We have learned that carbon gives the hardening quality to steel. Since
there is little carbon in soft steel and none in wrought iron, they
will not harden as carbon steel does when heated red hot and plunged
into water. But there are many small articles which are best made of
this soft steel and iron and which must be hardened in some way to
make them useful. In order to supply the lacking carbon the metal is
put through a process known as case hardening. There are two methods
of case hardening. The first method is to heat a piece of soft steel
or iron red hot and cover the part to be hardened with cyanide of
potassium. The metal will absorb the carbon out of the cyanide and
when cooled in water will have taken on a hardened surface. If this is
repeated two or three times the hard surface deepens.
Any drug store will sell you some cyanide of potassium. It comes in
cakes. The cakes are broken up into small pieces. Be careful to keep
the hands as much as possible from contact with the cyanide.