This is the process of rendering cast-iron castings malleable, or capable of bending without breaking. Castings are not only relieved of brittleness, but a considerable degree of toughness and ductility is imparted to them. Many small articles of iron in every-day use, notably iron-pipe fittings, are far more readily made as castings and malleableized afterward, than if made of wrought iron or mild steel at the beginning. This process practically converts them into a mild steel by the removal of carbon, and its method of application is as follows:

Iron boxes of convenient size, known as annealing pots, are filled with castings, each of which is entirely surrounded by some kind of iron oxide, usually mill scale, squeezer scale, or pure magnetic ore. The castings have been thoroughly cleaned of sand and fins or other projections of metal before leaving the foundry, and a good sprinkling with sal ammoniac will give them a coating of rust, which as iron oxide, assists in malleableizing.

Each pot is closed with a thin iron cover luted with clay and may be placed in any kind of a furnace in which a steady heat may be maintained. In malleableizing works, several hundred post are stacked in a large furnace heated with producer gas. The brick-lined iron door of this furnace is closed, the contents are heated up to about 1800° in 24 hours, are maintained at this heat about 48 hours, and then allowed to cool gradually. At the high heat maintained, the castings give up their uncombined carbon to the oxygen of the iron oxide, and when removed they have lost the brittleness of cast iron.

This process is not so much in general use as formerly. Many castings are now displaced by drop forgings and articles of pressed sheet steel. These are neater looking and of less bulk than the usual run of malleable castings. Some shapes are cheapest made as castings, however.