Case-hardened steel is produced by placing bone-dust, specially prepared for the purpose, or burnt leather scraps in a cast iron box together with the article to be hardened. The top of the box is covered with plenty of the hardening material to keep out the air. The whole mass is slowly and uniformly heated from two to five hours until it finally attains red heat. A few iron rods about 5/16 in. in diameter are packed in the box, one end of the rods reaching to the middle and the other projecting through the hardening material on the top. When the heat appears to be about right, these rods are pulled out one at a time so as to ascertain the heat in the center of the mass. When the box has been exposed to the fire the desired length of time, its contents are quickly dumped into cool water.
Sieves of iron netting are laid on the bottom of the tub into which the case-hardening material is dumped, so that the hardened articles may be conveniently taken up from the water by one of the sieves. The case-hardening material itself is taken out by another sieve which is of very fine netting and which is placed under the first one. The material is dried and used over again with the addition of a little new material.
When articles are finished before hardening, this process gives a very fine color to both soft steel and wrought iron.
Case-hardening may also be effected by packing the articles in soot, but this process does not give a good color. Horn and hoof are also used for case-hardening. Malleable iron may be case-hardened, but requires careful handling to prevent its cracking and twisting out of shape.