One-Coat Work ; known as "Lath and Plaster one Coat;" or, "Lath and Lay" - This consists of a layer of "coarse stuff" of an uniform thickness, spread over the laths with a smooth and even surface. The plaster should be stiff enough to hold together, but just sufficiently soft to pass between the laths, being worked well in between them with the point of the trowel, and bulging out behind the laths into excrescences, which form a key, and keep the plaster in position.
Two-Coat Work; described as "Lath, Plaster, and Set;" or, 'Lath, Lay, and Set."
The first coat is laid upon the laths as above described, but the surface, instead of being smoothed, is roughed over by scratching it with a birch-broom, so as to form a key for the second coat.
The second coat, or "setting," is a thin layer of fine stuff, or putty, or gauged stuff, and should not be trowelled on till the first layer is stiff. If the latter has become very dry, it must be moistened before the second coat is applied, or the latter in shrinking will have its moisture sucked out, crack, and fall away. As the fine stuff is laid on, the surface is smoothed by drawing backwards and forwards over it the wet brush used for damping the first coat.
Described as "Lath, Plaster, Float, and Set;" or, "Lath, Lay, Float, and Set."
Pricking-up is the name given in this case to the first coat, which is laid as before described ; but in order to form a good key for the next coat the surface is scored over with the point of a lath in deep scratches, crossing each other diagonally in sets of parallel lines about 3 or 4 inches apart.
Scratching tools, with several points, are sometimes used.
Floating) - The second, or "floated" coat, is applied when the pricking-up is sufficiently dry to resist pressure.
It consists of fine stuff, with the addition of a little hair, and derives its name from its being laid on with "floats" in the following manner: -
In order to ensure the surface of the plaster being in a true plane, narrow bands or "screeds" of plaster, about 6 or 7 inches wide, are formed at the angles, and at intervals of from 4 to 10 feeton the wall or ceiling. The surfaces of these are then brought into the required plane by passing long straight-edges over them.
1 Sc. Straightening.
Horizontal screeds for ceilings should moreover be levelled, and vertical screeds "plumbed" up from the skirting grounds (see page 80), before proceeding farther.
The spaces between the screeds are then "filled out" flush with the fine stuff, and smoothed off with straight-edges, or with a large flat board, having two handles at the back, and known as a "Derby float."
The surface is then gone over with a smaller hand float, and any lefects made good by adding a little soft stuff.
Before applying the third coat or setting, the floated surface should be scratched over with a broom, and then allowed to become perfectly dry.
If the surface is to be papered, it should be " set with fine stuff;" if it is to be whitened, it should be "set with putty and washed sand ;" and if it is to be painted, it should be finished with "trowelled stucco" or plaster.
"Set with Fine Stuff." - For surfaces to be papered the setting coat should be of fine stuff containing a little hair, and the finished work would be described as "Lathed, Plastered, Floated, and Set with Fine Stuff."
"Set with Putty and Plaster." - If the wall or ceiling is to be whitened or coloured, the third coat should be of plasterers' putty mixed with a little fine sand, and sometimes with a little white hair.
If required to set quickly, especially in damp weather, from 1/6 to 1/3 plaster of Paris is added to the stuff, which must be gauged (or mixed) in small quantities (see Gauged Stuff, p. 179).
Great care should be taken to ascertain that the floated coat is dry before this setting is applied, otherwise the coats will shrink unequally, and the last coat will be full of cracks.
Rendering is the term used for the process of applying plaster or cements to the naked surface of walls.
With regard to plaster, it is applied in exactly the same way as upon laths, excepting a slight difference in the first coat.
The surface of the wall to be rendered should be rough so as to form a key to which the plaster will firmly adhere. This may be secured by leaving the mortar joints unstruck and protruding when the wall is built; or the joints may be raked and the face hacked and picked over to give it the necessary roughness.
Rough Rendering is the first coat laid to receive more finished work.
It is of coarse stuff, but contains a little less hair than that used on laths, and is applied in a moister state, which causes it to adhere better to the wall.
The holes and crevices in the wall should be entirely filled up in applying this coat, but the surface of the plaster need not be scratched or scored over.
Floating and Setting are performed in exactly the same way as upon laths.
