The "Futu Yakiba" of the Japanese sword is a special blade treatment technique used to enhance the hardness, toughness, cutting performance, and durability of the blade.
In this treatment process, the surface of the Japanese sword blade is coated with a layer of "futu" (usually a mixture of clay, charcoal, and water), and the thickness of this layer can be adjusted according to the design and purpose of the blade. Then, the entire blade is heated to a very high temperature, allowing the "futu" layer to burn until the edge of the blade turns red. As the firing process progresses, different parts of the blade are heated to different degrees, creating an irregular pattern, which is one of the characteristics of the "Futu Yakiba."
After the blade cools, the "futu" layer is removed, and the blade is further processed and sharpened to achieve its final shape and sharpness. Due to the special treatment process of the "Futu Yakiba," the surface of the blade forms a quenched area with higher hardness, while the interior retains a certain toughness. This combination gives the Japanese sword excellent cutting ability, strength, durability, and fatigue resistance.