The Hidzukuri (火造り) Process — The Core Forging Technique That Creates the Shape of Japanese Swords

What is Hidzukuri — The Process of Giving Sword Form After Fold-Forging

After fold-forging (orikaeshi tanren) removes impurities and homogenizes the steel structure, the iron lump still lacks sword form. "Hidzukuri" (火造り) is the general term for the process of heating this steel lump in a furnace while hammering it into sword shape. If fold-forging is the process that determines material "quality," then hidzukuri is the process that determines the sword's "form" (katachi).

Hidzukuri is not merely a matter of shaping. The direction of hammer blows, the force applied, and the control of heating temperature determine the sword's curve (sori), width, thickness distribution, center of gravity, and bending characteristics. Much of a single sword's cutting ability, ease of handling, and durability are determined by the skill of hidzukuri. The sword blacksmith's mastery is most directly tested in the hidzukuri process.

Soshinobe (素延べ) — Drawing Out the Steel Lump into Sword Form

The first stage of hidzukuri is "soshinobe" (素延べ). The steel lump that has completed initial formation is reheated in the furnace and, while in a cherry-red state, is drawn out lengthwise using large hammers. At this stage, while keeping the final sword form in mind, the goal is to rough out the material's length and cross-sectional shape.

What matters in soshinobe is managing heating temperature. Overheating the steel causes "over-burning" (yakisugi), where steel crystals become coarse and brittleness increases. Conversely, if temperature is too low, steel's deformation resistance increases and the risk of cracks from hammering rises. The optimal hammering temperature ranges roughly between 900-1200°C, which craftsmen judge empirically from flame color (dark red to orange to yellow). Modern sword blacksmiths also rely fundamentally on visual temperature judgment, and mastering this sensory technique without thermometers takes years.

During soshinobe, the sword maker does not merely hammer-shape the steel but performs hammer work with conscious attention to cross-sectional form. Japanese swords possess a unique cross-sectional shape called "shinogi-dzukuri" (鎬造り)—a diamond-shaped cross-section with a ridge line (shinogi-suji) running along the blade's center—so from this stage onward, deformation is applied with awareness of the thickness ratio between the edge side and spine side.

Muneuchi (棟打ち) and Hatori (刃取り) — Precision Shaping of Cross-Sectional Form

Once rough length is established through soshinobe, the next step moves to precision shaping of cross-sectional form through "muneuchi" (棟打ち) and "hatori" (刃取り). Muneuchi is the work of hammering the spine side of the blade body to square it, while hatori is the work of thinning and hammering the edge side to form the blade shape.

These two processes proceed by alternately repeating. Striking the spine drives steel toward the edge side; striking the edge pushes steel back toward the spine side. While maintaining this "push-pull" balance, uniform thickness distribution is achieved along the entire length. To give appropriate thickness and width to each section—from edge tip to spine, from point to tang—the blade must be repeatedly reworked at different angles while being rotated.

Skilled sword makers at this stage design the form with foresight into polishing and the final finish after hardening. During hardening, the curve changes (the edge side shrinks, increasing the curve), and during polishing, the edge becomes thinner. Anticipating these changes beforehand, master craftsmen intentionally create "reverse curve" and "thicker edge" at the hidzukuri stage to compensate.

Soridashi (反り出し) — Hammer Technique for Controlling Blade Curve

The characteristic beautiful bow-shaped curve of Japanese swords contains both natural and intentional elements. First, as a matter of physical principle, during hardening the volume change differs between edge and spine sides, so steel tends to curve toward the edge side. This phenomenon can be controlled by working the steel in the opposite direction during the soshinobe stage to manage the final curve.

The technique for soridashi in hidzukuri is called "uchiori" (打ち反り)—a process of heating the blade body in the furnace, pressing the edge side against an anvil (daigane), and striking the spine side upward in an arc. How much uchiori to apply depends on the steel type used, the forging method, and the desired final curve, requiring each craftsman's unique technical judgment that cannot be expressed as a fixed formula.

Curve is not merely an aesthetic feature but has functional significance. Proper curve facilitates "pulling cuts" (hikigiri) during slashing, optimizing the contact angle between blade and target to enhance cutting ability. Excessive curve causes loss of strength and deterioration of handling; insufficient curve restricts thrusting techniques. The ideal curve changes according to era, school, and combat style, creating the diversity of Japanese sword forms (sugata) seen across different periods.

Sakimarume (先丸め) and Formation of the Point (Kissaki/鋒)

Once the blade body's form is established, the point section "kissaki" (鋒) is shaped. The kissaki is not a simple sharp point but has a distinctive form marked by a clear boundary line called "yokote" (横手), where the point face (kissaki-men) and edge join at a precise angle.

Formation of the kissaki is among the most technically demanding parts of the hidzukuri process. The sword's character changes greatly depending on the kissaki's "size," "roundness," and "point angle." Large kissaki (ōkissaki) excel in thrusting power and cutting range, while small kissaki (kokissaki) suit precise control and stabbing. The bold large kissaki of the large tachi (ōtachi) of the Southern and Northern Courts period and the delicate small kissaki of the small swords of the peaceful Edo period directly reflect the differences in period context and weapon requirements.

Special care in temperature control is essential when forming the kissaki. The point section has little steel mass and small heat capacity, so temperature drops quickly after removal from the furnace. Striking when temperature has dropped too much causes cracks, so the form is shaped gradually through repeated brief reheating. Some sword makers describe forming the kissaki as "the moment of creating the sword's face," showing their recognition of its importance and difficulty.

The Individuality of the Sword Determined by Hidzukuri — The Craftsman's Handiwork Lives On

After passing through the hidzukuri process, the blade body remains rough on the surface but has already fixed that sword's "structure" and "form." The blade body at this stage is called "kiji" (素地) or "shirotanaya" (白鍛え), and in subsequent stages of clay application, hardening, and polishing, the kiji's form and structure do not undergo major changes.

In other words, hidzukuri is the process that determines that sword's unique individuality. A craftsman's physique, strength, dominant-hand habits, the furnace's condition that day, the weight of the hammer used—countless elements combine to create a single blade of steel that will never take exactly the same form again. The reason Japanese swords are said to have "individuality" and "personality" derives largely from the subtle differences born from the hidzukuri process.