While the basic processes of forging a blade are universal, the Japanese swordsmiths employ a few variations in the forging process which result in a higher quality blade than the standard produced in the West during the Middle Ages.
Japanese swords are generally heavier, curved, and have only one cutting edge, which is ground with a convex, rather than concave, shape. They are forged with two types of steel, a soft inner layer (shingane) and a hard outer layer (hadagane). The two types of steel are hammered and folded individually. The steel is folded both transversely (end-to-end) and longitudinally (side-to-side). At each folding stage, the steel is cooled with water. Hammering the steel after oxidizing the surface by cooling with water helps to remove impurities from the steel. After repeated folding, the steel may be cut into small pieces and forged again. All of the folding, cutting and reforging results in a complex layered structure of the steel, which can total thousands of layers.
Before the final hardening, the hard outer layer is hammered into a U-shape and the soft inner layer is placed in the cavity. Then, the blade is hammered to ensure there are no air bubbles or impurities between the two steel layers. The smith’s skill can have a significant impact at this point in the process, as during hammering of the thicker back of the blade (mune), it will tend to force the blade to curve toward the cutting edge (ha). The smith will have to use his skill to prevent the warping and curving of the blade as he hammers the two layers of steel together. A triangular section is also cut out of the blade, at this point, and shaped to become the tip (kissaki). The tip can vary in length and even hook backwards.
Before the blade is heated for hardening, clay is applied to the blade. The clay slows the heating of the steel in the forge and the cooling of the steel in the water bath. Clay is applied in a thicker layer to the back and sides of the blade than to the cutting edge. This is achieved by adding a lighter layer of clay to the cutting edge with a brush or applying a thick layer of clay to the entire blade and scooping excess clay from the cutting edge. To this point, the blade remains a straight blade. However, once the heating process in the coals of the forge is begun, the lightly covered ha, or cutting edge, will heat more rapidly than the heavily clay-covered mune, or back of the blade. This causes the steel in the edge of the blade to expand more rapidly than the back, resulting in the iconic slight curve towards the back of the blade.
Once the blade has reached the appropriate temperature, it is plunged point first and edge down into a water bath. The edge and outer, hard layer of steel cool more rapidly than the soft inner steel resulting in more rapid contraction of the outer layer around the inner layer of steel. This pulls the hadagane, or outer layer, tightly around the shingane, or inner layer, like vacuum packaging. The softer, inner layer of steel acts as a shock absorber when the blade is used, reducing the frequency of breaks or bends in Japanese blades. The thicker blades also increase the resilience of Japanese blades in comparison with Western blades of the same time period. The very hard steel that forms on the cutting edge can be easily ground to a razor-sharp edge and can be reground many times. They are ground to a convex shape, rather than the straight or concave shape of the cutting edges of Western blades. The convex shape helps maintain the edge and helps the blade pass through material with less resistance, and therefore more cleanly and efficiently.
A hole was also punched in the tang (nakago); so when the hilt was added, a bamboo pin could be inserted to anchor the blade. The master smith would make his mark, which could also indicate the quality of the sword, on the tang. While nearly all swords are decorated, many include decorations only on the tang with the master smith’s signature. Forging of swords was considered a sacred art in feudal Japan, and the forging of a single blade could take months of work from numerous artisans.