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Efflorescence refers to the process where wet particles decrease in size through losing water to eventually crystallize out. An example of an efflorescent substance is the common Copper(II) sulfate crystal (CuSO4.5H2O), a blue crystalline solid that when exposed to air, slowly loses water of crystallization from its surface to crystallise to form a white layer of anhydrous copper(II) sulfate.
An efflorescent substance is a chemical which has water associated with its molecules, and which, when exposed to air, loses this water through evaporation. A common example of this phenomenon is the drying of cement. When cement is poured, it goes through an exothermic chemical reaction, which generates heat, in order to set. Then the set concrete also has to dry out by evaporation. The heat causes various mineral salts which are in solution in the wet concrete to migrate to the surface, where the evaporation of the water leaves a fine layer of crystals on the face of the concrete. In a thin layer these crystals are seen as a "blush" or "bloom"; in a heavier layer they can look like frost deposits or even flowers.
Efflorescence of different types has been discovered on the surface of a huge variety of art works, and is a serious concern for art conservationists. The link below from the cool conservation net has an interesting article with examples of efflorescence found on the surfaces of paintings.
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