349. This term, as commonly used in this country, refers to ornamental interior plaster work, such as cornices, mouldings, centrepieces, etc. For such work a mixture of lime paste and plaster of Paris is used, except for cast work, which is made entirely of plaster of Paris.
Plaster of Paris is produced by the gentle calcination of gypsum to a point short of the expulsion of the whole of the moisture. Paste made from it sets in a few minutes, and attains its full strength in an hour or two. At the time of setting it expands in volume, which makes it especially valuable for taking casts and for making cast ornaments for walls and ceilings, and also for patching and repairing ordinary plaster work.
When added to lime mortar, plaster of Paris causes the mortar to set or harden very quickly, and for this reason it is often mixed with mortar to be used for patching or repairing, or where it is necessary to have the plaster harden very quickly. When this is done it is called "gauged work."
Plaster of Paris is very liable to crack when used clear and in considerable thickness. Cast ornaments made of it are therefore usually made hollow, or with a thin shell. For work that is to be run, or worked by hand, it cannot be used clear, as it sets too quickly. It is for this reason that lime putty is mixed with it.
For mouldings, cornices, etc., about 2 parts of plaster of Paris to 1 of lime paste is used.
Plain mouldings, whether in a cornice, centrepiece, or on the wall or ceiling, are usually "run" in place by hand. The process consists in placing on the surface of the wall or ceiling a sufficient body of plaster and forming the mould by running along it a sheet iron template, cut to the reverse profile of the mould. The template is stiffened by wooden cleats, and provided with struts to keep the plane of the template always perpendicular to the plane of the surface on which the mould is run. The stucco work is always run before the finishing coat of plaster is applied, as it is necessary to fasten light pine straight-edges on the wall to form guides for the templates. In running the moulding two men are generally required, one to put on the plaster as it is needed and the other to work the template, which generally has to be worked back and forth several times before the moulding is finished.
The whole moulding or cornice between any two breaks or projections should be completed at once, so that the entire length may be uniform in shape and shade.
The mitres at the angles, both internal and external, have to be finished by hand, using a small trowel and straight-edge.
If the cornice or moulding contains much ornamental work, it is cheaper to cast it in sections of about 2 feet in length, and attach to the wall by means of liquid plaster of Paris. It requires great care in cast work to have the sections join nicely, so that the members will present a perfectly straight line.
If there are only one or two enriched members, the rest of the moulding or cornice may be run in the usual way, leaving sinkings to receive the enriched members, which are then cast and stuck in place, as at A, Fig. 232.
In designing cornices or belt mouldings, care should be taken not to have over 3 inches in thickness of plaster at any point. If the mouldings have greater than this the wall or angle should be blocked and lathed, as in Fig. 232, so as to reduce the amount of plaster required to a minimum. When the projection is only about 3½ or 4 inches, the back may be formed of brown mortar (Fig. 233), containing a little plaster of Paris, and held in place by projecting spikes or large nails, driven into the wall or ceiling before the mortar is put on.
Centre ornaments, when consisting only of plain circular mouldings, are run in the same way as other moulded work, except that the template is attached to a piece of wood which is pivoted at the centre of the ornament. Enriched centres are cast in a mould and stuck to the ceiling after the finish coat is on.
All kinds of ornaments, such as paneled ceilings, bas-reliefs, imitations of foliage, etc., may readily be executed in plaster of Paris, and when the ornament is placed in such positions that it cannot readily be injured by objects in the room, it answers as well as harder and more expensive materials.
Since hard wood finish has become so prevalent, however, it has largely supplanted the plaster cornices that were so common fifty years ago.
Stucco work is generally included in the plasterer's specifications. As it is much more expensive than ordinary plastering, the quantity and character of it should be clearly indicated on the drawings and in the specifications, and by full size details.
For enriched work the architect should require that the models be approved by him before the casts are made.