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.
239. While, by the use of the best materials and proper care in preparing and applying them, a good wall may be obtained with lime plaster, yet there are so many uncertainties about it that numerous substitutes have been brought into use, which have proved valuable and efficient. These are known under the general name of hard wall plasters, and may be divided into two classes: natural cement plasters, and chemical, or patented, plasters.
240. Natural plasters, such as Agatite, Royal, Acme, etc., are made from earths - found in Kansas and other states - which resemble plastic clay in appearance, and in chemical nature are similar to gypsum, from which plaster of Paris is made. They are produced by heating the natural earth to a high degree, which expels all the moisture. When mixed with water, they set like hydraulic cement, but much more slowly, thus allowing ample time for applying the mortar. These plasters have great strength and adhesive power; the full strength, however, is not attained till from one to two months after application. A sample of Agatite several weeks old showed a breaking strength of 370 pounds per square inch, a strength exceeding that of many cement mortars of the same age.
They work smoothly under the trowel, and adhere very firmly to wood, stone, or brick, without the use of fiber or any other binding material, although such is generally used when the plaster is applied to lathing. Most of these cement plasters must be mixed with sand before using, but are prepared with and without fiber; the former kind is used for the scratch coat on lathing, and the other for the brown coat, or for the first coat on brick or tile. As they are grayish in color, the finishing coat is made of lime and plaster of Paris, which is put on as before described.
In some respects these plasters are superior to natural cement, and have been substituted for it with considerable advantage in setting fireproof tiling, etc.
241. Chemical, or patent, plasters include Adamant, Windsor Cement, Rock Wall, Granite, Rockite, and others. While the exact composition of these plasters is unknown to the public, they appear to consist of plaster of Paris, with some material added to lessen the natural rapidity of setting of that substance, so that sufficient time will be had to apply the plaster. As far as known, the only difference in these plasters is in the materials used to render them slow setting.
Most patent plasters are sold containing sand and fiber, and only require mixing with water to be ready for use. Numerous grades are made for the different coats and application to various surfaces. The plaster known as Adamant was the forerunner of all hard plasters, and is probably more extensively used than any other; but, while the oldest are likely to be the most reliable, all of these plasters will doubtless give good results if properly applied.
Keene's Cement. When it is necessary to give walls, ceilings, etc. a hard and highly polished surface, a prepared plaster known as Keene's cement is generally used as a finishing coat. Strictly speaking, this is not a cement, but is made of plaster of Paris, soaked in a solution of alum and then recalcined. Applied to the walls, this material becomes very hard and takes a high polish, so that surfaces finished with it may be washed without injury. Its hardness also makes it very satisfactory to use for finishing the lower portions of walls where the surface is liable to injury by contact with furniture, etc.
Application Of Hard Plasters. The method of applying these plasters differs in no essential respect from that described as green work in lime plastering. As they are more like cement than lime, the hard plasters set instead of drying. The material should be mixed fresh every hour or two, only enough water being used to give them the proper consistency; no plaster should be remixed that has partially set. When the plaster is to be applied to stone, brick, or tile, the surfaces should be sprinkled before putting on the coat. Wood lathing should also be well moistened, so that they will swell before, and not after, the plaster has begun to set. The makers of hard plasters recommend that the laths be spaced 1/4 inch apart and that 3/4-inch grounds be used. A better wall, however, will be obtained by using 7/8-inch grounds and 3/8-inch key. All these plasters, except Adamant, can be finished with a third coat of lime, putty, and sand, which should not be applied until the second coat is thoroughly dry.
When not used properly, hard plasters are inferior to lime plaster, so that the manufacturers' directions for applying them should be carefully followed.
Advantages Of Hard Plasters. Hard plasters have many points of superiority over lime plaster, which more than balance their extra cost, and which will, in time, probably result in their almost exclusive use. Being machine-mixed, the plaster made from them is uniform in quality and strength, and has unvarying proportions of the constituents, while two batches of lime plaster may differ a great deal in this respect. They are also much harder and more tenacious, and resist fire and water better than lime, and the small quantity of water used in mixing enables them to dry much more rapidly. They are not injured by frost after they have begun to set, but should be protected from it for the first 36 hours after being put on the walls. Heat and moisture are not readily transmitted by hard plasters, and, being more dense than lime plaster, they do not absorb noxious gases or permit the entrance of disease germs.