Appreciating Japanese Swords
Japanese swords have been appreciated for their craftsmanship for as long as they have been weapons of war. Soldiers no longer carry swords into battle, but enthusiasts throughout the world continue to study the finer details of swordsmithing. When comparing different blades, three of the major elements to inspect are the grain pattern (jihada), the edge pattern (hamon), and the curvature (sori).
The jihada is formed during the forging process. The swordsmith repeatedly folds the steel, creating a dense piece of metal with around 33,000 layers. When the steel is flattened and polished, the layers are revealed in patterns that can be straight, wavy, or resemble different types of wood grain.
The hamon is a line down the edge of the blade that can be perfectly straight or follow a pattern, such as waves or zigzags, according to artistic taste. The degree of curvature and the width of the sword’s point varied over time, depending on the contemporary style of warfare.
A blade’s curvature and edge pattern are the result of a unique quenching process called yaki-ire. In this process, the nearly finished blade is coated with two different types of clay, which transfer heat at different rates. The blade is heated to around 800 degrees Celsius, then plunged into cold water. The layer of clay on the back of the sword causes it to contract more slowly, becoming a type of steel called “pearlite,” while the edge cools quickly and hardens into martensite. This uneven contraction tensions the blade and gives it its signature curve. Polishing reveals the blade’s hamon, which is determined by the pattern in which the clay was applied.