The ordinary methods of case-hardening are quite inadequate when large wrought-iron forgings of perhaps irregular shape require to be treated. These are box-hardened in the following manner. For the heaviest work, cast-iron boxes of circular form with east-iron covers are used. They are of sizes suitable for the work in hand, ranging between 1ft. and 2ft. 6in. iu diameter. For small work, tubes of wrought iron or old pulley bosses are used. The bottom of the box is covered with a thick layer of the hardening material, which may consist of bone dust, leather clippings, or hoofs, mixed with salt or charcoal powder. Care must be taken to give the forgings good support among the material, so that they shall not become distorted by their own weight while at a red heat. When the box is filled with alternate layers of metal and of material, the cover is put on, and luted with fireclay to make it nearly air-tight. It is essential that air be excluded. Then it is placed in a fire or, preferably, in a rever-beratory furnace, for from ten to thirty-six hours. The time during which the box is exposed to the heat of the furnace mainly regulates the depth to which the metal will be hardened. The chemical activity of the hardening agents, however, influences the result.

The addition of powdered yellow prussiate of potash is often an improvement. The forgings are turned out into cold water, and are thus hardened to a depth which ranges from 1/64 in- to nearly |in. But in the same forgings the depth of the hardening will not be quite uniform. For light articles, of course, a mere film of surface hardening is enough; for heavy work the steely casing should penetrate to nearly 1/8 in. Since hardening distorts the work, the minimum amount of penetration that is consistent with the purpose for which the forgings are required should be imparted to them - 1/16 in., or a bare skin., may be taken as a good average. The distorted outlines have to be corrected with an emery wheel or with emery paper.