David Deerson, age 10, of South River, N.J., for his question:
WHAT IS PLASTER?
When plaster is applied to a wall or ceiling, a number of special tools are used by the craftsmen who are trained to do this special work. The plaster is placed on a small platform called a hawk that is held by one hand. In the other hand the plasterer holds a trowel which is used to apply the mortar to.the desired surface. The plaster is then smoothed with a tool called a darby.
Plaster is a mortar coating that is put on the inside wall surfaces and ceilings of buildings to make them more airtight and to give a finished surface.
When used on walls and ceilings, plaster is made of sand and a cementing agent, such as gypsum, lime or portland cement. The ingredients are then mixed with water and either hair or fiber is mixed with the first and second coats to strengthen the plaster. The hair is goat or cattle hair and the fiber is manila, jute or wood fiber.
Sometimes lightweight material such as perlite or vermiculite is used in plaster instead of sand. These materials absorb sound and are also fire resistant.
Plaster is sometimes put directly on a masonry wall or a solid wood wall. Other times the plaster is put over laths that are made of metal sheets, pieces of gypsum or fiberboard or wooden strips. The laths on the surface to be plastered provide a better grip.
Wood laths are used much of the time. They are laid parallel, with narrow spaces between them. The plaster enters the spaces and forms wedges called keys. The wedges hold the plaster to the laths. Gypsum board or metal lathes are used in many modern buildings.
Plaster of Paris is something else. It is a white powder that, when mixed with water to form a paste, will turn hard in a few minutes. This substance is used for casting small statuary, for surgical casts, for enveloping the wax impressions of teeth made by dentists and for other purposes. Plaster of Paris is made by heating gypsum, a stone composed of calcium sulfate and water. When the water is driven off, the gypsum becomes a powder. When water is added again, the mass hardens to a stonelike substance similar to the original gypsum.
The plaster used as a building material can be applied in a number of different ways: either with a smooth surface or a texture that looks like natural stone.
Gypsum gauging plaster is added to a regular mixture sometimes to avoid hair cracking by reducing shrinkage.
Three coats of plaster are usually used on wood or metal laths but only two are usually needed on brick or tile surfaces.