Gypsum
The mineral known as gypsum, or alabaster, consists of calcium sulphate, or sulphate of lime, its composition being expressed by CaSO4 + 2H2O. In gypsum the crystalline structure is just discernible, whilst other varieties, such as the so-called “marine glass,” occur in considerable quantities as large, perfectly transparent masses. “Russian glass” consists of large, transparent lumps possessing the specific property of gypsum, viz. that of cleaving in two directions, in a high degree. Alabaster is composed of finely granular masses, which are either quite white, or else yellowish, or traversed by grey veins. This variety of gypsum is very abundant in central Italy, and the best blocks are employed for the production of works of art.
Ordinary gypsum, which frequently occurs in the vicinity of dolomitic limestones, is found in a great variety of colours, bluish-grey, yellowish or reddish tints being the most common. Pure white lumps, which are plentiful in some deposits, can be used as white pigment, the method of preparation being simple, viz. merely reducing the mass to powder. This is easily effected, the specific hardness of gypsum being only 2; and in many cases it is soft enough to scratch with the finger-nail.
If the original gypsum is white, the powder forms a dazzling white flour which, notwithstanding, is of comparatively little value as a pigment, on account of its low covering power. For this reason, powdered gypsum is chiefly used for making plaster of Paris (calcined gypsum) for plaster casts and stucco. Gypsum may also be employed to advantage for lightening various colours, since it is inert towards even the most delicate.