Fresco - The Lime

The best lime is made by burning calcium carbonate, found in nature in the form of limestone, marble, chalk, and shells. This burnt lime becomes calcium oxide and is called caustic lime, quick lime, or hot lime. It may be purchased from dealers in building supplies, in either lump or powder form, under the name of high-calcium lime. It should contain no clay, magnesia, or gypsum, since these may impair the setting qualities of the plaster or may case efflorescence. Powdered lime is to be preferred to lump lime. When water is added to quick lime or calcium oxide, it is changed to calcium hydroxide or slaked lime. This slaked lime, when properly aged, takes on the plastic putty consistency that makes a superior mortar. When the slaked lime putty is applied to a wall, it first sets by evaporation of the water content and then absorbs carbon dioxide from the air and returns once more to its original form of calcium carbonate. When it absorbs the carbon dioxide, it forms on its surface a porous mass of calcium carbonate crystals called a lime crust. It is this lime crust that holds the pigments of the fresco painting in the surface of the wall.

[Kay, Reed. The Painters Guide to Studio Methods and Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983. p. 173]

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