The hardening of a cutting tool makes it too brittle to stand up well when in use, and consequently it is necessary to reduce the brittleness somewhat. This process of softening, known as drawing the temper, is accomplished by reheating to the proper temperature, ordinarily determined by the color of the surface of the tool which must be brightened previous to the operation. As the piece of steel is heated, a light, delicate straw color appears; then, in order, a deep straw, light brown, darker brown, light purple, dark purple, dark blue, pale blue, blue tinged with green, and, finally, black. When black appears, the temper is gone. These colors furnish a guide to the condition of hardened steel, and indicate the tempers attained with the degrees of temperature used in the various connections shown in Table 1.
Augers; screw slotting saws; etc.
Blue, tinged with green
When steel is tempered in large quantities the method just described is expensive. It is not, moreover, so reliable as heating the articles in a kettle of oil, using a thermometer to indicate the temperature. A piece of perforated sheet metal or wire cloth should be used to keep the articles two or three inches from the bottom of the kettle. A perforated sheet-iron pail two inches smaller in diameter than the kettle, resting on a piece of iron, or a frame placed in the bottom, will keep the pieces from the sides and bottom of the kettle. The thermometer should be placed in the kettle outside the pail, in order that the bulb may be at the same depth as the lower pieces.
A piece of steel may be spring-tempered by first hardening and then drawing the temper to such a degree that the piece, when bent, will return to its normal shape after the pressure is removed. This may be accomplished by covering the surface with tallow or some animal oil, and then heating until the oil catches fire from the heat in the piece.