Title: Crystallites in Color Glass Beads of the 19th Century and Their Influence on Fatal Deterioration of Glass,
Abstract: 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 $\mu$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.