and, especially in England, bees-wax, while linseed oil was basically used to
make the paintings more brilliant.
After the application of surface consolidants that reduce porosity, prob-
lems arise in the presence of autochthonous salts (especially with changes of
the environmental climate). Additionally, the consolidants themselves react
to climatic changes by shrinking and swelling. This results in flaking of the
consolidant, as well as the pigments, after repeated cycles. Therefore clean-
ing, or at least reduction of the consolidation layer, must be undertaken to
preserve the paintings that have undergone this kind of consolidation.
In the second half of the twentieth century, natural polymers were subse-
quently replaced by synthetic polymers or derivatives of natural polymers, e.g.
based on cellulose, like methyl celluloses or hydroypropyl celluloses. The
most frequently applied synthetic materials range from polyvinyl alcohol and
polyvinyl acetate to acrylics. Even nylon was used. Deterioration by this type
of consolidant was enhanced at sites with autochthonous salt problems, and
this in turn enhanced the activity of bacteria and fungi. It could be clearly
demonstratedthat microorganisms could unexpectedly metabolise synthetic
polymers. All synthetic polymers investigated in laboratory experiments
turned out to be biodegradable and supported the growth of microorganisms.
It was not only the polymer itself but also numerous additives in the product
that supported growth. Microbial infestation of paintings with synthetic poly-
mer coatings can frequently be observed, especially by scanning electron
microscopy. However, although one can prove the use of the polymer as sub-
strate in laboratory investigations, this does not necessarily mean that the
microbes in situ can use the synthetic polymer as organic carbon source. It is
Wall Paintings: Aspects of Deterioration and Restoration 257
Figure 8Infested sample after consolidation with ethyl silicate, Clemenswerth Castle,
Soegel, Germany