CASE-HARDENING, the process of converting the
superficies or outer part of iron into steel. This is effected
by placing the article in a box with some animal or vege-
table charcoal, and exposing it to a red heat. It differs
only from the making of steel in the shorter duration of the
process. Iron tools, &c., when thus treated, combine the
toughness of iron with the hardness of steel.