168 Chapter 7
2.6 Degradation of Archaeological Glasses
In general, Roman glass (sodium rich) is quite stable. The only sign of alter-
ation after hundreds of years in the soil is often the formation of a thin altered
surface layer. For less durable glasses this layer can reach several hundreds of
micrometers, exhibiting “iridescence” (rainbow-like colouration), if several
thin layers of altered glass are superimposed (see Figure 7). Surfaces of archaeo-
logicalglasses may appear dull and pitted, with brown spots or dark stains.
Enamel-like surface layers render the glass into a completely opaque material
(Figure 8). Although the surface appears smooth, with no crystal deposits, the
degradation layer is thick (leaving only a minor core of bulk glass uncorroded)
Figure 6Glass panel with damaged paint, the paint lines are partly lost (19th century,
Netherlands)
Figure 7Fragment of an archaeological vessel glass damaged by “iridescence”: a network
of micro-cracks is covering the entire surface, which has already partly flaked off