- The process of giving the requisite degree of hardness
or softness to a substance, as iron and steel; especially, the process
of giving to steel the degree of hardness required for various
purposes, consisting usually in first plunging the article, when heated
to redness, in cold water or other liquid, to give an excess of
hardness, and then reheating it gradually until the hardness is reduced
or drawn down to the degree required, as indicated by the color
produced on a polished portion, or by the burning of oil.