from
The Collaborative International Dictionary of English v.0.48
Tempering \Tem"per*ing\, 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]