Efflorescence

A white powdery crust formed by extraneous salts evolving on the surface of stone, plaster, ceramics, or another material as the result of water migration and subsequent evaporation. Typical efflorescence on masonry surfaces, mural paintings, and frescoes occurs when moisture dissolves water soluble salts, then migrates to the surface, where the water evaporates leaving a salt residue. Efflorescence crystal growth can result from fluctuations in water vapor pressure that occur with changing humidity and temperature. The salts most commonly found in efflorescence are sulfates, carbonates, and chlorides of sodium, potassium, magnesium, calcium, and iron.