Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/567

Rh PLASTER-WORK.] BUILDING 505 of these are Keene .s marble cement and Parian cement. They become excessively hard in a short time, and are cap able of being painted in a few days. The principal com ponent is said to be obtained by the precipitation of alum by an alkali, which gives a white powder of great brilliancy. Tints can be made up with these cements for coloured decorative work. Cements made by the mixture of oil with various substances were formerly much used both here and abroad. The best known in England was called Hamelin s mastic, that in France the mastic de Dhil. These cements being very expensive, and requiring to be constantly painted, have now gone nearly out of use. For outside plastering they form a very fine clean surface, as may be seen in the quadrant in Regent Street and other buildings of that date, but in many instances it has been taken off and Portland cement substituted. Blue lias lime was formerly greatly esteemed as a cement for outside work, but the carelessness of the burners has tended much to limit its use, there being a large pro portion of unclerburnt stone left in it. The workmen also would not take the requisite pains for slaking this lime, and manufacturers therefore ground it, by which operation the core becomes mixed up with the properly burnt material, and the efficiency of the lime is destroyed. The lumps should be broken as small as nutmegs, then immersed in water on a sieve until air-bubbles freely rise to the surface ; the lime so wetted is to be left in a heap covered by damp sand for twenty-four hours, after which time it should be screened and mixed with the proper quantity of sand and the least possible quantity of water. To one of lime may be put not more than from two to two and a half of sand. When slaked it does not increase in bulk. The various coatings of plastering are distinguished thus. On laths, plastering in one coat simply is said to be laid, and in two coats, laid and set. In three-coat plaster ing on laths, however, the first is called the pricking up, the second is said to be floated, and the third set. On brick or stone walls, plastering in one plain coat is termed rendering ; with two coats, a wall is said to be rendered and set ; and in three, rendered, floated, and set. Before beginning to lath a ceiling, the plasterer proves the under face of the joists, to which he has to work, by apply ing a long straight edge, and makes up for any slight in equalities in them, when the work is not to be of a very superior description, by nailing on laths or slips to bring them as nearly even as he can. When the inequalities are great, or if the work is to be of fine quality, he recurs to the carpenter, who takes off inordinate projections with his adze, and nails on properly dressed slips where the joists da not come down low enough, and thus brings the whole to a perfect level. This operation is called fining, or furring. If it be a framed floor with ceiling joists the plaster has to work to, it is tolerably sure to be straight ; but the carpenter must have firred down on the beams or binders to the level of the ceiling joists, unless the ceiling joists have been nailed to the beams or binders, when nothing of this kind is necessary. If a ceiling is to be divided into compartments or panels, the projecting or depending portions must be bracketed or cradled down to receive the laths. It is an important point to be attended to in plastering on laths, and in ceilings particularly, that the laths should be attached to as. small a surface of timber as possible, because the plastering is not supported or upborne by its adhesion or attachment to the wood, but by the keying of the mortar itself, which passes through between the laths, and bends round over them. If then the laths are in constantly recurring contact with thick joists and beams, the keying is as constantly intercepted, and the plastering in all such places must depend entirely oil the portions between them which are properly keyed. Under a single floor, therefore, in which the joists are necessarily thick, a narrow fillet should be nailed along the middle under the whole length of them all, to receive the laths and keep them at a sufficient distance from the timber to allow the plastering to key under it ; thus too the surface may be made more perfectly even, as it is in single floors that inequalities mostly occur. This being all arranged, the plasterer commences lathing. The laths should be of the stronger sort. Thin weak laths, if used in a ceiling, are sure to produce inequalities, by sagging with or yielding to the weight attached to them ; one or two weak ones in a ceiling of otherwise strong laths may be the ruin of the best piece of work. They should be previously sorted, the weak, crooked, and knotty, if there be such, being re served for inferior works, and the best and straightest selected for the work of most importance, so that the workman shall find none to his hand that is not fit to be brought in. Taking a lath that will reach across three or four openings, he strikes a nail into it on one of the intermediate joists, at about three-eighths of an inch from the one before it, and then secures the ends of that and the one that it meets of the last row with one nail, leaving the other end of the lath he has just set to be secured in the same manner with that which shall meet it of the next bay in continuation. It is of importance also that in ceiling-work he pay attention to the bonding of his work. In lathing on quartering par titions and battened walls, the bonding is not a matter of much importance ; nor is the thickness of the timbers behind the latter of so much consequence as in a ceiling, because the toothing which the thickness of the lath itself affords to the plastering is enough to support it vertically ; but, nevertheless, the more complete the keying, even in works of this kind, the better, as the toothing above will not pro tect it from any exciting cause to fall forwards, or away from the laths. The thinner or weaker sort of lath is generally considered sufficiently strong for partitions. &quot;When the lathing is completed, the work is either laid Plastering or pricked up, according as it is to be finished with one, on laths. two, or three coats. Laying is a tolerably thick coat of coarse stuff or lime and hair brought to a tolerably even surface with the trowel only ; for this the mortar must be well tempered, and of moderate consistence, thin or moist enough to pass readily through between the laths, and bend with its own weight over them, and at the same time stiff enough to leave no danger that it will fall apart, a contin gency, however, that in practice frequently occurs in con sequence of badly composed or badly tempered mortar, unduly close lathing, or sufficient force not having been used with properly consistent mortar to force it through and form keys. If the work is to be of two coats, that is, laid and set, when the laying is sufficiently dry, it is roughly swept with a birch broom to roughen its surface, and then the set, a thin coat of fine stuff, is put on. This is done with the common trowel alone, or only assisted by a wetted hog s bristle brush, which the workman uses with his left hand to strike over the surface of the set, while he presses and smoothes it with the trowel in his right. If the laid work should have become very dry, it must be slightly moistened before the set is put on, or the latter, in shrinking, will crack and fall away. This is generally done by sprinkling or throwing the water over the surface from the brush. For floated or three-coat work, the first, or pricking up, is roughly laid on the laths, the principal object being to make the keying complete, and form a layer of mortar on the laths to which the next coat may attach itself. It must, of course, be kept of tolerably equal thickness throughout, and should stand about one- quarter or three-eighths of an inch on the surface of the laths. When it is finished, and while the mortar is still quite moist, the plasterer scratches or scores it all over TY. -- 64