Making a Samurai Sword

The hardening of steel involves altering the microstructure or crystalline structure of that material through quenching it from a heat above 1,472 °F (bright red glow), ideally no higher than yellow hot. If cooled slowly, the material will break back down into iron and carbon and the molecular structure will return to its previous state. However, if cooled quickly, the steel's molecular structure is permanently altered. The reason for the formation of the curve in a properly hardened Japanese blade is that iron carbide, formed during heating and retained through quenching, has a lesser density than its root materials have separately. The katana gets its gentle curve from quenching during forging, as it is straight prior to quenching. A process of differential tempering causes martensite to form predominantly in the edge of the blade rather than the back; as the spine has lower retained lattice strain, it cools and contracts, and the blade takes on a gently curved shape.

A coating of clay mixed with ashes and a small portion of rust is applied to every surface but the edge of the blade during hardening. This provides heat insulation so that only the blade's edge will be hardened with quenching. This process also creates the distinct swerving line down the center of the blade called the hamon which can only be seen after it is polished, a katana forger's signature.

After the blade is forged it is then sent to be polished. The polishing takes between one and three weeks. The polisher uses finer and finer grains of polishing stones until the blade has a mirror finish in a process called glazing. This makes the blade extremely sharp and reduces drag making it easier with which to cut. The blade curvature also adds to the cutting power.

The katana is characterized by its distinctive appearance: a curved, slender, single edged blade, circular or squared guard, and long grip to accommodate two hands. It has historically been associated with the samurai of feudal Japan and is also known as the "Soul of the Samurai", and has become renowned for its sharpness and cutting ability, to the point that its purported cutting capabilities have reached almost mythical status.