When it is desired to finish plaster walls, ceilings, columns, etc., with a very hard and highly polished surface, Keene's cement is generally used for the finishing coat. This cement is a plaster produced (in England) by recalcining plaster of Paris after soaking it in a saturated solution of alum. This material is very hard and capable of taking a high polish, and walls finished with it may be sponged with soft water without injury.
It is especially valuable for finishing plastered columns, the lower portions of walls, and wherever the plaster is liable to injury from contact with furniture, etc. It is also used in the manufacture of artificial marble.
The manufacturers of King's Superfine Windsor Cement claim that for finishing walls it is equal to the imported Keene's cement; it is considerably less expensive.
Neither of these materials should be used in situations much exposed to the weather, on account of their solubility.
351. Scagliola is a coating applied to walls, columns, etc., to imitate marble. The base or ground work is generally of rich lime mortar containing a large proportion of hair. After this has set and is quite dry it is covered with a floated coat, consisting of plaster of Paris or Keene's cement, mixed with various coloring matters in a solution of glue or isinglass, to give greater solidity and to prevent the plaster of Paris from setting too quickly. When the surface is thoroughly hard it is rubbed with pumice stone and then polished until it looks like polished marble. Columns can be made in this way that can hardly be detected by the eye from marble.
Imitation marble, when in flat slabs, is commonly made on sheets of plate glass. Threads of floss silk, which have been dipped into the veining colors, previously mixed to a semi-fluid state with plaster of Paris, are placed upon a sheet of plate glass so as to resemble the veins in the marble to be imitated. Upon this the body color of the marble is placed by hand. The silk is then withdrawn and dry plaster of Paris is sprinkled over to take up the excess of moisture and to give the plaster the proper consistency. A backing of cement or plaster of Paris is then applied of any desired thickness. Canvas is sometimes placed in the backing to give greater strength. After removing from the glass the slab is polished and set in place in the same manner as the genuine material. This work naturally requires much skill in the workman, besides practice and experience
A great deal of scagliola has been used in Europe, and in recent years several companies have been formed in America for making artificial marble, which is essentially the same thing. For interior work scagliola should be as durable as marble, and there are columns of it in Europe several hundred years old. It should not be used on the exterior of buildings, as it will not bear exposure to the weather.
352. 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. For casts about ¼ inch of plaster is put in the mould, and the canvas is then put on the back and slightly pressed into the plaster. Fibrous plaster is very light and strong, and can be easily handled without breaking. It is extensively used in England for ornamental work, and in Brazil it is said to be used extensively for external work.
Carton Pierre is a material used for making raised ornaments for wall and ceiling decoration. It is composed of whiting mixed with glue and the pulp of paper, rags and sometimes hemp, which is forced into plaster or gelatine moulds, backed with paper, and then removed to a drying room to harden. It is much stronger and lighter than common plaster of Paris ornaments, and is not so liable to chip or break if struck with anything.
Ornaments of carton pierre (under different names) are now extensively used in this country for decorating rooms, mantels, etc., and also to some extent on the exterior of buildings. If kept painted there appears to be no reason why it should not last for many years, except in very exposed positions.