areas with highly alkaline pore solutions, such fungi will outcompete other
species that are restricted by alkalinity. This should be considered when using
cleaningor consolidation materials providing alkaline pH.
The most important deterioration problems on wall paintings, induced or
influenced by microorganisms, can be summarised as follows: (i) deterioration
due to biofilms, (ii) metabolism of organic bindings as substrates, (iii) alter-
ations of mineral pigments, (iv) excretion of mineral or organic acids, and
(v) interaction of microbes and faunal elements.
Deterioration due to biofilms. When considering the biodeterioration of wall
paintings,we have to distinguish between alterations associated with living
and actively-metabolising cells and those parts of the biomass that are dormant,
or even dead. Microbial infestations of wall paintings will contain viable as well
as non-viable cells, and may lead to a completely different appearance of the
paintings. Algae and cyanobacteriaproduce chlorophyll to form green layers
upon the surface. Fungi are frequently pigmented by brown melanins, and
several bacteria express red or yellow carotenoids. However, even cells with-
out bio-pigments change the appearance by covering the painting, which
might be looked upon as being merely an aesthetical problem. Even if there
was no influence on the paintings,as was suggested for some bacteria causing
red discolourations on wall paintings, one has to consider possible effects on
humans, as will be discussed later.
Microbial growth frequently leads to the production of so-called biofilms
from extracellular polymeric substances (EPS) surrounding the cells from
which they have been produced. Biofilms, including the cells themselves as well
as surrounding EPS, may influence or induce deterioration, e.g. by swelling and
shrinking during climatic changes (Figure 2) that induce condensation or
Wall Paintings: Aspects of Deterioration and Restoration 249
Figure 1Algae growing on medieval paintings in a vault after infiltration of rainwater,
Eilsum, Germany