CEMENT PLASTER-HOW TO MIX AND APPLY. Many manufacturers of cements provide direc tions for the proper proportioning of their ma terials. It is taken for granted that their directions are the results of experiments and observations with their products, and they should therefore be considered reliable.
The following, from the annual report for 1904, of the Ohio State Geologist, will be of interest: First coat, one-half inch thick. For best results, the wall should be furred off with strips put on vertically twelve inches apart and well nailed. On these, fasten firmly metal lath. Add fiber to the mortar for lath work. Wet thoroughly the surface to be plastered. Mix one part of non-staining Portland cement with two parts medium sand. one part fine sand and one-half part lime flour. When this coat has set hard, wet the surface thoroughly, and apply the second coat with a wooden float.
Second coat, one-quarter inch thick. Mix one part cement as above, one part fine sand, and two parts medium sand or crushed granite. Before the second coat has set hard, it may be "joined" to present the appear ance of stone work. A small addition of lime flour increases the adhesion of the mortar.
The finished surfaces should he protected for at least two weeks with canvas curtains or bagging saturated with water.
Defects are liable to appear on cement plastered walls, (1) if too much cement is used; (2) if not applied with sufficient moisture; (3) if not troweled sufficiently; (4) if not protected from variations in temperature and draughts of air.
To this a prominent manufacturer of metal lath adds: In some sections a departure from this specification has been found preferable. It is the practice in the New England States, for instance, to staple metal lath directly to the studding, and then plaster with one heavy coat of Portland cement and lime mortar mixed, using one barrel of best Portland cement and three casks of hair and lime mortar made up in the usual manner, as if it were to be applied to wood lath. The lime mortar to be divided into batches so that the Portland cement can be added in small quantities just before using, that the cement may not have time to harden or set before the plasterer can use it.
After this coat has hardened sufficiently, it is back plastered on the key formed by the first coat, putting this back-plaster coat on with the same kind of mixture as the first coat on the outside, and covering the lath by at least one-half inch.
After these two coats have hardened sufficiently and dried out, the second or finish coat can be put on, either by slapdashing, or putting on one heavy coat with trowel finish, or applying any of the various attractive finishes which are possible by the use of cement.
The mixture of this final coat depends on the kind of finish desired ; but it is usually made with one barrel of Portland cement to two barrels of coarse, sharp sand. If a light color is desired, a hodful of lime putty is added to the mixture ; or, if a very rough finish is wanted, a propor tion of pebbles or crushed stone is mixed with the sand and cement. It is difficult to give a certain formula for the finishing coat, as nearly every plasterer or architect has his own ideas as to this finish.
The specification last mentioned is the one generally used on the cheaper grades of houses, from $5,000 to $7,500. On the higher grade of work, the plastering is done in four coats. There is first a scratch coat, which simply fills the meshes of the lath; second, the backing-up coat on the inside; third, what is called the brown coat, which is a heavy coat applied directly to the scratch coat, and which is floated or brought up to a straight, smooth surface, and left somewhat roughened to receive the final coat. The brown coat is often omitted on the cheaper class of houses. It is usually mixed with one barrel of Portland cement to two bar rels of sand, and a hodful of putty.
It may be added that improper gauging of cement and lime often causes an uneven color. Experienced plasterers overcome this easily. One who has done much of this says he thins down his lime putty so that it is so watery as to be used in mixing the cement.
The accompanying table shows the area which can be covered by one barrel of Portland cement mortar of various mixtures, with coats of various thicknesses.