and nitrates, nitrifying bacteria have also been shown to be active in building
materials in situand thereby contributing to biodeterioration directly. As well
as the nitrogen cycle, microorganisms also contribute to the sulfur cycle and
may produce sulfuric acid.
Salt ions are diluted in water and thereby transported into the pore system.
Whenever the water evaporates, the ions concentrate and crystallise. The
morphology of the crystals is influenced by the conditions at site, especially
by other ions present, the microclimate and the internal structures. The disin-
tegration and detrimental effect of subflorescences is mainly due to the pres-
sure produced by the growing crystals as well as hydration.
The process of deterioration happens over a period of time rather than being
the result of a single event. Seasonal and diurnal cycles have been reported to
be involved (Arnold and Zehnder, 1991). Initiation of hydration may take only
minutes rather than hours or days, depending on the specific environmental
conditions and material components at site. There are different approaches to
prevent these deterioration processes: (i) stabilisation by climate control,
(ii) stabilisation by chemical reactions, (iii) extraction of ions and (iv) microbial
activity.
Stabilisation by climate control. The most desirable method to prevent dam-
age induced by the repeated cycles of crystallisation and hydration would
probably be environmental control. However, neither the selection nor the
maintenance of such an ideal environment is possible if looked at realistically.
Predictions of salt crystallisation and hydration from mixed salt solutions are
more or less impossible, taking into account all the different parameters that
influence the process. Sawdy provides a brilliant overview of the subject and
considers relative humidity, temperature, air movement, type and structure of
the porous support, salt mixture compositionand salt concentration. It is nec-
essary to consider not only consolidation treatmentsof the plaster or the paint
layer, which as such may influence the transition behaviour of the salts, but
also the influence of microbial extracellular slimes on the porosity of the system.
The situation becomes even more unpredictable since crystallisation of
even pure salt solutions does not occur at a given point of relative humidity.
Also the behaviour of the solvent, in this case water, within the porous sys-
tem of the multi-layered plaster and paint layer system is different to, and much
more complicated than, in vitroexperiments. Therefore, we have to accept
that the prediction of crystallisation and hydration cycles of salt mixtures
within the porous wall painting system is one of the most difficult tasks one
can imagine.
Any kind of monitoring to verify or reject the predictions at site is restricted
by the fact that analyses of the efflorescences are necessary to identify precisely
which of the possible salts actually do crystallise. Most kinds of monitoring are
even restricted to those parts over the paint layer rather than underneath, where
244 Chapter 10