This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
245. Plaster ornaments are now much less used than formerly. They are usually made of plaster of Paris and lime, but for cast work, only the former is used.
A light and strong material, used to some extent for ornamental work, is made by applying a thin layer of plaster of Paris to pieces of stretched canvas, forming a kind of board. If casts are to be made, the plaster is poured into the molds, and, while it is still soft, the canvas backing is pressed slightly into it.
Another material, known as carton pierre and by various other names, is also much used for ornamental work. It is composed of whiting (ground chalk), paper pulp, hemp fiber, etc. mixed with glue, and pressed into molds, backed with strong paper, and dried. Ornaments made of this substance are much tougher and lighter than if cast in plaster of Paris, and are considerably used for interior decorations, and also for outside work; for the latter use they should be painted.
246. Scagliola is a material applied to columns, walls, etc., to imitate marble. The first, or ground, coat consists of lime mortar having a large proportion of hair mixed with it; this is applied in the usual way and allowed to become thoroughly dry. Another coat is then applied, composed of Keene's cement, or plaster of Paris mixed with glue, or gelatine, to make it more dense and compact, and to retard the setting. Various coloring matters are also added, to produce the required effect. The second coat is sometimes put on with a brush, a great many applications being necessary to properly blend the colors. To impart to the work the requisite polish, similar to marble, the workman rubs the surface, when it is hard, with pumice stone; then polishes it successively with tripoli, pulverized charcoal, and a piece of soft cloth, and finishes with oil. When well made and polished, scagliola can hardly be distinguished from marble. It is unsuitable for exterior decoration.
but for interior work is nearly as durable as good stone, and in Europe there are columns made of it which are hundreds of years old.