Gypsum

Gypsum, calcium sulfate, Calcium sul-furicum, CaSO₄+2H₂O, a white soft mineral that can be easily ground into powder, occurring in nature in large deposits; it is synthetically obtained by the action of sulfuric acid or its water-soluble salts on calcium salts; it is slightly soluble in water (1 part in 500 at 15°), insoluble in alcohol; when heated to 150°, it loses its crystallization water, giving the so-called burnt gypsum, corresponding to the composition 2CaSO₄+H₂O; the latter, when mixed with water, quickly reabsorbs the crystallization water, forming a hard, dense crystalline mass consisting of interwoven long needles; when heated above 200°, gypsum loses its ability to combine with water: the so-called overburnt G. is obtained. Gypsum water - a saturated aqueous solution of G. at room temperature - is used as a reagent for barium, strontium, tartaric, oxalic acids, etc. Burnt G. [Calcium sulfuricum ustum (Ph. VII)], obtained by heating aqueous G. at 110-130°, is a dry, fine, amorphous powder of white or grayish color, difficult to dissolve in water. A mixture of 2 parts of burnt G. with 1 part of water should harden within 10 minutes, forming a white dense mass; for slower hardening, glycerin is added to the water (a mixture of 100 parts G., 50 parts water, and 5 parts glycerin hardens within an hour). Burnt gypsum is used in surgery for dressings as is or in the form of gypsum bandages.