tempering

n. (Metal.) 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. [ 1913 Webster ]

Tempering color, the shade of color that indicates the degree of temper in tempering steel, as pale straw yellow for lancets, razors, and tools for metal; dark straw yellow for penknives, screw taps, etc.; brown yellow for axes, chisels, and plane irons; yellow tinged with purple for table knives and shears; purple for swords and watch springs; blue for springs and saws; and very pale blue tinged with green, too soft for steel instruments. [ 1913 Webster ]