Case-Hardening of Iron, is a superficial conversion of that metal into steel, by a cementation of it with vegetable or animal coals. This operation is usually performed on small pieces of iron, worked into tools and instruments, by putting them together with the cement, into an iron box., which is closely closely shut, and exposed to a red heat, for several hours. Thus, the surface of the iron, to a certain depth, is converted into steel, to which a proper degree of hardness may be given, by a sudden immersion of the heated pieces into a cold fluid. - See Iron.