A Structure Balancing Sharpness and Resilience: The Philosophy of Steel and Iron Composites

The structural foundation of Tosa cutlery is a traditional technique known as warikomi. This method is based on the concept of composite materials, where an extremely hard material called hagane (steel), which determines the blade's sharpness, is sandwiched between a softer material called jigane (soft iron), which absorbs shock and supports the entire blade.

A blade must simultaneously possess two fundamentally opposing properties: "hardness" to maintain a keen edge and "toughness" (or resilience) to prevent chipping or breaking under impact. It is incredibly difficult to achieve both of these with a single metal.

As one artisan explains the logic behind this structure, "The only part of a blade that truly needs to be hard is the very tip of the cutting edge that makes direct contact. The rest of the blade should be designed to flexibly absorb impact."

The warikomi technique masterfully solves this dilemma through the principle of tekizai tekisho, or "the right material for the right place." It involves using expensive, hard, high-carbon steel (hagane) exclusively for the cutting edge, while the bulk of the blade's body is made from a more affordable and resilient low-carbon steel (jigane). As a result, the jigane absorbs and disperses impact, preventing the blade from breaking, while the hard hagane edge retains its exceptional sharpness over a long period.

This structure is based on the very same philosophy as modern, advanced composite materials like carbon-fiber-reinforced plastic (CFRP). It’s a sophisticated engineering approach that overcomes the limitations of a single material by creating a structural design that combines materials with different properties.

Moreover, this method was also exceptionally brilliant from an economic standpoint. By minimizing the use of high-quality steel—a historically precious and costly resource—and bulking out the blade with inexpensive iron, it achieved incredible efficiency. The warikomi technique thus represents a sophisticated solution that simultaneously meets the demands of functionality, durability, and economy.