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

The "Pre-Stage" Before Folding-Forging Determines Quality

When discussing the manufacturing process of Japanese swords, the most noted step is "folding-forging" (orikaeshi-tanren). It is widely known that folding and forging tamahagane multiple times determines the quality of the sword. However, there are surprisingly few specialized explanations of the "pre-heating accumulation" (tsumi-wakashi) that precedes it. Recently, as reevaluation of Japanese sword manufacturing methods progresses from the perspective of metallurgical engineering, the importance of tsumi-wakashi has drawn increasing attention.

Tsumi-wakashi is the process of stacking (tumi) selected pieces of tamahagane, heating them in a furnace, and forge-welding (tansetsu) them together. This process, which creates a single mass of steel (kogane) that becomes the material for folding-forging, is a critical step directly linked to the final quality of Japanese swords.

From Selection of Tamahagane to Stacking

Tamahagane produced through tatara iron smelting arrives at the swordsmith's workshop as an inhomogeneous material with varying carbon content (carburization ratio).

The swordsmith crushes tamahagane with a small hammer and judges the carbon content by observing the cross-section's color, fracture patterns, and crystal coarseness. Through this selection process called "wari" (division), steel pieces are separated by their properties as follows.

The precision of the swordsmith's selection eye determines the quality ceiling for all subsequent processes.

In tsumi-wakashi, selected steel pieces are stacked on an iron plate (shitakura), covered with "mud" made of straw ash (warabai) and water, and placed in a furnace. This mud acts as a "flux" (fusing agent) to prevent oxidation of the steel during heating, dissolving the oxidation scale on the forge-welding surface and facilitating direct metal-to-metal bonding. The precision of this flux effect greatly influences the success of tsumi-wakashi.

Furnace Temperature Management—The Critical Point of 1300°C

The most crucial aspect of tsumi-wakashi is temperature management. While the temperature required for forge-welding varies by steel type, for tamahagane it is generally estimated at around 1200–1350°C (the state where the steel "boils" or niwaku).

The "boiling" state is determined by visual observation. When the steel inside the furnace changes from orange to nearly white incandescence and countless small sparks (waki-bana) scatter across its surface, this signals that the proper forge-welding temperature has been reached. This condition is called "waki-hada" (boiling skin), and the swordsmith judges when to remove it from the furnace based on this observation.

If the temperature is not appropriate, the following problems occur.

This balance cannot be measured numerically and depends solely on the experienced swordsmith's visual judgment. Some modern research indicates that the charcoal burning rate and subtle differences in furnace environment during this process affect quality.

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

The stacked heated steel removed from the furnace is placed on an anvil (kanatoko) and quickly struck with a large hammer (ootsuchi) to forge-weld it. Since this first blow determines the overall shape, synchronized continuous strikes with an apprentice (sakie) are required.

During forge-welding, the angle and force distribution of the blows are critical. While compressing the stacked steel, oxidation scale (noro) must be expelled outward to prevent it from remaining inside. If noro becomes embedded internally, it becomes a "forging flaw" (kitaekizu), resulting in a defect in the finished sword.

The work must be done quickly because only the first few tens of seconds after removal from the furnace maintains the proper temperature. As the steel cools, "toughness" (nebari) develops, making it harder to strike, requiring reheating. Conversely, if overheating continues, oxidation progresses. Synchronized continuous strikes between the swordsmith and apprentice maximize this brief window. This "composite perception" is the essence of tsumi-wakashi technology, with the swordsmith simultaneously sensing:

• The sound of the hammer
• The glowing color of the steel
• The spray pattern of the oxidation scale

The Forge-Welding Mechanism Revealed by Modern Science

Modern metallurgical engineering has partially clarified the forge-welding mechanism of tsumi-wakashi.

Steel forge-welding (solid-state bonding) is a phenomenon in which flux (SiO2 and K2O in straw ash) dissolves and removes the oxide film on the bonding surface, allowing clean metal surfaces to contact under high temperature and pressure, causing "diffusion bonding." Iron atoms mutually diffuse across the bonding surface, eliminating distinct boundaries and creating unified steel.

Recent research has focused on how the component ratio of straw ash used as flux affects forge-welding quality. The silica and potassium content varies depending on straw type and combustion conditions, altering melting point and viscosity. Traditional swordsmiths' attention to straw type likely reflects an empirical pursuit of optimal flux formulation.

However, despite this theoretical understanding, actual work is affected by variables such as climate, charcoal quality, furnace condition, and variations in tamahagane properties from batch to batch, making numerical management alone insufficient to achieve proper forge-welding. This is why the swordsmith's "empirical knowledge" remains indispensable.

Frequency of Tsumi-wakashi and Steel Homogenization

Since a single tsumi-wakashi and forge-welding cycle is insufficient for steel homogenization, the cycle of tsumi-wakashi → forge-welding → folding is typically repeated two or more times.

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

Each school of swordsmiths has unique approaches to the number of tsumi-wakashi cycles and combinations of materials, which are reflected in each school's "individual character of jigane" (blade steel). Even with the same tamahagane, different tsumi-wakashi procedures produce differences in the landscape of the steel surface (itame, ko-itame, mokume, etc.). It can be said that a school's individual character originates from the swordsmith's philosophy at the tsumi-wakashi stage.

Summary: The Hidden Process Determines Sword Quality

Tsumi-wakashi is one of the most inconspicuous steps in Japanese sword manufacturing, yet the carbon selection precision, forge-welding accuracy, and appropriate temperature management at this stage determine the quality foundation for all subsequent processes including folding-forging. As the saying goes, "fine swords begin with fine materials"; the swordsmith's eye for selecting tamahagane and forge-welding mastery are indeed the primary conditions for creating a masterpiece sword.

TOUKENZA's online catalog features numerous Japanese swords created through such meticulous processes. Understanding that the subtle landscapes visible in a sword's steel surface result from layers of craftsman's judgment and technique beginning with tsumi-wakashi deepens one's appreciation of the blade.