PLASTER MATERIALS Plaster is principally composed of lime, sand, hair, and water.
Lime is obtained in different sections of the country from calcined limestone, the carbonic acid and moisture contained in the stone being driven off by the burning process. The whole theory of plaster ing is based upon the reduction of limestone to lime, and its chemical recombination, when distributed upon the walls of a house, into something approaching its original state. The slaking of the lime provides the moisture necessary for the process of crystallization that produces the set of the mortar; while the sole purpose of applying it upon the wall in several coats is to present that much more surface to absorb the carbonic acid—of which it was originally deprived in burning—from the air. The thinner the coats and the larger their total exposed surface, the greater the absorption of this strengthening constituent. For this reason—and solely for this reason—is three coat plaster work to be considered as better than two.
Properly burnt lime slakes easily and completely, when water is added, until it is converted into a fine dust, which, in its turn, is moistened and turned into a paste under action of the water, which bubbles and hisses with the heat generated by the process. This is what is called the slaking of lime. Very rich and pure lime—the best for plastering—increases to about twice its original bulk by being slaked, and is then almost pure white in color. Lime should always be as fresh as possible, and must be delivered in tightly sealed barrels. Care should also be taken to ascertain that it has been burned with wood and not with coal.
Sand is broken or rotten rock which has become decomposed spontaneously or by the action of running water. That made by running water, or from stones worn small by rolling over and over upon the beach, is composed of particles so nearly round in contour and so lacking in angularities of surface that they are not good material for mixing in any mortar where strength is a requisite or necessity. The particles of rotten rock decomposed by exposure are better adapted to make good sand for mixing with mortar, their shape being more irregular, with many sharp and angular corners.
Sand obtained from ledge stones contains the essential elements of those stones, quartz, feldspar, and mica being present in granite formations, and lava, obsidian, etc., in volcanic sand. The sand coming from the softer stones is generally more thoroughly disinte grated, being frequently so rotten as to be entirely unsuitable for use in plastering. In most parts of the country the principal supplyof sand now comes from the beds of ancient lakes or rivers, and is called pit sand. True sand, no matter how fine, may always be distinguished from dust by dropping it into a glass of water, as it will invariably sink to the bottom without leaving any appreciable dirt upon the surface.
For plastering purposes, sharply angular sand is not absolutely essential. Good river sand, the coarser the better, is obtained so easily, and is so clean and free from dirt, clay, and earth stains, that it is most generally employed for plaster.
The third necessary constituent is hair. The best hair upon the market is cattle hair obtained from the tanneries. The hair should be of good length; and, if too lumpy or clotted, it should be separated by soaking in water the day before mixing it with the mortar, as this method of separating the hair is less dusty and more healthful than beating or whipping it dry to obtain the same result.
Occasionally brick dust is added to the mortar for coloring, when it is likely that the mortar will set more rapidly—especially if the dust is mixed in shortly before using and is dry at the time of mixing. All brick dust should be sifted through a fine sieve. Besides brick dust, a variety of colorings for mortar are used—such as lampblack, ivory black, powdered charcoal, Spanish brown, raw umber, burnt umber, red aniline, Venetian red, Indian red, vermilion, ultramarine blue, indigo blue, chrome yellow, and, occasionally, pulverized clay. Mineral colors should be preferred to earth colorings. The latter weaken the plaster, and fade rapidly. Variously colored sands —when they can be obtained—make the best and most durable ma terials possible for tinting the final plaster coat.