Plas′ter.
1. (Plastering.) a. A composition of lime, sand, and water, with or without hair as a bond, and used to cover walls and ceilings. b. Calcined gypsum or sulphate of lime, used, when mixed with water, for finishing walls, for molds, ornaments, casts, luting, cement, etc. It is also the basis for stucco and scagliola or artificial marble. The hydrated sulphate of lime is calcined at a heat of about 300° Fah., and parting with 20 per cent of water falls into a white powder. While it decrepitates it does not decompose, like limestone, but is greedily absorbent of water, and by combination therewith becomes again solid. Calcined with alum it forms Keene's cement, which solidifies more slowly than the common plaster of Paris, but is harder when set. See cement. Reburnt with borax and other substances, it produces other and fine cements. See cement. The mortars used for inside plastering are termed coarse, fine, gage or hard finish, and stucco. Exterior plaster or stucco: 1 volume of cement powder to 2 volumes of dry sand. In India, to the water for mixing the plaster is added 1 pound of sugar or molasses to 8 imperial gallons of water, for the first coat; and for the second or finishing, 1 pound of sugar to 2 gallons water. Turkish plaster or hydraulic cement: 100 pounds fresh lime reduced to powder, 10 quarts linseed-oil, and 1 to 2 ounces cotton. Manipulate the lime, gradually mixing the oil and cotton, in a wooden vessel, until the mixture becomes of the consistency of bread-dough. Dry, and when required for use mix with linseed-oil to the consistency of paste, and then lay on in coats. Water-pipes of clay or metal, joined or coated with it, resist the effect of humidity for very long periods. The names of plaster coatings are as follows:— First coat of two-coat plastering: laying, when executed on lath; rendering, when executed on brick. First coat of three-coat plastering: pricking-up, when executed on lath; roughing-in, when executed on brick. Second coat: set, in two-coat plastering; floating, in threecoat plastering. Third or finishing coat: stucco, when for painting; setting, when for papering. Or, as follows:— Lath, laid, and set: lath with two coats of plaster. Lath, laid, set, and colored: the addition of coloring to the former. Lath, pricked-up, floated, and set: three-coat work. Lath, floated, and set fair: same as the last. Roughed-in, floated, and set: three-coat work on brick walls. The Greeks used a very durable mortar for stucco and plastering. Their walls and floors are yet hard and white, and retain their polish in instances where they have not been mutilated. The Romans plastered their cisterns and reservoirs with a mixture of pure lime and pounded brick, which hardened into a mortar resembling breccia, which has nodules of red matter in a white matrix. This was beaten to render it plastic, and it was then floated on to the walls by sandstone trowels, which were rubbed over the surface to smooth it. Over this was laid a coat of fine plaster covered by one of red-lead and oil.
2. (Medicinal.) Plasters are spread by placing the material upon the wash-leather or paper, on which is laid a mask which determines the margin when the material is flattened by the hot spatula. One form of the device has a pair of masks which move over each other so as to expose an opening of any required size and proportions.
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| Plaster-spreader. |
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| Plaster-spread. |
