tempering : Idioms & Phrases

• 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.

Webster 1913