PLASTER OF PARIS, OR GYPSUM.
Plaster of Paris is sulphate of lime—a combination of lime and sulphuric acid; and exists abundantly in various degrees of purity; it occurs plentifully in the locality of Paris, and is brought here in masses of a greenish, pink, or brown color. To prepare the gypsum it is first made red hot, to separate the water which it contains, and then ground to powder of various degrees of fineness. Plaster of Paris has the peculiar property of uniting with a certain quantity of water and forming a solid compound with it; upon this property all its usefulness depends. If a portion of Plaster of Paris be mixed with sufficient water to form a liquid of the consistence of cream, in a few minutes it will harden and become quite set, and as it dries, it will harden still more, till it is of a hardness almost equal to stone: this useful property causes it to be much used for casting figures and ornaments; cheap plaster figures, &c., are generally cast hollow, to save the plaster; this is done by pouring into the mould a certain quantity of the plaster mixed with water, and quickly turning the mould about so that it shall adhere and form a layer on all the inside of the mould; when set hard, the mould is taken to pieces and the figure finished by scraping off the marks where the mould was joined. Plaster of Paris, combined with whiting, forms what is called by plasterers “putty,” and is much used for ceilings, and similar purposes.