166 Chapter 7
2.4 Degradation of Objects Indoors
In a solution, the reaction products are constantly diffusing away from the
glass, in contrast to natural weathering in the museum environment, where the
alkaline compounds accumulate on the surface. Water droplets appear due to
the hygroscopic nature of the degradation products, which is described as
“weeping glasses”. If the leached compounds react with air pollutants, sev-
eral secondary reactions take place to form, for example, carbonates that may
precipitate as crystals on the surface. In showcases without ventilation, spe-
cific pollutants such as acetic acid may accumulate leading to the formation
of acetates, which are again hygroscopic and appear as droplets. Apart from
the formation of crystals or droplets, the fracturing of the surface is a symp-
tom of decay, which is visible even without microscope and often described
as “crizzling”. Thicker layers tend to flake away, leading to a loss of the orig-
inal substance.
All examples mentioned above demonstrate that even glasses that have
never or rarely been in contact with liquid water can suffer from deteriora-
tion. Above a certain limit in relative humidity, the glass surface attracts
water molecules that form a film on the surface and are ready to initiate ion-
exchange reactions.
Apart from chemical degradation, there are examples of glasses in
museums with signs of damage connected to their former use. Many of those
examples are damaged by scratches that provide prominent spots for further
corrosion.
When looking at a specific museum collection, a large number of objects
are in good condition, with a variable number of “sick” glasses among them.
According to a survey of the National Museum of Scotland, for example,
around 400 out of 2000 objects of the collection showed signs of deteriora-
tion. These glasses require special treatments and are therefore the focus of
conservation research.
2.5 Weathering of Stained Glass Windows
Many stained glass windows are still set in their original surrounding and are
thus exposed to outdoor weathering (if not recently fitted with protective
glazing), which leads to a variety of degradation phenomena. The impact of
the environment on medieval glasses leads first of all to the formation of a
hydrated layer, exhibiting a crack pattern if a certain thickness has been reached.
In addition, the formation of crystalline encrustations is detected frequently:
leached K and Ca ions react with air pollutants to form carbonates, nitrates
and mostly sulfates. Soluble compounds are washed away by rainwater. Thus,
the crust analysed nowadays on medieval glasses consists mainly of gypsum
(CaSO 4 2H 2 O), which is the less soluble weathering product. Figure 5 shows