The substances which enter into the composition of mortar depend upon the nature of the surface to be coated, the order in which the layers are applied, and the desired finish. For ordinary work they are lime paste, sand, hair, and plaster of Paris.

Lime is the product of the calcination or burning of limestone. The material is heated in a kiln until it emits a red glow, thus expelling the carbonic acid and moisture; the residue is quicklime, lumps of which, after being removed from the kiln, are called lime shells. In preparing the mortar, the lime shells are deposited in a wooden slaking box, and are liberally sprayed with water; they soon begin to swell, crackle, and fall into a powdery mass. This process is called slaking and the powdered substance is slaked lime; during the process, the lime increases from two to time times in bulk and much heat is given out, which transforms the excess of moisture into steam. Mortar is usually mixed by manual labor, but on extensive works and in cities it is often prepared by mortar mills, a more thorough incorporation of the ingredients and a tougher paste being produced by the machine process.

Sand may be procured from the natural deposits in pits or along river shores. It should be clean; this can be determined by rubbing a moistened quantity of it between the hands; the grains should be sharp and angular, not round and polished. "Where the sand is coarse, it should be screened to the desired fineness, by being passed through a sieve. It should be free of salt, otherwise it will attract and retain moisture; this presence can be detected by tasting. Sand is mixed in the mortar for economy, and to check the excessive shrinkage of the lime paste. The sand in the mortar increases its bulk, while sufficient strength is retained, if each grain is well enveloped in a film of the paste.

Hair is employed to bind the paste together and to render it more tenacious. Cattle or goat hair is used for this purpose, but the latter is considered the best. The hair should be long, free from grease and dirt, of sound quality, and beaten up if matted. Owing to the presence of salt in salted hides, hair taken therefrom is undesirable.

Plaster of Paris is obtained from gypsum, by gentle calcination. It is very soluble in water, which renders it unfit for external use, but it is valuable for cornice molds and enrich-ments, and is also used in several plastic mixtures. The great value of plaster of Paris is that paste made from it rapidly sets and acquires full strength in a few hours. Its volume expands in setting, making it a good material for filling chinks and holes in repair work.

Mixing The Materials

The composition of the successive coats are generally classified as coarse stuff, fine stuff, plasterers' putty, gauged stuff, and stucco. For all of them it is essential that the lime should be thoroughly slaked. In much of the lime used there are more or less overburnt, hard, obstinate nodules which resist the permeation of water, and fail to disintegrate; these must be removed from the lime by screening, otherwise a pitted appearance of the finished work will invariably ensue from the future slaking of the particles.

Coarse stuff, used for the first coat, is composed of from one to two measures of sand to one of slaked lime. The lime paste and hair are well mixed; then, after adding the sand, the mass is worked together with a hoe, until the materials are completely combined. The mixture is then piled in a heap for a week or ten days, to allow it to sour or ferment, the lime expending its heat and becoming effectually slaked. The lime and sand, if mixed while the lime is still hot, produces a much tougher mortar, though some authorities hold that the sand should be added last, after the hair, which, to prevent being charred by hot lime, is always mixed in the mortar after the lime has been cooled. One pound of hair is usually added to every 2 or 3 cu. ft. of mortar, according to requirements, it being essential to add more for ceiling stuff than for walls. The consistency of the prepared mortar should be such that when the paste is made to fold over the edge of the trowel it will hang well together.

Fine stuff is the pure lime which has been slaked to a paste by the addition of a small quantity of water, after which it is further diluted until it is as thin as cream. When the lime held in suspension has subsided, the excess of water is drained off, and the moisture allowed to evaporate until the stuff is sufficiently stiff for use. When desired, a small quantity of white hair is added.

Plasterers' putty, which is always used without hair, is practically fine stuff, but the creamy paste, having been strained through a fine sieve, has become much more velvety.

Gauged stuff consists of about 3/4 of the foregoing putty and about 1/4 of plaster of Paris, which causes the mixture to set quickly, so that it must be immediately used, not more than can be applied in 20 or 30 minutes being prepared. An excess of plaster in the mixture will cause the coat to crack. This is used as a finishing coat for walls and ceilings, and also for running cornices; for the latter work, equal proportions of putty and plaster are used.

Stucco, for interior work, consists of 2/3 fine stuff and 1/3 sand, and is used as a finishing coat, the mixture being whipped and reduced, by the addition of water, to a thin paste.