Steel Structure Types in Blade Construction—What Shihozume, Honsammai, and Kabusae Reveal About Kawagane and Shingane Lamination Design

What is Kumihada?—Structural Design of Japanese Swords Through Multiple Steel Combinations

The technical answer to the seemingly contradictory requirement that Japanese swords be "hard yet unbreakable" is a method known as kumihada (造り込み). Because a single steel cannot easily achieve both hardness and toughness simultaneously, Japanese swordsmiths ingeniously combined multiple steel types with different carbon contents, creating a composite structure that distributes function throughout the blade.

The basic concept of kumihada is straightforward. Hard, high-carbon steel is placed at the edge (hasaki) to ensure sharpness and wear resistance, while soft, low-carbon steel is positioned along the spine and ridge (mune) of the blade to provide shock absorption and flexibility. This structure creates the ideal blade—one that resists breaking while maintaining a razor-sharp edge. The forms of kumihada developed diversely across different schools and eras. The major types include shihozume, honsammai, koubuse, and sanmaiawase.

Koubuse—The Simplest Two-Layer Structure

Koubuse represents the most fundamental form of kumihada, consisting of two layers in which hard outer steel (kawagane) wraps soft inner steel (shingane) from above. When viewed in cross-section, it resembles a rod of iron core topped with outer steel bent into a U-shape. The edge and front and back surfaces of the blade are carried by the outer steel (high-carbon), while the sides and interior expose the inner steel (low-carbon).

The advantage of koubuse lies in its manufacturing simplicity. With fewer material joints and relatively low welding difficulty, it is considered a kumihada form well-suited to apprentice swordsmiths. Simple ancient and medieval tachi swords also exhibit koubuse structure, and it is often positioned as the prototype of kumihada technique. However, because the inner steel along the blade's sides is easily exposed, this structure has inferior breaking strength compared to shihozume, discussed below.

Honsammai—Three Independent Steel Types for Edge, Core, and Ridge

Honsammai employs three types of steel—outer steel, inner steel, and ridge steel (munegane)—dividing the sword's cross-section into three vertical layers. The typical arrangement places the hardest high-carbon steel at the edge (hagane), medium-carbon steel in the center (shingane), and low-carbon soft iron along the ridge (munegane).

The characteristic feature of honsammai is its ability to precisely control the blade's weight distribution and deflection properties by placing an independent soft iron piece along the ridge. Because the edge steel, core steel, and ridge can each be designed individually, the swordsmith can adjust the carbon content of each section according to the intended use—prioritizing slashing or thrusting, for example. After the Edo period, through the New Sword and New New Sword eras, as technique became more refined, an increasing number of swordsmiths adopted honsammai.

Shihozume—The Highest Form of Structure, Where Ridge Steel Encloses Inner Steel on All Four Sides

Shihozume is the most advanced kumihada form, with outer steel encasing inner steel from all four directions (top, bottom, left, right). Viewed from any cross-sectional direction, outer steel (hard) forms the perimeter, and inner steel is completely contained within.

Realizing this structure demands considerable technical skill. Rather than simply laying outer steel over the core, the smith must uniformly weld outer steel to all four faces of the inner steel, then forge-weld it so that the boundary remains evenly thick even after polishing. Any gap or defect in the welded joint becomes a point of weakness where cracks can propagate, making welding precision crucial to performance.

Shihozume is chosen for durability. No matter the direction of impact, hard outer steel absorbs the first blow while soft inner steel toughly absorbs the energy. Its advantages are especially apparent in side blows and parries against armored opponents—situations where the blade receives strong lateral impacts. The widespread adoption of shihozume from the late Kamakura period through the Muromachi era corresponds to this intensified period of warfare.

Sanmaiawase—An Armor-Plating Structure Where Inner Steel is Sandwiched

Sanmaiawase features inner steel sandwiched between two outer steel plates on the left and right, sometimes called "warikomi" (割り込み) in smithing circles. While this arrangement may be considered a variation of honsammai, it differs in that it makes no clear distinction between edge and ridge, instead placing outer steel equally on both sides.

Sanmaiawase is particularly common in larger weapons such as naginata (polearms) and odachi (great swords), which require near-double-edged functionality. The cross-section is left-right symmetric, giving it distinct aesthetic and functional characteristics compared to the standard single-edged sword. Additionally, the larger welded surface area between outer and inner steel provides manufacturing advantages in terms of stable forge-welding strength.

Reading Kumihada—The Perspective of Polishers and Appraisers

The type of kumihada can be discerned by skilled polishers (togishi) and sword appraisers from the blade's surface after polishing. The boundary between outer and inner steel appears as variations in the thickness of the ridge meat (muneniku) and changes in the grain pattern of the ground steel (jigane). In shihozume particularly, the steel quality transition from the ridge line (shinogi) to the ridge itself remains clearly observable after polishing.

In appraisal, the kumihada type serves as valuable information for identifying the swordsmith and estimating the era. For example, Yamashiro school smiths (Kyoto tradition) are said to have favored honsammai, while Soshu school smiths (Kamakura tradition) frequently employed shihozume. However, determining a smith's school solely by kumihada type is dangerous; proper appraisal requires comprehensive judgment considering ground steel, hamon (edge pattern), dimensions, and sword profile together.

Kumihada provides the clearest evidence that the Japanese sword transcends mere weaponry, bridging material science and structural engineering as a refined craft. From the perspective of modern materials science, kumihada's composite steel structure—employing tatara steel—ranks among the pinnacle achievements of pre-modern ironworking technology.