Exfoliation is the irreversible loss of scales, flakes, or layers from a surface. A common example of exfoliation is on stone and ceramics where weathering, crystallization and dissolution of salts, or freeze-thaw action cause pieces of the surface to flake off (CAMEO 2014).
Synonyms in English
flaking, spalling, peeling, shed
Two of the primary causes of exfoliation are salts and freeze-thaw. Salts absorbed into porous materials (like stone and ceramics) crystallize and dissolve in response to changes in moisture content and relative humidity. Solid salt crystals occupy greater volume than salt in solution, and the pressures they exert when they form that cause exfoliation – especially if they form below the surface (Davison, Buys and Oakley 23).
Freeze-thaw causes exfoliation in a similar way as the salts. When an object gets wet and the temperature declines to below freezing, the frozen water will expand. The expansion causes pressure on the object's surface and can cause pieces to flake off.
Another form of exfoliation takes place when certain metals corrode. If the metal is part of a larger structure, such as a building, then the corrosion can also cause the metal to expand and then push on the attached materials, much in the way that salt or ice would, and cause exfoliation and spalling of areas around the metal (Fidler, Wood, and Ridout, 2004).
Material types that are especially susceptible to exfoliation are ceramics, porous rocks, metals, and anything that has a coating that prevents salts or ice from reaching the surface (paintings, rock art, et cetera).
There are ways to prevent or at least reduce exfoliation. The most effective way is to remove any salts that might be present, and then store in a controlled environment where humidity and temperature are regulated. However, that is not always possible. The use of "breathable" coatings (such as limewash) is also a possible way to prevent exfoliation (Fidler, Wood, and Ridout, 2004).
Buys, S. and V. Oakley. 1996. Conservation and Restoration of Ceramics. Butterworth - Heinemann, Oxford.
Exfoliation. 2004. Art and Architecture Thesaurus Online. Getty Research Institute. http://www.getty.edu/vow/AATFullDisplay?find=exfoliation&logic=AND¬e=&english=N&prev_page=1&subjectid=300264844 (accessed March 18, 2015).
Exfoliation. 2014. CAMEO (Conservation and Art Materials Encyclopedia Online). Museum of Fine Arts, Boston. http://cameo.mfa.org/wiki/Exfoliation (accessed March 18, 2015).
Fidler, J., C. Wood, and B. Ridout. 2004. Flooding and Historic Buildings. English Heritage. http://ncptt.nps.gov/wp-content/uploads/Flooding-and-Historic-Buildings-Technical-Advice-Note-2004.pdf?1ef327
U.S. Congress, Office of Technology Assessment. 1986. Technologies for Prehistoric and Historic Preservation. U.S. Government Printing Office. http://ncptt.nps.gov/wp-content/uploads/OTA-Technologies-for-Prehistoric-and-Historic-Preservation-1986.pdf?1ef327