Gypsum, or terra alba is calcium sulphate containing moisture. It is found in a commonly oc-curing class of minerals, both crystalline and fibrous, and in the form of gypsum earth or land plaster.

Gypsum, from which all the water has been removed by burning, is termed burnt gypsum, or plaster of paris. It has the property of again combining with water to form a solid mass, which property is taken advantage of in many ways.

Unburnt gypsum, when treated with a dilute solution of potassium sulphate, or carbonate, will harden even more quickly than burnt gypsum with pure water. Thus, plaster, which has not hardened properly, can be made to do so by coating it with a solution of the potash salts mentioned, which will also assist in rendering mild or less alkaline any caustic lime which may be used with the plaster. Gypsum is used for building and decorative purposes, in stucco work and in some kalsomines and fresco paints.

Gypsum can be hardened in several ways. Burnt gypsum may be mixed with lime water or a solution of glue in water. A very good method is to mix the gypsum with a solution of alum water containing 3 to 4 ounces of alum to the pint of water used. This mixture hardens completely in a few minutes, and is known as marble cement. Borax water, containing 2 ounces of borax to the pint, will act in the same manner.