Air furnace iron anneals rather more readily than does the product of the cupola, and the strength and malleability are usually greater. The former requires a temperature of about 1350° F., while the latter must have 1500° F., a difference of about 150° F. Whether this results alone from the somewhat higher sulphur of cupola malleable is not definitely known, but it is probable that, also, the slightly higher total carbon gives the iron-carbon chemical compound a tendency to persist more strongly. The open-hearth furnace is sometimes used for making malleable cast iron. It melts much more quickly than does the air furnace which requires from three and one-half to nine hours per heat, depending upon the size. The quality of the product which the open-hearth furnace produces is of the best, but on account of the continuous operation necessary, this type of furnace is not largely used. Malleable iron has also been made in the Bessemer converter, and, occasionally, in the crucible furnace, but in this country the practice is not at all common. In Germany a great deal of malleable iron is made in the crucible furnace. When dumped from the molds the castings are extremely brittle. The sprues are knocked off and the castings go to the “tumbling” mills where they are tumbled, either with the sprues, with hard iron (white iron) shot or star-shaped pieces of iron which quickly clean the sand from them and give smooth, clean surfaces. At the chipping and sorting benches any remaining pieces of gates and other excrescences are removed while the castings are being handpicked and sorted. White iron, because of its brittleness, breaks easily and small protruding parts can more readily and cheaply be removed before annealing than after the castings have been thereby toughened. [Illustration: ANNEALING FURNACES, SHOWING SETS OF POTS OR “SAGGERS” IN WHICH THE CASTINGS ARE ANNEALED ] Having the cleaned castings of white iron of proper composition, malleability is given to them by the heat treatment known as “annealing.” Through the influence of a cherry-red heat continued over a sufficiently long period the iron and carbon, which in the white iron are chemically combined, gradually become divorced, and, after complete annealing, the casting will be found to consist of free iron in which are imbedded throughout very small particles of coke-like carbon. Castings that before this heat treatment were so brittle that they broke into many pieces under a blow and so hard that they might scratch glass, are now found to be capable of withstanding considerable distortion without fracture and so softened that a needle may scratch them.