on Fatal Deterioration of Glass
Glass corrosion is a crucial problem in keeping and conservation of beadworks in museums. All kinds of glass beads undergo deterioration but blue-green lead-potassium glass beads of the 19th century are subjected to the destruction to the greatest extent. Blue-green lead-potassium glass beads of the 19th century obtained from exhibits kept in Russian museums were studied with the purpose to determine the causes of the observed phenomenon. For the comparison, yellow lead beads of the 19th century were also explored. Both kinds of beads contain Sb but yellow ones are stable. Using scanning electron microscopy, energy dispersive X-ray microspectrometry, electron backscatter diffraction, transmission electron microscopy and X-ray powder analysis, we have registered the presence of crystallites of orthorhombic KSbOSiO$_4$ and cubic Pb$_2$Sb$_{1.5}$Fe$_{0.5}$O$_{6.5}$ in glass matrix of blue-green and yellow beads, respectively. Both compounds form at rather high temperatures obviously during glass melting and/or melt cooling. We suppose that the crystallites generate internal tensile strain in glass during its cooling which causes formation of multiple microcracks in inner domains of blue-green beads. We suggest that the deterioration degree depends on quantity of the precipitates, their sizes and their temperature coefficients of linear expansion. In blue-green beads, the crystallites are distributed in their sizes from $\sim\,$200 nm to several tens of $μ$m and tend to gather in large colonies. The sizes of crystallites in yellow beads are several hundreds of nm and their clusters contain few crystallites. This explains the difference in corrosion of these kinds of beads containing crystals of Sb compounds.