Deterioration Patterns Wallpaintings

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Author: Nicole Riedl

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Calcareous sinter crusts

Calcareous sinter crust, Roman wall-painting, grave chamber, Nehren

Calcareous sinter crust, Roman wall-painting, grave chamber Nehren

Repeated precipitation of calcium carbonate on the surface of wall paintings forms a white, dense calcareous sinter crust. It is characterized by a high stability and strong bond with the painting. The vapor permeability of the wall painting is reduced and the layered crust deposits affect the aesthetic appearance considerably.

Cauliflower crust

Cauliflower crust, grave chamber Nehren

Cauliflower crust, grave chamber Nehren

The formation of firmly adhering pustules and stable crust deposits, is a characteristic of the cauliflower crust. The composition may consist of calcium carbonate or calcium sulphate. Depending on the contaminants present, the cauliflower crusts appear whitish, yellowish, reddish or brownish. Biogenic precipitate can be stored within the crust deposits.

Hygroscopic salts

Gypsum crust, hygroscopic, grave chamber Nehren

Gypsum crust, hygroscopic, grave chamber Nehren

Darkened areas within the layers of crust deposits are visible alongside fissures in the paint layer and the topcoat of the plaster. Increased humidity, originating from the plaster or the ambient air, is bound by the salt crystals and appears macroscopically as a dark, blotched composition.

Powdery salt deposits

Powdery salt efflorescence, sodium sulfate, grave chamber Nehren

Powdery salt efflorescence, sodium sulfate, grave chamber Nehren

White, loosely adhering salt efflorescence on the plaster and the paint surface. A characteristic of this efflorescence is the formation of small crystalline, soft, powdery salts, which can differ considerably in their chemical composition. The example shows sodium sulfate deposits on a repair plaster made of trass cement.

Scaling due to salt activity

Scaling, grave chamber Nehren

Scaling, grave chamber Nehren

The crystallizing salts burst the plaster structure. During the phase transition from liquid salt solution to the formation of salt crystals, an expansion takes place and pressure is applied to the adjacent plaster matrix. Hence, entire layers of plaster are lifted from the substructure and become crumbly and fragile.

The permeability is a measure of the ability of a porous material (often a rock or unconsolidated material) to allow fluids to pass through it. Am material with a high permeability lets the fluid pass through faster than one with a low permeability.

crystallization of a dissolved salt