working them while cold.

_Tempering_ is the process of making steel tough after it has been
hardened, so that it will hold a cutting edge and resist cracking.
Tempering makes the grain finer and the metal stronger. It does not affect
the hardness, but increases the elastic limit and reduces the brittleness
of the steel. In that tempering is usually performed immediately after
hardening, it might be considered as a continuation of the former process.

The work or tool to be tempered is slowly heated to a cherry red and the
cutting end is then dipped into water to a depth of 1/2 to 3/4 inch above
the point (Figure 6). As soon as the point cools, still leaving the tool
red above the part in water, remove the work from the bath and quickly rub
the end with a fine emery cloth.

As the heat from the uncooled part gradually heats the point again, the
color of the polished portion changes rapidly. When a certain color is
reached, the tool should be completely immersed in the water until cold.

For lathe, planer, shaper and slotter tools, this color should be a light
straw.

Reamers and taps should be cooled from an ordinary straw color.

Drills, punches and wood working tools should have a brown color.

Blue or light purple is right for cold chisels and screwdrivers.

Dark blue should be reached for springs and wood saws.