GYPSUM, ALABASTER, ETC.
Gypsum. — Gypsum is a mineral which is found in nature in several forms. Chemically it consists of bi hydrated calcium sulphate. It occurs in translucent crystals, as selenite; in massive forms and opaque crystals, as gypsum; in fine-grained partially translucent masses, as alabaster, and in a fibrous form, as satin spar. When heated, gypsum gives off the water it contains and pulverizes into an opaque powder which, if moistened, quickly solidifies. This powder is known commercially as plaster of Paris. It is made by heating gypsum and grinding it to a fine powder. Gypsum is also used as a fertilizer, and in the manufacture of glass and porcelain. Plaster of Paris is chiefly used for making models and casts, and as a cement for marble and ala baster.
Among the most celebrated gypsum beds in the world are those of Mont martre near Paris, which have given plaster of Paris its name.
Alabaster. — This is a variety of gypsum of great beauty and easily carved on account of its softness; hence it is extensively employed for ornamental purposes.
To Use Plaster of use plaster of Paris, mix it with water to the consistency of thick cream and apply at once, as it sets quickly. No more should be mixed at a time than will be required for immediate use.
Or, if additional strength is desired, mix with a thin glue size, or a solu tion of 1 or 2 ounces of gum arable in 1 pint of water. These not only harden the plaster but give its sur face a desirable smoothness.
Or mix plaster of Paris with an equal weight of sulphate of potas sium, and thin with 2 to 6 parts of water. This mixture sets very quick ly and is harder than plaster of Paris alone.
To Prevent Plaster of Paris from Setting. — Mix 2 per cent of alum, sulphate of potash, or borax, with plaster of Paris before adding water, and it will be prevented from solidi fying for 3 or 4 hours. Thus it can be handled much more readily. When dry, it will also be much harder than plaster of Paris alone.
Or mix with vinegar diluted with water. The more vinegar used the
longer the plaster will require to set.
Molds for Plaster of Paris. — To make molds for casting plaster or naments, mix 12 ounces of rosin, 8 ounces of tallow, and 6 ounces of beeswax.
Or use plaster of Paris itself.
Or mix equal quantities of clean white sand and pulverized limestone or marble dust, and to 14 pounds of this mixture add 1 pound of litharge and 4 pounds of linseed oil. Grind or rub up the litharge with the oil, heat the mixture of sand and lime stone to dryness, stir into a stiff mor tar, and pour in an oiled mold.
To Cast Plaster of Paris.—Oil the object to be copied and cover it to a sufficient depth with one of the above mixtures, so that when removed the mold will be strong enough to han dle. If plaster of Paris is used to make the mold, the object to be cop ied mast be moistened, but no drops must be allowed to stand on the surface.
The mold should be made in two parts, which may be done by molding first one side up to given line, and then the other. Leave an opening through which to pour the plaster.
To make castings, it is only neces sary to oil or moisten the inside of a mold. Lock the two parts of the mold together, oil the edges so that they can be readily separated, and pour the mold full of plaster from a funnel through the opening left for that purpose. After the plaster has set, the mold may be removed and used repeatedly.
To Harden Plaster-of-Paris Ob jects.—Put in an iron kettle 2 pounds of a solution of caustic potash. Add in shavings 2 ounces of stearin and 2 ounces of castile soap, and boil for half an hour, stirring constantly. Add 1 ounce of pearlash dissolved in a lit tle soft water. Let the whole boil up, remove from the fire, and stir un til cold. Now stir in cold potash lye until the mass becomes perfectly liquid and falls in drops from the stirrer. Let stand for several days tightly covered before using. This preparation will keep for years.