Degradation and destruction of historical blue-green glass beads: a study using microspectroscopy of light transmission

Abstract

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.

KSbOSiO4 microcrystallites as a source of corrosion of blue-green lead-potassium glass beads of the 19th century

“…2) likely may be explained by purely optical effect conditioned by light reflection on sides of large cracks; light is also scattered by small cracks. 6 Both these phenomena reduce light transmittance as a result of which beads look blackened. An experiment on immersion of a blackened bead in vaseline oil which we carried out supported this statement.…”

“…2, Table I). 6,22 They were examined by means of the scanning electron microscopy (SEM), X-ray microanalysis and X-ray diffractometry. Beads of different colors (yellow, white and red) were also investigated using SEM but only yellow ones were analysed using the X-ray diffraction.…”

“…That is why the study of causes of degradation of the blue-green glass beads is strongly required for the conservation of authentic historic beadworks in museum collections. 6 The majority of data on this issue existing presently in the scientific literature have been obtained for earlier archeological or ethnographic beads (see, e.g., Refs. 7-15).…”

“…6 Bead degradation may occur due to internal stress, resulting in local inelastic strain and rupture of glass. It is assumed that some sufficiently large precipitates, probably crystalline, may be sources of the internal stress.…”

“…6 and structure of the secular blue-green beads of the 19th century. We have demonstrated that the previously observed micro inclusions are crystalline.…”