Glass and Ceramics 167
Figure 5Medieval stained glass window from Erfurt (Germany) with a dense crust of
weathering products, a cross section, SEM picture (left) and element mapping
(EDX) (right); (Note: elements are specified in the picture (Ca, K, S, Si))
the cross section of a fragment from Erfurt Cathedral (Germany): in the SEM
picture it can be seen that the degradation layer has a total thickness up to
200 m and that this layer is not homogeneous, with many micro-cracks pene-
trating deep into the glass. The leached layer and the corrosion crust can be
distinguished only with an additional element mapping (with energy disper-
sive X-ray analysis (EDX)). The layer adjacent to the bulk glass has a high
Si-content and is thus defined as the “gel layer” (or “depleted layer”). The crust
on top is rich in Ca and S, therefore, identified as gypsum (or weathering crust).
In the sample above, both layers are mixed within a broad transition area.
Other types of glasses tend to develop different symptoms of decay: the
formation of pits is related to droplets of leached compounds accumulated on
the surface or to inhomogeneities in the glass. Other case studies have connected
pitting with the attack of microorganisms. Local pits can be quite large (up to
3 mm in diameter). Smaller pits can accumulate and cover the whole surface
of a fragment, leading to the loss of the original surface. Another remarkable
phenomenon of degradation is limited to manganese-containing glasses: due
to oxidation processes underneath the surface, glasses that were almost trans-
parent turn brown.
Potassium-rich medieval glasses are very sensitive to corrosive attack.
Only a few examples of glass segments with an even and almost unaffected
surface are documented. Heavily corroded glasses have lost up to one-third of
their original thickness. In contrast to the potassium-rich medieval windows,
the soda lime silicate glasses of the 19th century are more stable and show
only minor signs of degradation. Microorganisms may have settled on the
surface and may have produced a bio-layer that reduces the transparency of
the glass and has to be removed. Furthermore, for 19th century windows, the
degradation of paint is considered to be the most serious damage phenomenon
that poses serious conservation problems (Figure 6).