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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