The plaster used for covering the walls of buildings is a mortar composed of lime or cement and sand, mixed in various proportions, generally with a little hair or some such material to. give it elasticity. It is laid on by hand with a trowel in several thicknesses of about 1/8 to 1/4 inch each, and either on the bare masonry wall or on a special screen of lathing made for it, to either of which it adheres by entering into and keying itself in the joints and openings, and by its adhesive quality. With some variations in the materials and mixing, it is used for exterior and interior work and for ceilings. For the purpose of assisting to keep the interior of the rooms of a house dry, it is advantageous to employ lathing, which being detached from the masonry of the walls, forms a lining, distinct in itself, and not liable to the effect of moisture which may be in the walls. It is of the utmost importance, in plasterers' work, that the lime should be most thoroughly slaked, or the consequence will be blisters thrown out upon the work after it is finished. Many plasterers keep their stuffs a considerable period before they are wanted to be used in the building by which the chance of blistering is much lessened.
When a wall is to be plastered, it is called "rendering "; in other cases the first operation, as in ceilings, partitions, etc, is lathing, nailing the laths to the joists, quarters, or battens. If the laths are of oak, wrought-iron nails must be used for nailing them, but cast-iron nails may be employed if the laths are of fir. The lath is made in 3 or 4 foot lengths, and, according to its thickness, is called single, something less than 1/4 inch thick, lath and half, or double. The first is the thinnest and cheapest, the second is about one-third thicker than the single lath, and the double lath is twice the thickness. When the plasterer laths ceilings, both lengths of laths should be used, by which, in nailing, he will have the opportunity of breaking the joints, which will not only help in improving the general key (or plastering insinuated behind the lath, which spreads there beyond the distance that the laths are apart), but will strengthen the ceiling generally. The thinnest laths may be used in partitions, because in a vertical position the strain of the plaster upon them is not so great; but for ceilings the strongest laths should be employed.
In lathing, the ends of the laths should not be lapped upon each other where they terminate upon a quarter or batten, which is often done to save a row of nails and the trouble of cutting them, for such a practice leaves only J inch for the thickness of the plaster; and if the laths are very crooked, which is frequently the case, sufficient space will not be left to straighten the plaster.
After lathing, the next operation is laying, commonly called plastering. It is the first coat on laths, when the plaster has two coats or set work, and is not scratched with the scratcher, but the surface is roughed by sweeping it with a broom. On brickwork it is also the first coat, and is called rendering. The mere laying or rendering is the most economical sort of plastering, and does for inferior rooms or cottages. What is called pricking up is the first coat of three-coat work upon laths. The material used for it is Coarse Stuff, being only the preparation for a more perfect kind of work.
Coarse stuff is made with chalk-lime prepared as for common mortar, but slaked with a quantity of water, afterwards evaporated, mixed with an equal quantity of clean sharp sand, and ox-hair at the rate of 1 lb. of hair to 3 cub. feet of stuff. After the coat is laid on, it is scored in diagonal directions with a scratcher (the end of a lath), to give it a key or tie for the coat that is to follow it.
Lath layed or plastered and set is only two-coat work, as mentioned under laying, the setting being the gauge or mixture of putty and plaster, or, in common work, of Fine Stuff, with which, when very dry, a little sand is used. Fine stuff is a mortar made of fine white lime exceedingly well slaked with water, or rather formed into a paste in water to make the slaking complete: for some purposes a small quantity of hair is mixed up with it. Fine stuff very carefully prepared, and so completely macerated as to be held in solution in water, which is allowed to evaporate till it is of sufficient consistence for working, is called putty, plasterers' putty.
Setting may be either a second coat upon laying or rendering, or a third coat upon "floating," which will be hereafter described. The term " finishing " is applied to the third coat when of stucco, but "setting" for paper. The setting is spread with the smoothing trowel, which the workman uses with his right hand, while in his left he uses a large flat-formed brush of hog's bristles. As he lays on the putty or set with the trowel, he draws the brush, full of water, backwards and forwards over its surface, thus producing a tolerably fair face for the work.
