Early iron melting made use of bloomeries, which produced two layers of metal: one with a very low carbon content that is worked into wrought iron, and the rest a high carbon cast iron. Since the high carbon iron is hot short, meaning it fractures and crumbles when forged, it was not useful without more smelting. The wrought iron, with nearly no carbon in it, was very malleable and ductile, but not very hard.
Case hardening involves packing the low-carbon iron within a substance high in carbon, then heating this pack to encourage carbon migration into the surface of the iron. This forms a thin surface layer of higher carbon steel, with the carbon content gradually decreasing deeper from the surface. The resulting product combines much of the toughness of a low-carbon steel core, with the hardness and wear resistance of the outer high-carbon steel.
The traditional method of applying the carbon to the surface of the iron involved packing the iron in a mixture of ground bone and charcoal, or a combination of leather, hooves, salt and urine, all inside a well-sealed box. This carburizing package is then heated to a high temperature, but still under the melting point of the iron, and left at that temperature for a length of time. The longer the package is held at the high temperature, the deeper the carbon will diffuse into the surface. Different depths of hardening is desirable for different purposes: sharp tools need deep hardening to allow grinding and resharpening without exposing the soft core, while machine parts like gears might need only shallow hardening for increased wear resistance.
The resulting case hardened part may show distinct surface discoloration. The steel darkens significantly, and shows a mottled pattern of black, blue and purple, caused by the various compounds formed from impurities in the bone and charcoal. This oxide surface works similar to bluing, providing a degree of corrosion resistance, as well as an attractive finish. Case coloring refers to this pattern and is commonly encountered as a decorative finish on replica historic firearms.
With modern steelworking techniques, it is possible to make homogeneous steels of low to high carbon content, removing much of the original motivation for case hardening. However, the heterogeneous nature of case hardened steel may still be desirable, as it can combine both extreme hardness and extreme toughness, something which is not readily matched by homogeneous alloy
