Carbon Distribution Control in Core and Jacket Steel of Japanese Swords

Tamahagane Composition and Carbon Inhomogeneity—The Starting Point of Japanese Sword Materials

Tamahagane produced by tatara ironmaking is distributed over a wide range of carbon content from 0.6% to 1.5%, and possesses fundamentally different material properties from homogeneous Western steel (carbon steel). This inhomogeneity is the very source that generates the two-layer structure of shingane (core steel) and kawagane (outer steel) in Japanese swords, and moreover, the technical significance of the folding and hammer technique.

The work of breaking tamahagane and striking it with a chisel to separate high carbon content sections (high-carbon steel) from low carbon sections (low-carbon steel) is called "kozawari." The high-carbon sections contribute a sharp edge due to high hardness, while the low-carbon sections contribute durability by resisting blade breakage with high toughness (resilience). By intentionally combining these two types of steel, conflicting characteristics of sharpness and durability are realized in a single blade—this is the fundamental design philosophy of Japanese sword forging.

The Science of Folding and Hammer Technique—Impurity Removal and Steel Homogenization

The greatest challenge of tamahagane is the inclusion of slag and impurities, as well as the localized bias of carbon distribution. The "folding and hammer" process resolves this.

By repeating the operation of heating steel, striking and drawing it out, folding it back, and striking again, three effects are obtained.

• Slag is forced out and the steel is refined
• Carbon atoms diffuse evenly within the steel's crystal lattice, eliminating localized hardness irregularities
• Steel crystal particles become finer (fine-graining), and toughness improves

Theoretically, homogenization progresses with more folds, but excessive folding causes the counterproductive effect of decarburization making the steel too soft. According to modern metallurgical analysis, the 8 to 16 folds (approximately 256 to 65,536 layers) performed in traditional sword-making are shown to approach the optimal solution for carbon control and impurity removal.

Number of Folds and Quality of Forging—The Misconception of "More is Better"

It is premature to judge a sword's quality based solely on the number of folds. Folding is a means for impurity removal and carbon homogenization; the comprehensive power of the entire process—including the quality of the tamahagane material itself, the smith's skill in fire management and striking force control, and temperature management during quenching—ultimately determines the final blade's strength and the beauty of its edge pattern. The number of folds is merely one element composing the process, and it alone does not guarantee the sword's quality. When selecting a sword, it is practically important to view it comprehensively, including the condition of the jigane and edge pattern, rather than being caught up in the single metric of fold count.

Positioning Design of Shingane and Kawagane—The Rationality of "Koubuse Tanae"

The process of combining low-carbon steel (shingane) and high-carbon steel (kawagane) that have undergone folding and hammer technique is called "tsukuri-komi" or "koubuse tanae" (there are other methods such as "hon sanmai" and "shihou zume").

The most typical koubuse tanae employs a U-shaped cross-sectional structure with shingane wrapped in kawagane. Shingane absorbs impact within the blade and prevents breakage, while kawagane maintains high carbon concentration and hardness toward the edge. This arrangement is rational because placing high-toughness shingane near the shinogi-suji (ridge line), where stress concentration easily occurs, minimizes breakage risk in actual combat.

Moreover, by designing the shingane's carbon content lower than the kawagane, during quenching only the shingane receives a "mild" heat treatment, controlling the effect so that the brittleness from martensite transformation does not reach the blade's core. This is the same principle as modern case-hardening technology, and it is remarkable that empirical practice aligns with metallurgical knowledge.

Quenching and Carbon Phase Transformation—The Scientific Basis of Edge Patterns

Tsuchimi (clay coating)—the process of applying "coating clay" mixed with clay and whetstone powder to the blade—is a technique to vary the cooling rate during quenching for different areas. By applying clay thinly (or not at all) on the edge and thickly on the spine side, the edge is rapidly cooled during quenching to generate martensite (high-hardness phase), while the spine side cools more slowly and remains as pearlite and bainite structure that retains toughness.

Hamon (edge pattern) is formed at the boundary between the rapid-cooling zone and the slow-cooling zone. The higher the carbon content, the more white-shining nie and nioi appear; diverse hamon patterns such as midare-ba, choushi-ba, and suguha emerge as the combined result of tsuchimi shape, quenching temperature, and tamahagane's carbon distribution. Each time the same smith forges with the same design, a different hamon appears because it reflects the "natural variation" of tamahagane's inhomogeneity.

Traces of Forging Visible in Jigane and Hamon—Points for Appreciation

The layered steel structure created by folding and hammer technique appears as patterns of "jigane" (steel grain) on the polished blade surface. All are traces reflecting the nature of tamahagane and the care taken in forging.

Regarding hamon, the white-shining granular aspect is called "nie," and the misty continuous aspect is called "nioi"; the appearance changes according to differences in carbon distribution at the boundary and cooling rate during quenching. Observing jigane and hamon together provides clues to understanding what steel and techniques were used to make the sword.

Dialogue Between Modern Science and Traditional Technique—Remaining Mysteries

Through modern analysis using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), cross-sectional structure and carbon distribution of masterpiece swords can now be visualized. This is confirming that the carbon control empirically mastered by master craftsmen before the Edo period approaches optimal conditions even from modern materials science perspective.

On the other hand, many aspects remain completely unexplained. The influence of trace elements unique to tamahagane (titanium, vanadium, etc.) on quenching properties, the visual effect of localized carbon segregation on the "scenery" of hamon, and the individual sword-maker's tacit knowledge—the technique of judging quality from striking sound, flame color, and steel's viscosity—belong to the domain of tacit knowledge difficult to quantify.

The technology of carbon distribution control in shingane and kawagane is a uniquely Japanese materials engineering invention born from the constraints of the inhomogeneous material tamahagane. Understanding its scientific rationality provides a perspective to evaluate Japanese swords not merely as weapons or art objects, but as intellectual achievements in human technological history.

NBTHK Certificates and Proof of Traditional Technique

The certificate issued by NBTHK (Society for Preservation of Japanese Art Swords) is issued after experts comprehensively examine the quality of jigane and hamon, appearance, signature, and other elements, and shows authentication categories such as period, origin, and school. Traditional techniques such as folding and hammer technique and the construction of shingane and kawagane are closely linked to the evaluation of jigane and hamon recorded in the certificate, making it one of the indirect supporting materials for these techniques being properly applied. When selecting a sword, confirming the condition of jigane and hamon with one's own eyes, along with the certificate's contents, becomes a practical clue for discerning authentic works made based on traditional technique.