paint layer may be endangered by this intervention itself, and even microbial
surface colonisation has been observed, even in several situations where none
existed before the application of the wet compresses.
To reduce this problem and reduce stress to the paint layer, methods were sub-
sequently developed to include climate control, so that ions are mobilised and
transferred to the rear support, in order to reduce salt concentration in the paint
layer.This results in a more even salt distribution in deeper parts of the plaster
or brick. Ideally, a new layer of plaster can be applied to the back, e.g. the sup-
porting brick. The aim of this treatment is to ensure crystallisation within or
upon this plaster, but it is difficult to achieve. Investigations nevertheless
demonstrated a remarkable reduction of salt ions near the highly-vulnerable
paint layer and a more even distribution in deeper parts, which may give more
time to search for other solutions. Obviously this technique is restricted to rela-
tively thin supports and cannot be applied in the case of cavity walls.
Other methodologies, such as electro-osmosis, have been reported to be suc-
cessful on certain objects. In this case, electrodes were introduced in areas where
the paint layer was already lost completely to reach beneath the remaining
parts of the paintings. Migration of anions and cations of detrimental salts
was induced viaan electrical field. Whereas cations were transported toward
earth, anions were attracted by the iron anode. The system is reported to be
under observation, with repeated exchange of the iron at the end of its serv-
ice life. The ions reacting with the iron can be removed from the system and
analysed further. A drop in the current of about 300 mA was observed during
salt reduction. However, up to now the method has only been applied to a
small number of objects so the results will have to be studied carefully.
Microbial activity. Microorganisms are involved in the global sulfur and
nitrogen cycles through a wide range of biochemical reactions. In the context
of deterioration problems of wall paintings (and stone) caused by repeated
crystallisationsand hydrations of salts, bacteria involved in desulfurylation
and denitrifying processes have been considered for their use in salt-reduction
methodologies of bioremediation (see also page 233).
In aerobic respiration, the electrons from carriers at the end of an electron
transport chain are finally accepted by molecular oxygen. With oxygen serving
as an electron acceptor, as much energy as possible is released from the elec-
tron donor. When the environment is depleted of oxygen, alternativeelectron
acceptors, such as sulfate or nitrate, may be reduced. However, in the case of
denitrifying bacteria, the aerobic and anaerobic respiration pathways compete,
and under normal conditions oxygen as an electron acceptor will succeed
(facultative denitrification). In other microorganisms like sulfate-reducing
bacteria, the reaction is obligatory and oxygen cannot be used. Hydrogen or
organic compounds, like lactate or acetate, function as electron donor, with
246 Chapter 10