For tempering many kinds of tools, the steel is first hardened by heating it to a cherry red, and plunging it into cold water. Afterward the temper is drawn by moderately heating the steel again. Different degrees of hardness are required for different purposes, and the degree of heat for each of these, with the corresponding color, will be found in the annexed table:
Very pale straw color, 430°—the temper required for lancets.
A shade of darker yellow, 450°—for razors and surgical instruments.
Darker straw-yellow, 470°—for penknives.
Still darker yellow, 490°—chisels for cutting iron.
A brown yellow, 500°—axes and plane-irons.
Yellow, slightly tinged with purple, 520°—table-knives and watch-springs.