Deterioration by Soluble Salts

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 disintegration and detrimental effect of subflorescences is mainly due to the pressure 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 damage 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 composition and salt concentration. It is necessary to consider not only consolidation treatments of 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 system of the multi-layered plaster and paint layer system is different to, and much more complicated than, in vitro experiments. 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 

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This chapter is presented from the point of view of a microbiologist but it will not be confined to the known facts of deterioration by soluble salts and biodeterioration. It will necessarily focus on the interactions between the different causes of damage and the effects of interventions, which may exacerbate the deterioration processes. 

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