LATHING AND PLASTERING are the processes of finishing walls and ceilings with lath and plaster. The very general practice of plastering walls and ceilings dates back only about a century; pre vious to that time walls and ceilings were either wainscoted, boarded. or covered with canvas or tapestries, or else left unfinished. Brick walls and hollow tile, plastic composition and concrete ceilings and partitions have an affinity for plas ter, and it may be applied directly to them: all other constructions require some form of lathing to serve as a ground to receive and hold the plaster. Practically, all dwelling, of moderate cost, and a large proportion of other buildings, are lathed with wooden lath. Wooden lath is furnished the builder in strips about one-quarter of an inch thick and I% inches wide and 4 feet long.
Lathing is the process of nailing those strips parallel to each other, with one-quarter of an hull space between them to the ceiling or parti tion studding. In recent years the tendency toward fireproof construction has led builders and manufacturers to devise various styles of metal lathing. Coarse-mesh wire cloth, ex panded metal (Fig. 13), and perforated metal (Fig. 14) are the most familiar kinds of metal lathing.. These metal lathings are made in strips or sheets which are cut to the proper dimensions and applied to the ceiling-joists and partition studding by the lather who fastens them with nails or with wire clips of some form.
The purpose of lathing, as already stated, is to form a ground to receive and bold plaster. Plas ters for interior work may be divided into lime plasters and bard-wall plasters, and hard-wall plasters are susceptible of division into natural cement plasters and chemical or patented plas ters. Lime plaster is composed of quicklime, sand, and hair, or some fine fibre. For the best lime plasters the proportion of the ingredients is generally about one part lime and two parts sand, with bushels of hair to each 200 ponnds of lime. The mixing of lime plaster is usually done on the building-site; but in some of the large cities there are special' establishments whose business it is to mix plaster by machinery and furnish it to builders ready for use. The
general mode of procedure in mixing Hine plas ter on the building,-site is as follows: The lime is first slacked in a tight box by adding water and then the hair is added and thoroughly in corporated with the lime; lastly, the sand is added and thoroughly mixed with the lime and hair. Usually. the whole process occupies but one or two hours; but to get the very best plas ter possible, the slacked lime should be allowed to stand from twenty-four hours to seven days be fore the hair and sand are added, and then the mixture should stand about a week; it is then mixed with water to the proper plasticity and immediately applied to the lathing.
Natural-cement plasters are made from certain kinds of earths found in Kansas and Texas, which assume a pulverized form when calcined, and Nvhen mixed with \rater will set like hydrau lie lime or cement, but much more slowly, so that ample time is afforded for applying the plaster. The chemical or patented plasters commonly used arc of a secret composition; but it is known in a general way that they are compounds of gypsum, to which some chemical is added, which has the effect of retarding the naturally quick setting of that material. Plastering on lathing is gen erally done in three coats, known as the first or scratch coat, the second or brown coat, and the third or white coat, skim coat, or finish. The scratch coat is always made rich, and contains plenty of hair or fibre, since it forms the founda tion for the brown and finish coats. The scratch coat is generally put on from three-sixteenths to one-quarter inch thick over the laths, and is pressed by the trowel with sufficient force to squeeze it between and behind the laths so as to form a key or clinch. It is this key which holds the plaster to the laths.