This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
If burnt gypsum is stirred up with water containing formaldehyde and with a little alkali, and the quantity of water necessary for the induration of the plaster containing in solution a reducible metallic salt is added thereto, a plaster mass of perfectly uniform coloring is obtained. The hardening of the plaster is not affected thereby. According to the concentration of the metallic salt solutions and the choice of the salts, the most varying shades of color, as black, red, brown, violet, pearl gray, and bronze may be produced. The color effect may be enhanced by the addition of certain colors. For the production of a gray-colored gypsum mass, for example, the mode of procedure is as follows: Stir 15 drachms of plaster with one-fourth its weight of water, containing a few drops of formaldehyde and a little soda lye and add 10 drops of a one-tenth normal silver solution, which has previously been mixed with the amount of water necessary for hardening the gypsum. The mass will immediately upon mixing assume a pearl-gray shade, uniform throughout. In order to produce red or copper-like, black or bronze-like shades, gold salts, copper salts or silver salts, bismuth salts or lead salts, singly or mixed, are used. Naturally, these colorings admit of a large number of modifications. In lieu of formaldehyde other reducing agents may be employed, such as solutions of sulphurous acid or hydrogen peroxide with a little alkali. Metals in the elementary state may likewise be made use of, e. g., iron, which, stirred with a little copper solution and plaster, produces a brown mass excelling in special hardness, etc. This process of coloring plaster is distinguished from the former methods in that the coloration is caused by metals in the nascent state and that a very fine division is obtained. The advantage of the dyeing method consists in that colorings can be produced with slight quantities of a salt; besides, the fine contours of the figures are in no way affected by this manner of coloring, and another notable advantage lies in the mass being colored throughout, whereby a great durability of the color against outside actions is assured. Thus a peeling off of the color or other way of becoming detached, such as by rubbing off, is entirely excluded.
Frequently, in order to obtain colored plaster objects, ocher or powdered colors are mixed with the plaster. This method leaves much to be desired, because the mixture is not always perfect, and instead of the expected uniform color, blotches appear. Here is a more certain recipe: Boil brazil wood, logwood, or yellow wood, in water, according to the desired color, or use extracts of the woods. When the dye is cold mix it with the plaster. The dye must be passed through a cloth before use. One may also immerse the plaster articles, medals, etc., in this dye, but in this case they must be left for some time and the operation repeated several times.