This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Brown Coat. The second coat, consisting of fine stuff, to which a little hair is sometimes added, is applied from 1/4 to 3/8 inch thick, when the scratch coat has become sufficiently firm to resist pressure. The second layer is frequently called the floated coat, because its surface is worked by means of board-shaped trowels, called floats; it is also known as the straightening coat, since, by its application, all the wall surfaces are straightened and made true. On walls, if the grounds are set true and the surfaces are not too large, the plastering can usually be brought to a true plane without much difficulty. On the ceilings, however, there is nothing to guide the plasterer, and in many cases, if great care is not taken, the ceilings have a rolling surface, as may be seen at the edges.
In order to get perfectly level walls and ceilings, especially in cases where the grounds are insufficient, resort is had to what is known as screcding. Screeds are plaster bands, 5 or 6 inches in width, formed on the surface to be floated. The surfaces adjacent to the angles are carefully plumbed up from the plaster grounds e, Fig. 105, but kept about 1/8 inch back from the face, to allow for the finishing coat, and vertical screeds f are formed by means of the straightedge g. Similar screeds h are formed along the ceiling angles; these screeds are made straight, and coincide with those at the opposite angles. Intermediate horizontal or vertical screeds, as i and j, are then formed between the screeds adjacent to the ceiling and the plaster grounds, as preferred by the plasterer; these are usually placed from 4 to 8 feet apart, and are gauged to line by means of the straightedge. The screeds thus form a system of framing which has been reduced to a true plane; the panels may then be filled in flush, in line with the screeds, and firmly rubbed down with the two-handled, or derby, float k. The surface is then worked over with a wood hand float, the coat being firmly compacted, by incessant rubbing; when the coat becomes dry, during the process, it is moistened with water, applied by a wide brush. A close, firm layer can be obtained only by the thorough, laborious operation of pressing and rubbing the particles of the mortar together; and herein lies the secret of strong, durable work - plenty of "elbow grease." In order to form a rough base for the subsequent coat, the surface is scratched over with a broom. The ceiling surfaces are treated in a similar manner, and the screeds are carefully leveled so as to secure true and level planes.
The space between the plaster grounds and the floor is usually finished with a scratch and a brown coat of plaster, so as to prevent air-currents entering the room through the spaces between the furring strips; in cheap work this filling is omitted, the spaces being covered by the finished wood base.
233. Very often the brown coat is put on immediately after the scratch coat, without allowing time for the latter to dry; this is known as green work. In such a case, the first coat is made very rich, while the brown coat contains a large proportion of sand, and is worked into the first coat so as to really form but one. While it saves time, this practice cannot be commended; it is much better to allow the scratch coat to dry before the brown coat is put on, although more labor and lime are thus required. Another objection to green work is that the excess of moisture causes the laths to swell badly, which, in drying, shrink and produce cracks in the plastering. Nearly all lime plastering is green work, unless otherwise specified.
234. Cornices are usually molded before the finishing coat is put on; the operation of making them is about as follows: Longitudinal strips b, b are first attached to the wall, as at b, Fig. 106, or f g, Fig. 103 (u), on which the mold guide runs. Sometimes a strip is also attached to the ceiling, but more often the ceiling guide is merely a line, as de, Fig. 103 (u). The coarse stuff is made to conform to the approximate profile with a muffled mold, that is, by forming a layer of plaster of Paris along the edge of the mold, about 1/8 inch in thickness, or an extended profile can be cut out of zinc and attached, temporarily, to the correct mold. The mold is placed in position and pushed along the angle of the wall, as indicated in Fig. 106. When the coarse stuff has been correctly profiled, the surface is coated with gauged stuff and carefully worked over with the correct mold, until an exact and perfect finish is obtained. The internal and external angles cannot be finished by means of the molds, but require to be carefully molded and mitered by hand, using jointing tools, such as shown in Fig. 103. Some plasterers prefer to push the mold with the left hand, instead of with the right, as shown in the Figure, so that they can handle the trowel with the right hand, when applying the stuff, to make up any deficiency; therefore the molds require to be made to suit the direction in which they are intended to be driven.
235. Sometimes the clinch, or key, is reinforced and the plaster held more firmly in place by the use of projecting spikes or large nails, driven into the wall or ceiling before the mortar is applied, as in Fig. 107, in which a shows the mortar; b, the laths; c, the spikes; and d, the finished surface.
236. When the cornice projects considerably, the angle must be blocked and lathed, to reduce the quantity of plaster required, as there should be no thickness of plaster much over 3 inches. This method is represented in Fig. 108, in which a shows one of the blocks which are nailed to the floor joists; b, the lathing; c, the cornice; d, a dentil course; and e, an egg-and-dart molding. If the cornice is to be ornamented, as at d and e, recesses are left in the plastering, to receive the pieces to be inserted, which are formed separately and then stuck in place by-liquid plaster.
When there is much ornament, it is cheaper to cast the cornice of plaster of Paris in sections 2 feet or more long; these may be attached to the wall by thin plaster. Careful work is required to make the molded pieces come together, or match properly.
237. Centerpieces, consisting only of plain circular moldings, are formed in the same way as cornices, except. that the mold, or templet, is so fastened as to swing around the center of the ornament.
When decorated centerpieces are used, they are usually cast in a mold and stuck on the ceilings with liquid plaster of Paris. Nearly all kinds of ornamental plastering, such as paneled ceilings, bas-reliefs, imitations of foliage, etc., may be easily cast of plaster of Paris, and serve the purpose as well as more costly decorations.