consideration, as well as salt and microbial deterioration processes that may
be occurring in the painting.
4.2 Cleaning Methodologies
Cleaning methods are subject to fashion. It is quite clear that any kind of dust
upon the surface should be removed from the paintings, not only for aesthetic
reasons, but because it attracts humidity and may serve as a nutrient source
for microorganisms.
There are many different approaches, for example, application of cattle faeces
to remove soot (which is one of the most predominant surface pollutions on
paintings), application of acids or application of organic solvents. Even bread
or dough was commonly used in previous times and it is still a practice in
Scandinavia at the present.
Cleaning by aqueous and organic solvents. Mineral acids such as fluoric and
hypochloric acid, as well as organic acids like formic and acetic acids, have been
used for cleaning purposes, especially after mural paintings have been uncovered
from the superficial layers of whitewash, with detrimental effects. When applied
in liquid form or in poultices, it is not only the direct attack on carbonate that
occurs. As described for alkaline compounds and ionic tensides, ions remain in
the paintings and form salts that cause new problems. Although this is known,
it is often forgotten that neutralisation of such treatments entails the formation
of salts. Application of ion exchange resins has already been mentioned.
Before organic solvents are applied, the chemical nature of the materials to be
removed should be intensively investigated. Only then can the solvents appro-
priate for application at site be tested. This is especially important, since most of
the popular solvents bear enormous health risks. The risk may be reduced by the
application of solvents in the form of gels as introduced by Wolbers.
Cleaning by biochemical methodologies. Recently, reports on the applica-
tion of bacteria for cases of flaking mural paintings caused by consolidation
with glue, have been published. Attempts were made to solve the problem by
microbial activity. It was suggested that the bacteria applied are washed away
after treatment. However, wall paintings provide a more or less rough sur-
face, so that it is impossible to remove all bacteria completely. Consequently,
the application of biocides has to be considered to prevent the problems aris-
ing from the microorganisms themselves. Moreover, how can it be guaran-
teed that amino acids, resulting from the extracellular enzymatic cleavage
of protein, do not remain in the paintings as a readily-available substrate for
microbes? And how is it possible to guarantee that the bacteria only reduce
surface consolidants, and do not attack the original binding in cases of paint-
ings not executed a fresco? There are still concerns about this approach.
260 Chapter 10