Plaster is the popular name for the ordinary covering of lime and sand applied to internal walls. Technically, the word "plaster" often signifies plaster of Paris. Ordinary plaster consists of slaked lime and sand. The lime is a "fat" lime, devoid of hydraulic properties, and slaked by being mixed with a large quantity of water and allowed to stand in a "pit" for some weeks, - the longer the better. The sand should be free from clay, soot, organic matter, and other impurities, and must be washed before being used if these are present. Sea-sand must be avoided, as the salt in it has a great affinity for moisture. "Two-coat" work consists of a first coat of coarse stuff- - i.e. slaked lime and sand mixed in the proportion of about one to three, a little cow-hair being usually added to bind the mixture together, - and a second and much thinner coat of fine stuff which contains only a little sand. "Three-coat" work ensures a truer and better surface, and consists of a first or "rendering" coat, a second or "floating" coat, and a third or "setting" coat The last coat may be finished either with a wooden float or a steel trowel, the latter giving the smoother surface.

On ceilings and wood partitions, it is necessary to provide a backing, to which the plaster will adhere. This is usually formed with laths of oak or fir about an inch wide, and nailed to the studs or ceiling-joists about a quarter of an inch apart. Single laths are about 3/16-inch thick, lath-and-half -inch, and double laths -ineh. More fire-resisting substitutes for wood laths may be obtained in the form of Johnson's metal lathing, Banks's "Helical" metal lathing, and the "Expanded" metal lathing, all of which have been severely tested in actual fires. Another form of metal lathing is the "Jhilmil", but this contains much more metal than any of the others.

Ordinarv plaster is undoubtedly porous, and absorbs both moisture and organic matter. Sir Douglas Galton says that "in a discussion, in 1862, in the French Academy of Medicine, a case was mentioned in which an analysis has been made of the plaster of a hospital wall, and 46 per cent of organic matter was found in the plaster". This is scarcely credible, but undoubtedly contamination by organic impurities given off by the lungs and skin does take place, and may prove a source of discomfort and ill-health. A harder and more impervious wall-covering may be obtained by using either Portland cement, or one of the so-called "cements" which have plaster of Paris for their base, - Martin's. Keene's, Parian, and Robinson's cements. These have the further advantage that they set at once and can be painted in a few hours after being laid, whereas ordinary plaster frequently takes weeks to harden, and cannot be satisfactorily painted for months.

The danger of occupying newly-plastered houses is a very real one, and it would be a blessing if sanitary authorities were careful to withhold certificates as to the suitability of houses for occupation, until the walls were thoroughly dry. Dr. Richardson has said: " I once visited a new and pretty row of houses in a London suburb to see a young lady there who was suffering from pulmonary consumption. The house was literally saturated with moisture. This patient died from the disease that had been lighted into activity there. On making further inquiries, I found that in the same row of houses, twenty in number, there occurred within the first two years of their occupation six other instances of pulmonary consumption and fourteen instances of acute rheumatic fever." Here is the case of another lady, - a confirmed cripple from rheumatic disease following upon acute rheumatic fever. "Newly married, she and her husband bought a new house, which, in their desire to settle quickly, they inhabited while the walls were still bedewed with moisture. She sickened with acute rheumatic fever, and never fully recovered from its effects. Worse than all, every one of her children - and she gave birth to seven after her attack - were affected with rheumatic disease, three dying from heart affection dependant upon the rheumatic constitution." Before occupying a new house, fires should be constantly maintained in all the rooms for several weeks, and streams of air should be passed through the rooms, by keeping all windows and internal doors constantly open, whenever the weather permits. Sunshine and change of air are powerful aids to dryness; indeed, without change of air, fires will be of little service. If the bricks and stone, of which the house was built, were thoroughly soaked with moisture before the plaster was applied, drying may be a question not of weeks but of months.

Plaster moulds and enrichments are often receptive of dust and difficult to clean, but undoubtedly a great change for the better is now taking place in this respect. The days of deep hollows in cornices, and of "bold" enrichments, appear to be numbered, and taste has reverted to the more refined detail of a century or two ago. A simple but effective cornice, without any dirt-catching members, was illustrated in Fig. 70, page 135, and others arc shown in Plates III. and V. The proper treatment of plaster lies undoubtedly in elaboration of surface, and not in constructing massive beams and ornament in high relief.

Fibrous plaster has a plaster face on a canvas backing, and can be obtained in plain slabs of various sizes, which are simply fixed to the wood ceiling-joists or studs by screws. A thin coat of plaster of Paris, applied to the whole, covers the joints and renders the surface uniform. A great .saving of time is effected by the use of fibrous plaster Blabs, and the house can be safely occupied much sooner than when ordinary plaster, or even cement, is used. Ornamental til nous-plaster slabs are now made in endless variety to architects' designs, and are fixed to form the ceilings and friezes of important rooms.

Glazed wall-tiles are now largely used in bath-rooms, lavatories, water-closets sculleries, pantries, vestibules and halls, &c; they are made in numerous plain colours, or decorated and embossed in a variety of ways, the more expensive tiles being hand-painted. They are laid on walls on a bed of quick-setting cement, the joints of the brickwork being first raked out to afford a key for the cement. The backs of the tiles have sometimes undercut recesses, which, when the tiles are pressed into the wet cement, obtain a firm hold, and prevent the tiles falling off. Wood partitions, on which tiles have to be laid, may first be boarded, and the tiles fixed by brass-headed screws at the corners, each screw head catching the corners of four tiles. Special tiles, however, have been devised, which dispense with these visible screw Fig. 97 illustrates Halls new improved patent hanging-tiles, fixed to wood battens by nails or screws passing through the holes CC. The two nails in each tile are covered by the tiles in the coarse above, When these tiles are used for walls, they are bedded on neat Portland cement, and (irmly secured by pressing the tile against the wet cement, so that some of the cement passes through the splayed holes at a a, forming a dove-tail key. The tiles are made in stretchers (9" x 27/8" on face),headers (4." x 27/8' on face), and in square and bull-nosed quoins; tiles 6" x 6* on the nice are also made.

Fig. 97.   Hall's New Improved Patent Hanging t&ea.

A new material, named "Opalite", has recently been introduced for wall-surfaces. It is a glazed plate, about one-sixteenth of an inch thick, and can be attached to walls after the manner of tiles. The plates need careful bedding, or cracks will be sure to occur sooner or later.

Fig. 97. - Hall's New Improved Patent Hanging-t&ea.