they are contaminated or when they have been in use for an extended time,
and then consolidation is required. Microbial contamination of organic bind-
ings leads to enzymatic breakdown of macromolecules like proteins, fatty
acids or carbohydrates because microorganisms are unable to absorb these
large macromolecules. They produce exoenzymes to break them down into
smaller monomers, which can then be absorbed by cells as carbon source. The
polymers then disintegrate and can no longer function as consolidants. In any
paintings not executed as fresco, or at least as lime-painting, organic bindings
guarantee the consolidation of paints. During microbial infestation, these
organic bindings will be decomposed and lose their consolidation effect.
Metabolism of organic bindings is not only restricted to natural organic bind-
ings but also affects synthetic polymers applied for consolidation purposes.
Alteration of mineral pigments. Besides the production of bio-pigments,
microorganisms may also interfere with the mineral pigments of the painting
composition itself and thereby completely change its appearance. Sulfur-
reducing bacteria may produce black sulfides from lead white. A summary of
pigments transformed by microbes is given by Lyalikova and Petrushkova.
Lead oxides,frequently suspected of being transformed by microbial activity,
however, couldnot be detected in the darkened ceiling of Sala Terrena in
Weißenfels Castle, Pommersfelden, although microbial infestation could be
shown. In laboratory experiments, production of acetic acid by fungi isolated
from wall paintings resulted in the formation of green copper acetate from blue
azurite. The reactionin situ, however, is still in question. Other pigments like
manganese or iron maybe oxidized by fungi isolated from building materials
and thereby change their colour. As mentioned above, microorganisms require
certain metals for growth. If these elements are limited, the microorganisms
are able to extract the minerals from the pigments that the paintings are com-
posed of, and to incorporate these into biomass.
Excretion of mineral and organic acids. Bacteria growing by nitrification
and sulfur oxidation excrete mineral acids into their surroundings while gain-
ing energy from these reactions. Sulfuric acid and nitricacid are strong mineral
acids, which induce deterioration of components of the painting, plaster or
stone, particularly lime, but also organic bindings in the objects.
Excretion of organic acids may be connected to the turnover of specific nutri-
ents and to mineral limitation. Acids frequently excreted by microorganisms,
especially when growing on carbohydrates, are, among others, oxalic acid,
citric acid and acetic acid, which damage wall paintings by the same mecha-
nisms described above. In some cases, however, the excretion of oxalic acid into
calcareous materials may result in the formation of resistant oxalates. After
oxalates had been detected on many ancient marble statues, it led to the sugges-
tion that microbial activity might have been responsible. This will be discussed
252 Chapter 10