This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
The mass used by Beaumel consists of alum and heavy spar (barium sulphate) with addition of water and the requisite pigments. The following proportions have been found to be serviceable: Alum, 1,000 parts; heavy spar, 10 to 100 parts; water, 100 parts; the amount of heavy spar being governed by the degree of translucence desired. The alum is dissolved in water with the use of heat. As soon as the solution boils the heavy spar is mixed in, stirred with water and the pigment; this is then boiled down until the mixture has lost about 3 per cent of its weight, at which moment the mass exhibits a density of 34° Bé. at a temperature of 212° F. The mixture is allowed to cool with constant stirring until the substance is semi-liquid. The resultant mass is poured into a mold covered on the inside with several layers of collodion and the cast permitted to cool completely in the mold, whereupon it is taken out and dried entirely in an airy room. Subsequently the object may be polished, patinized, or finished in some other way.
Imitation Black Marble. — A black marble of similar character to that exported from Belgium—the latter product being simply prepared slate —may be produced in the following manner: The slate suitable for the purpose is first smoothly polished with a sandstone, so that no visible impression is made on it with a chisel—this being rough—after which it is polished finely with artificial pumice stone, and lastly finished with extremely light natural pumice stone, the surface then presenting a soft, velvet-like appearance. After drying and thoroughly heating the finely polished surface is impregnated with a heated mixture of oil and fine lampblack. This is allowed to remain 12 hours; and, according to whether the slate used is more or less gray, the process is repeated until the gray appearance is lost. Polishing thoroughly with emery on a linen rag follows, and the finishing polish is done with tin ashes, to which is added some lampblack. A finish being made thus, wax dissolved in turpentine, with some lampblack, is spread on the polished plate and warmed again, which after a while is rubbed off vigorously with a clean linen rag. Treated thus, the slate has the appearance of black marble.