is the most important concern for preventive conservation of sensitive glass
objects.
For freshly-prepared glasses or glasses with a rather stable composition, as
well as objects not affected by previous degradation, the rate of degradation
below 35% relative humidity is very low (in the absence of acid pollutants).
Therefore, these objects are protected best in a dry atmosphere. In contrast,
glasses with a hydrated layer on the surface, built up during previous exposure
to atmospheric weathering or to humid burial environments, require storage
at higher values of relative humidity. Recommendations in the literature vary
between 35–40% RH (for Egyptian faience), 42% RH (for excavated glasses),
37–42% RH (glass collection of the Veste Coburg in Germany) and 50% RH
(for crizzled glasses at the Corning Museum, USA). Dry conditions provoke
dehydration of these surfaces, followed by the formation of micro-cracks and
peeling effects. In any case, rapid changes of relative humidity have to be
strictly avoided to allow for a slow adaptation of the glass to gain its equilib-
rium with the new environment.
At moderate levels of relative humidity, acid pollutants accelerate the degra-
dationof glass, notably in display cases with a high concentration of acetic
acid (released from showcase materials or from other objects) where heavy
damage has been reported on sensitive glasses even after exposure periods of
a few months only.
The attack of humidity and outdoor acid pollutants is an even more import-
ant issue for stained glass windows that are exposed in the cathedrals, in their
original architectural setting. For these masterpieces, protective glazings are
installed as a preventive conservation measure: the original window is moved
a few centimetres towards the interior of the church and installed in a new frame,
whereas a new transparent and stable glass is placed in the original setting as
a weather shield. For these installations the ventilation of the interspace has
to be optimised to allow sufficient air circulation and to reduce condensation
effects on the original glass window.
Special care has to be taken for waterlogged glasses: fragments exca-
vated from a wet soil environment are normally heavily degraded, exhibiting
thick laminated layers (see Figures 7–9). These fragments may have to be
temporarily stored in water and then adapted slowly to museum conditions
by a special drying process: the water is replaced by water–ethanol mix-
tures and exchanged gradually with organic solvents that allow consolidation
with an organic polymer to stabilise the fragments, before regular storage is
possible.
In general, it must be stressed that glass is considered as a relatively stable
material, although various groups of objects do have a special need for pre-
ventive conservation. Although a large part of a particular collection might
be easy to keep, inappropriate environmental conditions may cause severe
Glass and Ceramics 173