more likely that natural organicmaterials like casein or glue may function as
a carbon source. Nevertheless, excreted microbial intermediates like organic
acids may attack the synthetic polymers and especially synthetics applied as
a coating. This may result in some kind of greenhouse effect by reducing des-
iccation, when applied over an existing microbial infestation (see Figure 9).
Application of wax and various wax resin mixtures was predominantly
employed in England until the middle of the last century as a result of mis-
understanding of the ancient Roman techniques. The application caused detri-
mental results. After it was agreed that wax as a consolidant was not appropriate,
subsequently techniques of de-waxing had to be devised, which may, how-
ever, only reduce the amount of wax in the painting, not remove it completely.
Moreover all those removal techniques applied until now involve use of organic
solvents, with enormous risks both to paintings and conservators.
Consolidation by alternative approaches. The latest approach is the appli-
cation of oxalic acid to wall paintings, as well as stone monuments. This
method is still under investigation in Italy. After intensive laboratory investiga-
tions, somein situapplications on stone monuments were executed. The pro-
posed advantage is the controlled transformation of the uppermost layer of
calcium carbonate into oxalate, which is much more resistant to any kind
of aggressive environmental attacks. This can be observed on a large number of
architectural surfaces with compositional oxalate films. As compared to cal-
cium carbonate, the oxalate is much more resistant to acids. At neutral pH the
solubility of calcium oxalate is about two orders of magnitude smaller than
that of calcium carbonate. It still has a very low solubility down to pH values
of about 2–3. However, the treatment by oxalic acid is completely irreversible
258 Chapter 10
Figure 9SEM of microorganisms growing beneath Klucel surface consolidation,
Salzhemmendorf, Germany