HARDENING

, the giving a greater degree of hardness to bodies than they had before.

There are several ways of Hardening iron and steel; as by hammering them, quenching them in cold water, &c.

Case-Hardening, is a superficial conversion of iron into steel, as if it were casing it, or covering it with a thin coat of harder matter. It is thus performed: Take cow horn or hoof, dry it well in an oven, and beat it to powder; put equal quantities of this powder and of bay salt into stale urine, or white wine vinegar, and mix them well together; cover the iron or steel all over with this mixture, and wrap it up in loam, or plate iron, so as the mixture touch every part of the work; then put it in the fire, and blow the coals to it, till the whole lump have a blood red heat, but no higher; lastly, take it out, and quench it.—See Steel, under which article are described other processes for this purpose.