Cast iron may be hardened as follows: Heat the iron to a cherry red,
then sprinkle on it cyanide of potassium and heat to a little above red,
then dip. The end of a rod that had been treated in this way could not
be cut with a file. Upon breaking off a piece about one-half an inch
long, it was found that the hardening had penetrated to the interior,
upon which the file made no more impression than upon the surface. The
same salt may be used to caseharden wrought iron.
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