Raman Microspectroscopy Of Blue-Green Historical Beads: Comparative Study Of Undamaged And Strongly Degraded Samples

Confocal Raman microspectroscopy is used to substantially supplement existing experimental data on historical blue-green beads with respect to the presence and distribution of potassium antimony silicate (KSS) crystals. Problem of degradation of blue-green beads on historical artifacts accounts for topicality of the work. The reasons for possible short-term (several years) degradation of beads and the presence of neighboring damaged and undamaged beads on a single item remain unknown in spite of multiple experimental works on various glasses. Presence of KSS microcrystals may serve as a reason for degradation of beads. Raman measurements at several depths are used to determine the distribution and estimate sizes of KSS microcrystals. Such distributions are different for undamaged and strongly degraded beads. For degraded beads, Raman data show the depth dependence of the concentration of (non)bridged oxygens in the near-surface layer with a thickness of 3-4m (i.e., inhomogeneity of glass matrix). The results prove the growth of KSS microcrystals in historical blue-green beads, which may cause internal stress and, hence, degradation. The results can be used for optimization of storage conditions and restoration procedures for historical items.