GYPNUM, or PLASTER-STONE, in natural history, a genus of fossils, naturally and essentially simple, not inflammable nor soluble in water, and composed of flat small particles, which form bright, glossy, and in some degree transparent masses, not flexible or elastic, not giving fire with steel, nor fermenting with or being soluble in acid menstrua, and very easily calcined in the fire.
Of these gypfums, some are harder, others softer, and are of several colours, as white, grey, red, green, &c. sometimes distinct, and sometimes variously blended together.
The origin of all these gypfums is from the vitriolic acid and calcareous earth. See CHEMISTRY, no 127. They are much used for stuccoing rooms, and for casting busts and statues; for which last purpose they are excellently adapted by the property they have of expanding when they set, or become solid, after being mixed with water. See PLASTER.
Gypfum by itself is very difficult of fusion: yet if a piece of forged iron is surrounded with gypfum in a crucible, and urged with a vehement heat, that metal, though otherwise unfusible, will be melted, and retain its malleability, though some say it assumes the nature of cast iron. Another very remarkable property of gypfum is, that when mixed with chalk, clay, limestone, and some other unfusible earths, they melt, in a heat not very great, into a yellowish glass. It is impossible, however, either to reduce this glass to a sufficient degree of fineness and transparency by itself, or by means of it to give a good yellow colour to other glass.