Darker colors than this, ranging through green and gray, denote that the
piece has reached its ordinary temper, that is, it is partially annealed.

After properly hardening a spring by dipping in lard or fish oil, it should
be held over a fire while still wet with the oil. The oil takes fire and
burns off, properly tempering the spring.

Remember that self-hardening steels must never be dipped in water, and
always remember for all work requiring degrees of heat, that the more
carbon, the less heat.

_Case Hardening._--This is a process for adding more carbon to the
surface of a piece of steel, so that it will have good wear-resisting
qualities, while being tough and strong on the inside. It has the effect of
forming a very hard and durable skin on the surface of soft steel, leaving
the inside unaffected.

The simplest way, although not the most efficient, is to heat the piece to
be case hardened to a red heat and then sprinkle or rub the part of the
surface to be hardened with potassium ferrocyanide. This material is a
deadly poison and should be handled with care. Allow the cyanide to fuse on
the surface of the metal and then plunge into water, brine or mercury.
Repeating the process makes the surface harder and the hard skin deeper
each time.

Another method consists of placing the piece to be hardened in a bed of
powdered bone (bone which has been burned and then powdered) and cover with
more powdered bone, holding the whole in an iron tray. Now heat the tray
and bone with the work in an oven to a bright red heat for 30 minutes to an
hour and then plunge the work into water or brine.