Work which consists of three coats is called floated: it takes its name from an instrument called a float, which is an implement or rule moved in every direction on the plaster while it is soft, for giving a perfectly plane surface to the second coat of work. Floats are of three sorts: the hand float, which is a short rule that a man by himself may use; the quirk float, which is used on or in angles; and the Derby, which is of such a length as to require two men to use it.
Plaster, float, and set is the term for three coats of plaster on laths. The first or pricking-up coat is of coarse stuff put on with a trowel to form a key behind the laths, and about 1/4 or 3/8 inch thick on the laths: while it is still moist it is scratched or scored all over with the end of a lath in parallel lines 3 or 4 inches apart, the scorings being made as deep as possible without exposing the laths; the rougher the edges are the better, as the object is to produce a good key for the next coat. When the pricking-up coat is sufficiently dry not to yield to pressure in the slightest degree, the second coat or floating is put on. The floating is of fine stuff with a little hair mixed with it; ledges or margins, 6 or 8 inches wide, and extending across the whole width of a ceiling or height of a wall, are made at the angles and at intervals of about 4 feet apart throughout: these must be made perfectly in one plane with each other with the help of straight-edges. These ledges are technically called "screeds." They form gauges for the rest of the work, and when they are a little set the spaces between them are filled up flush, for which a Derby float or a long straight-edge is used. The screeds on ceilings ought to be levelled, and those on the walls plumbed.
When the floating is sufficiently set it is swept with a birch broom for the third coat or setting. The third, or setting coat, should be of plasterers' putty if the ceiling or wall is to be whitened or coloured. If it is to be papered, the third coat should be of fine stuff, with a little hair in it. If it is to be painted, the third coat should be of bastard stucco trowelled.
Bastard stucco is of three coats, the first is roughing in or rendering, the second is floating, as in trowelled stucco; but the finishing coat contains a small quantity of hair behind the sand. This work is not hand-floated, and the trowelling is done with less labour than what is termed trowelled stucco.
Trowelled stucco, which is the best sort of plastering for the reception of paint, is formed on a floated coat of work, and such floating should be as dry as possible before the stucco is applied. In the last process, the plasterer uses the hand float, which is made of a piece of half-inch deal, about 9 inches long and 3 inches wide, planed smooth, with its lower edges a little rounded off, and having a handle on the upper surface. The ground to be stuccoed being made as smooth as possible, the stucco is spread upon it to the extent of 4 or 5 feet square, and moistening it continually with a brush as he proceeds, the workman trowels its surface with the float, alternately sprinkling and rubbing the face of the stucco, till the whole is reduced to a fine even surface. Thus, by small portions at a time, he proceeds till the whole is completed. The water applied to it has the effect of hardening the face of the stucco, which, when finished, becomes as smooth as glass.
Ceilings are set in two different ways; that is the best wherein the setting coat is composed of plaster and putty, commonly called gauge. Common ceilings are formed with plaster without hair, as in the finishing coat for walls set for paper.
Pugging is plaster laid on boards, fitted in between the joists of the floor to prevent the passage of sound between two stories, and is executed with coarse stuff. In the country, for the interior coating of dwellings and outbuildings, a species of plastering is used called " roughcast." It is cheaper than stucco or Parker's cement, and therefore suitable to such purposes. In the process of executing it, the wall is first pricked up with a coat of lime and hair, on which, when tolerably well set, a second coat is laid of the same materials as the first, both as smooth as possible. As fast as the workman finishes this surface, another follows him with a pailful of the roughcast, with which he bespatters the new plastering, so that the whole dries together. The roughcast is a composition of small gravel, finely washed, to free it from all earthy particles, and mixed with pure lime and water in a state of semi-fluidity. It is thrown from the pail upon the wall, with a wooden float, about 5 or 6 inches long, and as many wide, formed of 1/2-inch deal, and fitted with a round deal handle.
With this tool the plasterer throws on the roughcast with his right hand, while in his left he holds a common white-washers' brush dipped in the roughcast, with which he brushes and colours the mortar and the roughcast already spread, to give them, when finished, a uniform colour and appearance.