The plasterer requires a great variety of mouldings, ornaments, pateras, flowers, and other enrichments for the decoration of his work.
The ornament is in either case first modelled in clay and well oiled.
In making wax moulds, the wax is melted, mixed with rosin, and poured in upon the model, arrangement having been made to prevent its escape; the whole is then steeped in water, and the wax becomes detached in one mass.
In casting, the plaster in a semi-fluid state is dabbed with a brush into the mould.
The oil and resin are melted together and added to the glue, which has been dissolved in water separately. This mixture is then poured upon pounded whiting, well mixed, and kneaded up with it to the consistency of dough.
When used the material is warmed to make it soft, and is forced into boxwood moulds carved to the patterns required.
Papier-Mache is a much lighter material for ornaments than either composition or plaster, and it is much used for the purpose.
Cuttings of paper are boiled down and beaten into a paste, mixed with size, placed in a mould of metal or sulphur, and pressed by a counter-mould at the back, so as to be reduced to a thickness of about1 /4 inch, the inner surface being parallel to the outer surface, and roughly formed to the same pattern.
In some cases a composition of pulp of paper and rosin is first placed in the mould. This adheres to the paper ornaments moulded as above described, and takes the lines and arrises of the mould more sharply than the paper alone would do.
Carton Pierre is a species of papier mache made with pulp of paper, whiting, and size, pressed into plaster moulds.
Fibrous Plaster consists of a thin coating of plaster of Paris on a coarse canvas backing stretched on a light framework, and formed into slabs.
This material has great advantages. Large surfaces can be quickly covered without much preparation for fixing, as it is less than 1/4 the weight of plaster, and it can, if required, be painted at once.
The material used for Dennett's patent fireproof construction is a concrete of broken stone or brick imbedded in a matrix of plaster produced by calcining gypsum at a strong red heat (see p. 372, Part II.) Being fireproof, it is much used for theatres. It sets at about the same rate as ordinary Portland cement, and attains a strength nearly equal to that of the original gypsum.