The forging, hardening and tempering of steel tools for cutting metals have long been practiced as a part of blacksmithing. The process used by blacksmiths to give an edged tool the correct degree of hardness is accomplished in two steps vaguely designated as "tempering." To illustrate blacksmith-shop hardening and tempering, which, though crude, is highly useful and very convenient, an example of hardening and tempering a cold chisel will be given:
Having forged the cutting end to shape, place this end in the fire and heat about half the length of the chisel to cherry red (the critical temperature, as near as can be judged). Plunge most of the red hot part into water, holding it still until cold, then withdraw and quickly rub a bright metallic spot near the cutting edge with a piece of grindstone or other abrasive material. This quenching gives the cutting end the maximum hardness which can be given it, but this is too hard for use, and the heat left in the un-quenched part of the chisel is now used for the purpose of tempering this extreme hardness, or "drawing the temper."
Holding the chisel in the tongs, watch the gradual changes of color on the rubbed soot, as the heat travels down from the un-quenched end. As soon as the color reaches that denoting the hardness desired (dark purple in this case), plunge the whole chisel at once into water to prevent further "drawing of temper."
The water must not be extremely cold, as the sudden change of temperature may be great enough to cause small cracks over the surface of the steel.