If avoidable, the artificial drying of plaster by salamanders should not be employed; natural drying by sun and air is, under all circum stances, preferable. The salamander not only dries the room in which it is placed, too quickly—especially the ceiling above—but fills the air and the plaster itself with gas fumes, and, by steaming, is frequently the cause of the rotting of plaster or hair, thus reducing its vitality and life. Heating a house to dry out the plaster by means of the regularly installed heating plant, is preferable to the use of salamanders, the chief objection in this case being occasioned by the unduly rapid drying-out of wall plaster back of or above registers and radiators. The situation is helped if the radiator is set out from the wall and some screen is placed between it and the plaster. A screen may also be employed against the wall over a hot-air register; but there is no means of protecting the plaster on either side of a partition through which a hot-air or steam pipe passes. Such plaster is bound to be severely strained by being dried too quickly.
If plaster is frozen when wet, it is likely to loosen up and injure the whole mass so that it may eventually fall off. The effects of freezing are less troublesome if the wall is frozen after it is dried and has once set. If only slightly frosted, and thawed immediately and floated again, it may often be saved, the effect in that case being not much different from what it would be if the wall had been surface moistened and refloated.
Plaster Moulding. Plaster mouldings upon ceilings and walls are less frequently employed now than a few years ago, when, espe cially at the intersection of wall and ceiling, a heavy cornice of plaster was the common method of finish. Nowadays a cornice of wood is more commonly used.
Briefly described, the running of a moulded plaster cornice is as follows: Two parallel strips, or screeds, are run on the ceiling and the side wall, with their nearer edges evenly straightened. These edges are then fitted to the mould—a piece of metal cut out to a reversed section of the cornice outline. The mould is run along the strips fastened to the wall for guiding it, the lower edge being cut out and fitted to run upon them.
The plaster necessary to fill up the mouldings of the cornice may be tied back to the wall and ceiling by rows of nails driven so as to stand at about the location of its greatest thickness; while a strip of metal lath, filling in the angle upon projecting furrings, will offer the best possible clinch, and will help to reduce the thickness of the plaster and render its drying and shrinkage more equable and its sur face less likely to crack.
When all is ready, enough putty and plaster are gauged hi about equal parts to run the cornice down the length of one side of the room.
The moulding form is then rested upon the supporting and guiding strip against the wall, and drawn along from right to left, pressed against the mass of mortar which is thrown into the angle just ahead of it by the trowel, the space immediately in front of the moulded strip being kept sufficiently full of plaster mortar to fill out the moulding entirely at all times. When the length is completed, or the gauged material is used up, the mould is moved back and forth along the length of cornice that has just been run, scraping away all the plaster except that included within the outline of the mould.
Where hollows occur, the gauged material scraped off by the mould should at once be thrown on again at these places, so that they may be immediately filled and brought up to the right section outline by again running the mould over these portions. The gauged putty will set in a few moments, and each side of the room or section of the moulding must be run and completed or filled out very rapidly. The corners at the angles of the room may be filled in by hand, or a section of the mould may be separately run upon the floor, sawn in a mitre box, mitred and fitted in place upon the wall, the joint between the cast and run moulding being then carefully patched and evened off.
The extra amount of plaster included in the thickness of extreme projecting mouldings is the cause of occasional surface cracking; while other cracks are occasioned by the settlement, shrinkage, and move• went of the house frame. For these and other reasons, it is now gen erally considered that a wooden cornice, despite its defects of shrink age, is better suited than plaster to this purpose.
Finally, the moulding may be sprinkled with the brush and the mould may be run over it several times more, ending by finishing with a brush so as to give the moulding a gloss just as on the wall plastering. The same process is repeated for different kinds of plas ter moulding, merely varying the method to provide for the different conditions set by circumstances, a circular moulding around the lighting outlet in the middle of the room, for instance, being swung from a peg driven into the center of the gas pipe or outlet box. Other kinds of plaster mouldings are run by unimportant variations of the processes described.
Cast ornaments are made separately in moulds, into which the plaster is poured. Most of these separate moulds are made of plaster hardened with glue or shellac, or surfaced with beeswax, and are generally oiled before being used. Plaster ornaments are fastened in place with fresh plaster or glue; occasionally a few screws are used, in which case the heads should be countersunk and covered in with plaster so as not to show.