Deterioration Patterns Wallpaintings
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Examples of damage by salt crystallization
Calcareous concretions (sinter crusts)
Precipitation of calcium carbonate on the surface of wall paintings generally begins by the formation of a fine white veil and may continue to form a white, dense calcareous concretion. It is characterized by its high stability and strong bonding with the painting. The water vapor permeability of the wall painting is reduced with the negative consequences that this may induce and the layered crust deposit(s) considerably affects their aesthetic appearance.
Framboidal efflorescence (cauliflower crust)
The formation of firmly adhering framboidal efflorescence is characteristic for some conditions. The composition may consist of calcite (calcium carbonate) or gypsum (calcium sulfate dihydrate). Depending on the contaminants present, the efflorescence may appear whitish, yellowish, reddish or brownish. Biogenic material may be included within the crust deposits as they form.
The photograph shows darkened areas along fissures in the paint layer and topcoat of the plaster that also has crust deposits on it. This is caused by the presence of hygroscopic salts that will tend to bind any moisture available in the plaster itself or originating from the ambient air. The moisture film will reflect light more giving the surface its darker appearance.
Powdery salt deposits
White, loosely adhering salt efflorescence on the plaster and the paint surface. It is characterized by small crystalline, soft, powdery efflorescence which may be formed by several different salts. The example shows sodium sulfate deposits on a repair plaster made with trass cement.
Scaling due to salt crystallization
The crystallizing salts can detach the paint layer and even the topcoat of the plaster. This occurs as a result of the liquid to solid phase transition during salt crystallization so that depending on where the salts crystals form, a thinner or a thicker layer may be detached.