with the mercury bath, as the fumes that arise are poisonous.

Should toughness be desired, without extreme hardness, the steel may be
cooled in a bath of lard oil, neatsfoot oil or fish oil. To secure a result
between water and oil, it is customary to place a thick layer of oil on top
of water. In cooling, the piece will pass through the oil first, thus
avoiding the sudden shock of the cold water, yet producing a degree of
hardness almost as great as if the oil were not used.

It will, of course, be necessary to make a separate test for each cooling
medium used. If the fracture of the test piece shows a coarse grain, the
steel was too hot at that point; if the fracture can be cut with a file,
the metal was not hot enough at that point.

When hardening carbon tool steel its heat should be brought to a cherry
red, the exact degree of heat depending on the amount of carbon and the
test made, then plunged into water and held there until all hissing sound
and vibration ceases. Brine may be used for this purpose; it is even better
than plain water. As soon as the hissing stops, remove the work from the
water or brine and plunge in oil for complete cooling.

[Illustration: Figure 6.--Cooling the Tool for Tempering]

In hardening high-speed tool steel, or air hardening steels, the tool
should be handled as for carbon steel, except that after the body reaches
a cherry red, the cutting point must be quickly brought to a white heat,
almost melting, so that it seems ready for welding. Then cool in an oil
bath or in a current of cool air.

Hardening of copper, brass and bronze is accomplished by hammering or