Degradation and destruction of historical blue-green glass beads: A study by microspectroscopy of light transmission

Blue-green historical beads are sometimes referred to as instable ones because of their degradability. At present, the cause of the phenomenon of deterioration of the blue-green beads is unknown. We explore internal microstucture of degrading blue-green historical beads and its evolution in the process of bead deterioration. Investigating transmittance and scattering spectra of visible and near infrared light we observe formation of microscopic internal inhomogeneities with the sizes less than 150 nm in the glass bulk and growth of their density with increase in degree of bead degradation. By means of laser scanning microscopy we also observe numerous microinclusions and microcracks on the cleavage surface of a partially degraded bead. We discuss possible physical factors resulting in destruction of the blue-green beads.