A type of plaster composed of calcium oxide (lime). Lime plaster has been used since antiquity. It is prepared by heating limestone to high temperatures over a long period to remove the chemically bound water, thus producing calcium oxide (quicklime). When quicklime is mixed with water, it converts to calcium hydroxide (slaked lime). Slaked lime hardens by the evaporation of water. The set of lime is very slow and requires the presence of carbon dioxide. Also, without additives the plaster is weak and prone to shrinkage and cracking. Often aggregates or binders are added to increase the strength and decrease porosity. Lime plasters were used for wall coverings, frescos and wall decoration. For wall plaster, animal hair gives the lime and sand greater toughness and cohesion; in medieval work gypsum plaster was frequently added to lime as an accelerator of set. Additives to provide water-repellency, such as tallow or linseed oil, or to entrap air and thus improve the set, such as urine or beer, were traditionally used. For hand-modelled ornament gypsum was added, with a retardant such as glue, sour milk or wine, to allow a long working time. Marble powder was added as an aggregate because it permitted the rendering of fine detail. Lime plasters were commonly used in Europe, both internally and externally, but their long setting time of several months made decoration with paint or wallpaper difficult. Synonyms: lime; quicklime; slaked lime. Trench (2000); Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]