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.