HARDENING. In metallurgy this term has two meanings. First, the hardening of tool-steel throughout in order to fit it for cutting purposes, or to withstand wear when used as a pin, roller, cam, tappet, etc. Second, the surface or case-hardening or car burising of wrought-iron or mild-steel to give it a hard skin suit able for wearing purposes. The first-named treatment is per formed by heating the steel to a red or white heat and cooling in water, oil, lead, mercury or in air or a blast according to circum stances. The second by heating the article in a furnace in con tact with a case-hardening material which thus penetrates and adds carbon to the surface, and then quenching the steel while red hot. Leather cuttings, hoof parings, horns, bones are packed in a box with the steel and put in the furnace for a period, or a prepared powder is used instead of those substances. The fur naces are much the same as those described under ANNEALING. The principal difficulties to contend with in the process are crack ing in the case of tool-steels, and warping in case-hardening. See TEMPERING.