Tsumiwakashi: The Forge-Welding Science Behind Stacking and Heating Multiple Steel Pieces in Japanese Swordmaking

The "Pre-stage" of Folding and Forging Determines Quality

When discussing the manufacturing process of Japanese swords, the most commonly discussed is "ori-kaeshi tanren" (folding and forging). It is widely known that the process of repeatedly folding and forging tamahagane influences the quality of the blade, but there is little specialized explanation of its "pre-stage," the "tsumi-wakashi" (stacking and heating) process.

Tsumi-wakashi is the process of stacking selected pieces of tamahagane, heating them in a forge, and forge-welding (tansetsu) them together. This process creates "a block of steel (kogane)" that becomes the material for folding and forging—an important step that directly affects the final quality of the Japanese sword.

From Tamahagane Selection to Stacking

Tamahagane produced by tatara smelting arrives at the swordsmith's hand as heterogeneous material with varying carbon content (carbon percentage). Parts with high carbon content are hard and brittle, while those with low carbon content are flexible but tough.

The swordsmith breaks the tamahagane with a small hammer and assesses the carbon content by observing the color of the cross-section, the pattern of cracks, and the coarseness of the crystals. In this selection process, called "wari" (splitting), the material is separated into high-carbon steel for the outer layer (kawagane) and low-carbon steel for the inner core (shingane).

In tsumi-wakashi, the selected steel pieces are stacked on an iron plate (shitakura), covered with "mud" made from a mixture of straw ash (warabai) and water, and placed in the forge. The mud acts as a "flux" that prevents the steel from oxidizing during heating and helps dissolve the oxidation scale on the forge-welding surface to facilitate direct bonding between metals.

Forge Temperature Control—1300°C as the Critical Point

The most critical aspect of tsumi-wakashi is temperature control. The temperature required for forge-welding varies depending on the type of steel, but for tamahagane, the target range is generally 1200–1350°C (the state where the steel is "boiling" or niwaku).

The "boiling" state is determined by visual observation. When the glow of the steel in the forge changes from orange to near-white and countless small sparks (waki-hana) begin to scatter on the surface, it signals that the appropriate temperature for forge-welding has been reached. This state is called "waki-hada," and the swordsmith judges the timing to remove the steel from the forge by observing this condition.

If the temperature is too low, the forge-welding will be incomplete, and "hagaré" (delamination) will occur during the subsequent folding and forging process. Conversely, if the temperature is too high, "yake-futori" (carbon loss and grain boundary fusion) occurs, weakening the steel. This delicate balance cannot be measured numerically and depends entirely on the experience and visual judgment of a skilled swordsmith.

The Moment of Forge-Welding—Precise Control of Hammer Force and Angle

The heated stacked steel removed from the forge is placed on an anvil (kanatoko) and quickly forge-welded by striking with a large hammer (ōtsuchi). Because the initial strike determines the overall shape, coordinated successive blows between the master and apprentice (sakite) are required.

During forge-welding, the angle and force of the strikes are critical. As the stacked steel is pressed together, oxidation scale (noro) is driven outward to prevent it from remaining inside. If noro becomes trapped inside, it becomes a "forge flaw" (kitakizu), resulting in a defect in the finished blade.

This work must be done quickly because only the few tens of seconds immediately after removing the steel from the forge maintain the appropriate temperature. As the steel cools, it develops "viscosity" (nebari) and becomes harder to strike, requiring reheating. Conversely, if excessive heating continues, oxidation advances. The coordinated successive blows between the master and apprentice maximize this brief window of opportunity.

The Forge-Welding Mechanism Revealed by Modern Science

Modern metallurgy has partially elucidated the forge-welding mechanism of tsumi-wakashi. Forge-welding of steel (solid-phase bonding) is the phenomenon where flux (SiO₂ and K₂O in straw ash) dissolves and removes the oxidation film on the bonding surface, and clean metal surfaces come into contact under high temperature and pressure to create "diffusion bonding." Iron atoms mutually diffuse across the bonding plane, eliminating distinct boundaries and creating unified steel.

However, even with this theoretical understanding, in actual practice, variations in climate, charcoal quality, forge condition, and the properties of individual tamahagane lots make it impossible to achieve proper forge-welding through numerical management alone. This is why the swordsmith's "empirical knowledge" remains indispensable even today.

The Number of Tsumi-Wakashi Operations and Steel Homogenization

Because a single tsumi-wakashi and forge-welding operation does not sufficiently homogenize the steel, typically 2–3 or more cycles of tsumi-wakashi → forge-welding → folding and forging are repeated.

The first tsumi-wakashi begins with varying carbon percentages and carbon distribution, and gradual homogenization progresses through repeated forge-welding and folding. Complete homogenization is not necessarily the goal; rather, it is important to intentionally separate the materials at the tsumi-wakashi stage to achieve the desired carbon distribution of "low-carbon shingane and high-carbon kawagane."

Each school of swordmaking has its own approach to the number of tsumi-wakashi operations and the combination of materials, which is reflected in each school's distinctive "jihada character." Even when using the same tamahagane, differences in the tsumi-wakashi procedure produce variations in the jihada pattern (itame, kō-itame, mokume, etc.).

Conclusion: The Invisible Process Determines Blade Quality

Although tsumi-wakashi is one of the most inconspicuous processes in Japanese sword manufacturing, the selection of carbon content at this stage, the precision of forge-welding, and proper temperature management determine the quality foundation for all subsequent processes after folding and forging. As the saying "great blades begin with materials" suggests, the swordsmith's eye for selecting tamahagane at the tsumi-wakashi stage and forge-welding technique are the first conditions for creating a masterwork blade.