Differential Hardening

Differential hardening is a heat-treatment technique that gives a single blade two different hardnesses along its length: the whole blade is heated evenly to a red heat, then coated with an insulating layer, usually clay, applied more thinly along the cutting edge and more thickly along the spine, so that when the blade is quenched the exposed edge cools quickly into hard, brittle martensite while the insulated spine cools more slowly into softer, tougher pearlite, combining a hard cutting edge with a spine resistant to snapping. The boundary between the two zones forms a visible wavy line down the blade, known in Japanese sword-making as the hamon, where the technique became closely associated with the katana and other traditional Japanese blades. According to the traditional account of its spread, differential hardening technology originated in China, where it was applied to blades such as the jian and dao, and later spread to Korea and then to Japan, where swordsmiths refined it into the form associated with the classical Japanese sword.