This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
217. Up to quite a recent period, practically all the interior plastering in this country was composed of lime, sand, and hair. When plaster is made of a good quality of lime, well slaked and properly mixed with the other constituents, it is very durable; but much of the lime plaster now used is made of inferior materials, so that much of it is practically valueless as far as durability is concerned.
The substances which enter into the composition of the mortar will depend upon the nature of the surface to be coated, the. order in which the layer is applied, and on the desired finish. For ordinary work, these are fane, water, sand, hair, and plaster of Paris.
218. Lime may be briefly defined as the product resulting from the calcination of the natural limestone, calcination being the process of heating the material in a kiln or oven until it emits a red glow, thus releasing the carbonic acid and moisture. The product is quicklime; the lumps of the material, after being removed from the kiln, are called lime shells. In preparing the mortar, the lime shells are deposited in a wood trough called the slacking, or slaking, box (so as to keep them clear of any earthy or loamy matter), and are liberally sprayed with as much water as they will absorb, when they soon begin to swell, crackle, and fall into a powdery mass. This process is called slaking, and the powdered substance is termed slaked lime, or hydrate of lime, from its admixture of water; during the process, the pure lime increases from two to three times in bulk, and much heat is given out, which transforms the excess of moisture into steam.
Many limes that make good mortar for other purposes, are unfit for use in plaster, as there are more or fewer over-burned, hard, obstinate nodules which resist the permeation of the water, and fail to readily disintegrate; eventually they will slake, or what plasterers call "pop," causing a small piece of plaster to fly off. Walls and ceilings may sometimes be seen pitted all over from such checking.
In many places a number of different brands of lime may be had, and care should be used in selecting one for plastering purposes, unless its slaking qualities are known. In any case, it is not best to use lime until it has been slaked at least a week, as even with the best limes considerable time is required for the slaking of all the particles.
Lime is frequently sold by weight in the Western states, but elsewhere it is usually put up in barrels and sold by measure.
Sand. This should be angular, of medium fineness, and free from earthy matter. Before use, all sand should be well screened, to remove the coarse particles, and if used for hard finish, it should be sifted. Its cleanness may be tested by rubbing a small quantity between the hands; if it contains much dirt, it will stain them. Another simple method is to squeeze some of the moist sand in the hand; if it cakes together, and retains the impression of the fingers, it very likely contains clay; but if it falls apart loosely it may be considered clean. The best plaster is made of screened, washed, and dried sand. In most cases, the sand is merely screened river or pit sand; the first is the best, as the water has washed away most of the clayey matter, while pit sand is likely to contain it. Sea sand is not desirable to use, on account of the salt contained in it, and the roundness of the grains. If used at all, it should first be well washed in fresh water. The chief uses of sand in mortar are to prevent excessive shrinkage and to lessen the quantity of lime required. It also greatly strengthens the mortar, owing to the formation of a very hard silicate of lime.
Hair And Fiber. These are used in plaster for the reason that ordinary lime mortar alone does not possess sufficient cohesive power to bind it together firmly. The first named is used almost exclusively for this purpose, but in the Eastern states Manila fiber, cut into short pieces, is frequently substituted for hair. The patent ready-mixed mortars usually contain either Manila jute or asbestos as a fibrous material. Cattle hides are the chief source of plastering hair, which, after being cleaned from grease, washed, and dried, is packed in bags containing a bushel (measured loose), and weighing from 6 to 8 pounds. The hair of salted hides is undesirable, owing to the presence of salt, which is not easily extracted. Goat hair, when it can be had, is preferable to cattle hair, as it is longer and of better quality.
221. Plaster of Paris is obtained by gentle calcination of a crystalline limestone called gypsum, which is classified as a hydrated sulphate of lime, its constituents being water, salts of sulphuric acid, and lime. Extensive deposits of the stone exist in the environs of Paris, in France, from which fact the trade name is derived. The chief value of this plaster is that paste made from it rapidly sets and within a few hours acquires its full strength. It is very soluble in water, which renders it unfit for external use, but it is invaluable for cornice moldings and enrichments, and is also used in several plastic mixtures. Its volume expands in setting, and on this account it is a good material for filling chinks and holes in repair work. Plaster of Paris is much used for making casts, etc., which are generally molded hollow, owing to its liability of cracking when formed into pieces of any considerable thickness. When plaster of Paris is mixed with lime mortar, in which case it is said to be gauged, the setting is considerably hastened.