This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
Sharp impressions of coins, medals, etc., are obtained, according to Böttger, with the following: Mix molten, thinly liquid sulphur with an equal quantity of infusorial earth, adding some graphite. If a sufficient quantity of this mass, made liquid over a flame, is quickly applied with a spatula or spoon on the coin, etc., an impression of great sharpness is obtained after cooling, which usually takes place promptly. Owing to the addition of graphite the articles do not become dull or unsightly.
Bronze and silver medals should always be coated with a separating grease layer. The whole coin is greased slightly and then carefully wiped off again with a little wadding, but in such a manner that a thin film of grease remains on the surface. Next, a ring of strong cardboard or thin pasteboard is placed around the edge, and the ends are sealed together. Now stir up a little gypsum in a small dish and put a teaspoonful of it on the surface of which the mold is to be taken, distributing it carefully with a badger's-hair brush, entering the finest cavities, which operation will be assisted by blowing on it. When the object is covered with a thin layer of plaster of Paris, the plaster, which has meanwhile become somewhat stiffer, is poured on, so that the thickness of the mold will be about 1/20 of an inch. The removal of the cast can be effected only after a time, when the plaster has become warm, has cooled again, and has thoroughly hardened. If it be attempted to remove the cast from the metal too early and by the use of force, fine pieces are liable to break off and remain adhering to the model. In order to obtain a positive mold from the concave one, it is laid in water for a short time, so that it becomes saturated with the water it absorbs. The dripping, wet mold is again provided with an edge, and plaster of Paris is poured on. The latter readily flows out on the wet surface, and only in rare cases blisters will form. Naturally this casting method will furnish a surface of pure gypsum, which is not the case if the plaster is poured into a greased mold. In this case the surface of the cast contains a soapy layer, for the liquid plaster forms with oil a subsequently rather hard lime soap. The freshly cast plaster must likewise be taken off only when a quarter of an hour has elapsed, after it has become heated and has cooled again.