Briefly, it is a method of casting printing plates of aluminum alloy in molds made from a composition of plaster-of-paris. In its essential points it is a modern adaptation of the process credited to William Ged of Edinburgh in 1730 and afterwards modified and improved in the early 19th century by Earl Stanhope of England.

In practice, the original to be duplicated is placed on a molding-slab. A molding frame is set upon the slab and enclosing the original. A special kind of oil is then sprayed on the face of the original. This is to facilitate the release of the plaster mold so that it will not “tear” when it is ready to be lifted off the original after solidifying, and at the same time to retain the sharpness of the mold.

The molding medium of plaster composition in a semi-liquefied state is then poured on to the original in the molding frame. The surplus plaster is scraped off flush with the top of the molding frame. After the plaster matrix in its molding frame has set sufficiently it is released by means of cams from the working pattern on the molding-slab.

The plaster matrix is then placed in a drying oven, through which a forced draft of hot air is kept circulating at high pressure. The thorough drying of the mold takes approximately ninety minutes.

When the plaster mold has become sufficiently dried, a round hole is cut through the bottom of the matrix in an offset of the molding frame. This hole is the gate through which the molten aluminum is forced. The mold is then securely locked upright in a specially designed casting machine.

The Aluminotype is cast by pressure and not by pouring as in the case of stereotypes, which depend entirely upon gravity. Fused aluminum alloy is poured into a hopper on the casting machine. A piston operated by the agency of compressed air forces the aluminum evenly into all parts of the plaster matrix.

When the cast is completed the molding frame is taken from the casting machine and the Aluminotype removed from its plaster-of-paris matrix.

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