Glass and Ceramics 169
and exhibits a laminated structure, due to precipitation processes. The fluctu-
ations in chemical composition within this surface layer can be visualised in
a cross-section (Figure 9). The degradation mechanism of buried glasses is
complicated by the fact that transition metal ions from the ground may migrate
into the altered glass.
It is difficult to estimate whether the archaeological record stored in our
museums is representative of what was produced and used originally. The finds
have gone through various selection processes conducted by archaeologists,
restorers and curators. Highly sensitive glasses, especially in waterlogged
environments, may have degraded completely and have never been identified
as glass during the excavation.
Figure 8Archaeological glass, medieval, with enamel-like surface layer, an overview
Figure 9Archaeological glass, medieval, with enamel-like surface layer; optical
microscopy of the cross-section, showing parallel laminated layers