Gauged Work is formed by the addition of a proportion of plaster of Paris to any coat of plaster, in order to cause it to set more rapidly. Unless the process is very carefully conducted cracks will occur in the plaster. The quantity of plaster added depends upon the rapidity of setting required, the dampness of the weather, etc.
If they do not project more than 2 inches, a backing of coarse stuff will be sufficient to support them; but if the projection is 6 or 8 inches, or more, brackets of wood, roughly cut to the section of the intended cornice, must be fixed along the wall at intervals of 10 or 12 inches. Upon these laths are nailed and "pricked up " - that is, covered with a thick coat of coarse stuff, so that a rough edition of the future cornice is produced. A mould made of zinc, or of beech with zinc or brass edges, is then for the time "muffled" by covering the profile edge with a layer of plaster of Paris about 1/8 inch thick. The mould is then drawn along over the surface of the rough cornice of coarse stuff already formed. It is guided by battens fixed along the lines where the cornice will cut the ceiling and wall, and the effect produced by it is to remove the superfluous stuff and leave the cornice moulded approximately to the form required, the surface all over being about 1/8 inch within the intended profile. The muffling is then removed from the mould, and the surface of the cornice covered with gauged stuff, over which the mould is worked until the exact form of the cornice is produced.
As the stuff sets very quickly, it should be frequently sprinkled, and portions between projections or other breaks in the line should be finished off at once.
Where a portion of the moulding projects 3 or 4 inches beyond the general surface, it may be sustained by nails driven into the wall or bracket about 6 inches apart, and connected by tarred string.
Mitres in angles and small breaks are finished by hand, and indentations are left for enrichments, which may be cast in plaster of Paris, or composition, and cemented into their place.
These indentations in the plaster are formed by projections left on the mould.
Ornaments of various kinds are made of plaster of Paris cast in bees'-wax moulds. When large and heavy they should be secured by screws to woodwork.
Carton pierre, or Papier Mache, consisting of paper formed into pulp and forced into moulds, is also used for ornaments. Though not capable of receiving so sharp an outline as plaster of Paris, it is more easily transported without breaking, lighter, easier to fix, and very useful - especially in the country, where skilled workmen to cast plaster ornaments are not easily obtainable.
There are several other materials used for making the mouldings and ornaments required by the plasterer, which it would be beyond the province of these Notes to describe.
Fig. 371 shows a cornice at the angle of a room in sectional elevation.
B is the rough bracket cut approximately to the shape of the cornice. This bracket is attached to the fillets //, which are fixed as shown, and carefully levelled. In some cases the bracket is nailed to the bottom or sides of the ceiling joists, and it is very frequently built into the wall, as shown at C in Fig. 372.
11 are the laths nailed to the bracket to form the surface and key for the plastering.
m is the moulding, which is made separately, and fixed (after the cornice is run and set) into the recess left for it, shown in Fig. 371 in dotted lines.
An ornament is also shown in the recess left in the soffit of the cornice. Holes are broken through the plaster forming this soffit, so that the soft plaster at the back of the ornament may pass between the laths themselves, and thus form a key, which secures it directly to the lathing.
The coved portion of the cornice is sometimes formed in papier mache or light plaster casts, and fixed without any supporting brackets, being fitted in between the projecting mouldings above and below, and secured with plaster of Paris.
Large Coved Cornices are supported by brackets or cradling, built up of pieces of board. Two or more are used, according to the size of the cornice, and the surface is covered with lathing, and finished in the same way as small cornices.
Fig. 3721 shows a bracket built up in three pieces, which about against one another, other pieces being nailed over the joints, the upper extremity being nailed to the joists of the floor above, and the lower end resting upon the rough bracket of the projecting cornice below, which is built into the wall.
1 The scale of this figure is so small that neither the lathing nor counter-lathing is shown.
This figure shows also the method of covering a beam projecting below the surface of the ceiling. The sides and soffit of the beam are counter-lathed, plastered, and decorated by mouldings struck on the plaster, or attached to it.
In some cases the beam is covered with cradling, which makes it up to the size required for the design of the decoration, and, by keeping the laths at a distance, affords a key without counter-lathing.
"Very large projecting mouldings and cornices inside buildings are even made of coarse canvas strained over a light framework, and washed over with gauged stuff. They are easily carried up and fixed in position."1