The 2 Key Features You Should Know: "Integrated Production" & "Warikomi Structure"

To truly understand how Tosa Uchihamono is made, you first need to recognize its two major characteristics.

The first is their system of "integrated production," in which nearly every stage is handled by a single artisan. Whereas many other knife-producing regions divide tasks such as forging, sharpening, and attaching the handle among different specialists, in Tosa, each craftsman masters a diverse skill set and is wholly responsible for every aspect of a product from start to finish.

The second is a traditional method called the "Warikomi" structure. This technique involves encasing the hard "Hagane" steel—responsible for sharpness—between layers of softer "Jigane" iron that absorbs shock.

This blend of materials allows the tool to achieve the sharp, hard edge needed for excellent cutting, and the flexibility (toughness) to resist chipping or breaking under use—qualities impossible to combine with a single kind of metal.

Together, these two features form the bedrock that supports the quality and unique character of Tosa Uchihamono.

[Process 1: Forging Bond] When Steel & Iron Meet—The Foundation-Building Skill

The knife-making journey starts with cutting charcoal from pine into uniform pieces—a crucial preparatory step that ensures stable heat and requires significant experience.

Next comes the material selection: high-carbon "Hagane" steel for the blade, and low-carbon "Jigane" iron for the body. Premium steels like Yasuki steel produced by Proterial (formerly Hitachi Metals), with subtypes such as Aogami (blue paper steel) for long-lasting sharpness and Shirogami (white paper steel) for razor-thin edges, are chosen based on the intended use.

Once the materials are prepped, they’re joined through a process called "forging bond."

The Jigane is heated to around 1000°C and scored with a chisel. Then, the Hagane is inserted. Borax or other fluxes are sprinkled to promote bonding, and then it’s heated again.

The craftsman reads the color of the fire and the way sparks fly to judge the precise moment when steel and iron truly fuse—a stage known as "Wakashi." Seizing this exact timing, they strike rapidly with a power hammer called a belt hammer, press-welding the materials at an atomic level.

This phase is absolutely crucial, as it establishes the basic structure of the blade.