Glass and Ceramics 181
salts in the pores (Figure 16). More severe changes are to be expected with
seawater, being an even more aggressive chemical environment.
For architectural elements, the access to rainwater or rising damp is decisive
for degradation. Here, salt crystallisation is an even more damaging factor
than for archaeological ceramics. Water-soluble salts can enter the body, and
then recrystallise during drier periods. The expansion in volume of the freshly-
formed crystals causes stress to the porous host structure, depending on the tem-
perature and the relative humidity of the environment. Every salt has a critical
relative humidity at which crystallisation occurs, although this can be affected
by the presence of other salts. Chlorides, nitrates, sulfates and phosphates are the
most important soluble salts to consider. The worst scenario is an environment
with constantly changing temperature or relative humidity, where salts can pre-
cipitate in cycles. Salts are introduced into ceramics at the exposure site, but also
during conservation treatments,e.g.by using acids for removing stains.
Damage caused by temperature changes can also be connected to frost and
thaw cycles, due to the formation of ice in the pores, thus it is more important
for architectural ceramics, exposed to severe weathering conditions.
The degradation of glazes follows similar mechanisms as described for glass
corrosion, based on leaching and dissolution reactions, dependent on the pH of
the attacking solution. Degradation also leads to similar phenomena, such as
micro-cracking or iridescence effects. In the case of glazed ceramics, the adhe-
sion of the glaze is an important stabilisation factor, although salt precipitation
can lead to exfoliation of even large surface areas (Figure 17). Firing of glazes at
lower temperatures allows a broader choice of colours, but it makes the surface
more susceptible to an environmental impact. A special phenomenon observed
on lead glazes is described as blackening: leached lead ions form lead sulfide,
through a reaction with sulfides in the soil, resulting from bacterial metabolism.
Finally, it should be mentioned that apart from chemical and physical deteri-
oration processes, damage on ceramics occurs very often simply by fracturing,
during handling in domestic use or due to disasters, such as floods or fires.
Figure 16Salt efflorescence on ceramic body (shown by optical microscopy of the surface)