Note:  Do not rely on this information. It is very old.


Case-hardening is the conversion of the surface of wrought-iron objects into steel by the addition of a small percentage of carbon. This is effected by heating them to a red-heat in contact with charcoal powder, leather or horn parings, or other matter containing the carbon required to effect the change. The objects are then cooled in water or oil, and will be found to be encased in a thin skin of steel, ordinarily 1/16 to 1/32 of an inch thick. The depth of the steel coating depends on the nature of the wrought iron and on the duration of heating. Objects so treated are more durable and better capable of receiving polish.