172 Chapter 7
adhering crusts may require a treatment with glass-fibre brushes, scalpels or
air-abrasive techniques, bearing a higher risk of damage for sensitive glasses.
Scratches on the glass surface are prominent areas enhancing degradation,
which can be detected by crystal growth, even occurring years after the damage
was initiated (Figure 11).
“Wet” cleaning is preferred for several cleaning problems: sound glass can be
washed with water and, even for more sensitive glasses, water is a commonly
used solvent, providing it is used with care, e.g. when removing hygroscopic
droplets from “weeping” glasses. Water may also be necessary to desalinate
finds from marine sites. Complexing agents like ethylenediamine tetra-acetic
acid (EDTA) or its sodium salts, as well as special gels containing ammonium
carbonate, are highly effective for cleaning a gypsum crust from stained glass
windows. However, the risk of further leaching of calcium ions from the glass
limits the applicability of these chemicals. Ultrasonic bath treatment is extremely
dangerous, when paint layers are present underneath the dirt. Organic solvents,
used to remove organic polymers originating from previous conservation treat-
ments, are rated harmless for glass. As an exception, acetone has to be singled
out, as it might have the potential to dry out hydrated layers on the surface
and thus induce flaking. Most recently, lasers have been investigated for the
cleaning of historic glass, but no general recommendation for this technique
can be given at this stage of research.
2.9 Conditions for Storage and Display
Water is the most influential parameter directing the environmental attack on
glass. Therefore, the optimisation of relative humidity during storage and display
Figure 11Scratches on a stained glass from Cologne Cathedral, resulting from a previous
cleaning process