Fundamentals of Fire-Forging: How Temperature Control and Hammer Timing in the Forge Determine Blade Shape
* Image is for illustrative purposes only.Table of Contents
What Process Is Hi-zukuri? (Which Stage Does It Refer To?)
The manufacturing process of Japanese swords generally follows this sequence: "tsukurikomi (construction) → sunobe (initial stretching) → hi-zukuri (heat forging) → tsuchidoki (clay application) → yakiire (hardening) → kenma (polishing)." Of these stages, "hi-zukuri" is the process in which the steel, having undergone tsukurikomi and sunobe to become rod-shaped, is formed into the actual shape of a sword—its blade length, curve, blade width, and the position of the shinogi ridge.
Tsukurikomi is the process of inserting shingane (core steel) into kawagane (outer steel) after folding and repeated forging. This creates the structure unique to Japanese swords, combining hard and soft steels. After tsukurikomi, the process called "sunobe" stretches the combined steel block to an extended length to establish the overall cross-sectional shape. Hi-zukuri is the work of bringing the sunobe-prepared material closer to the final sword shape using hammers (ootsuchi and kotsuchi) and a tagane (chisel).
Whether this stage is executed with precision has enormous influence on the quality of the sword's final shape. For this reason, hi-zukuri is regarded as one of the moments where the sword-maker's skill is most directly tested.
Temperature Control — Reading Flame Color and Steel Condition
Temperature control in hi-zukuri is not done using precision thermometers as in modern industrial manufacturing. The sword-maker observes the color of the flame heating the steel and the brightness and hue of the steel itself with the naked eye, judging the optimal temperature for striking.
| Color | Temperature Range | State |
|---|---|---|
| Black-red (kuroaka) | ~400–600°C | Steel is hard and resists deformation when struck with a hammer. Striking in this temperature range creates a risk of internal cracks, making it a forbidden temperature range. |
| Red (aka) ~ Orange-red (daidaiaka) | ~700–900°C | Steel begins to soften and can be shaped with hammer blows. This temperature range is often used in the sunobe stage. |
| Yellow-red (kiaka) ~ Yellow (ki) | ~900–1100°C | The most frequently used temperature range in the heat-forging of Japanese swords. It is considered the optimal forging temperature for tamahagane, and the steel softens sufficiently to follow hammer blows obediently. |
| White (shiro) ~ Bright white (hakuko) | 1200°C and above | Steel enters an overheated state (overcooking), risking becoming brittle as carbon burns away. Striking must be avoided at this temperature. |
The sword-maker discerns these flame colors in a dim blacksmith's forge with minimal natural light. For this reason, traditional forges are designed to minimize window placement, often deliberately constructed with few windows.
The "Maai" of Hammer Work — System of Timing and Force Distribution
Alongside temperature control, the "maai" (interval/timing) of hammer strikes forms the very foundation of hi-zukuri. The window of time when steel is at optimal striking temperature is extremely brief—only a few to a dozen seconds or so—and the necessary deformation must be achieved within that limited timeframe.
Division of roles between ootsuchi (large hammer) and kotsuchi (small hammer): In traditional forges, the basic working system involved two or more people: the sword-maker (smith) holding the small hammer and an apprentice called the "uchiko" (striker) wielding the large hammer. The sword-maker used the small hammer for "direction strikes" (指示打ち)—indicating the location and force of each strike—while the uchiko followed these directions, swinging the large hammer. In modern times, many sword-makers work alone, sometimes using electric hammers as an auxiliary tool.
Relationship between striking force and deformation: Striking harder produces larger deformation, but shape precision decreases. Lighter strikes maintain shape more easily, but processing efficiency suffers. The sword-maker adjusts the striking force in real time based on the amount of deformation desired at each point along the blade.
Positioning the shinogi ridge: The unique Japanese sword cross-sectional shape—shinogi-zukuri—featuring a ridge line (shinogi) at the center with different face angles on the cutting edge and spine sides, is determined by how the tagane and hammer are used during the hi-zukuri stage. Even a slight misalignment of the shinogi ridge becomes difficult to correct in the subsequent polishing stage.
Formation of Sori (Curvature) — Utilizing Cooling and Stress
The beautiful curve of a Japanese sword is formed through manipulation during the hi-zukuri stage and stress changes from cooling during hardening. At the hi-zukuri stage, the sword-maker forms the blade into a shape somewhat close to a straight sword, but during the rapid cooling of hardening, the edge side contracts significantly, and the curve forms naturally.
However, the curve is not born from hardening alone. During the hi-zukuri stage, deliberate forging of "uchizori" (inner curve)—maintaining the edge side in a subtly inward-facing posture—is performed to control the amount and shape of the curve after hardening.
The differences between koshizori (belly curvature, with the peak of the curve near the hilt) and sakizori (point curvature, with the peak near the tip) arise from differences in material shape control during hi-zukuri and differences in cooling direction and cooling speed during hardening.
Blacksmith Shop Equipment — Ho-do, Fuigo, and Anvils
Understanding the basic equipment of the blacksmith forge that enables hi-zukuri is also necessary.
The ho-do (forge/fire bed) is the furnace for heating steel, fueled by charcoal (primarily pine charcoal). The amount and arrangement of charcoal directly affect heating temperature and uniformity. Confirmation that "the steel is being heated uniformly throughout" is performed by the sword-maker carefully observing the interior of the furnace while adjusting the charcoal supply.
The fuigo (bellows) is the air-supply device for the forge. Historically, hand-operated box bellows were used. Increasing the air supply raises temperature; reducing it lowers temperature. In modern times, electric fans are often used, but fine temperature adjustment still depends on the experienced craftsman's intuition.
The kinuta (anvil block) and kanatoko (anvil) are the striking surfaces that receive hammer blows, and their hardness and shape influence how the steel deforms.
Experiencing Hi-zukuri — Modern Apprenticeship and Viewing Opportunities
Today, "Sword-Forging Technique Preservation Training" (刀鍛冶技術保存研修) is conducted based on systems established by the Japanese Agency for Cultural Affairs. Apprentice sword-makers training under their masters experience actual hi-zukuri hands-on learning at their masters' forges. Several facilities also offer general public forging experience programs, conducted in regions closely connected to sword-craft and tatara steel-making, such as Seki in Gifu Prefecture and Okutamazumo in Shimane Prefecture.
For collectors and enthusiasts, witnessing an actual hi-zukuri workshop is invaluable for deepening understanding of sword-craft. By directly observing the sword-maker's hand movements and the color of the steel rising from the fire bed, one gains intuitive understanding of questions such as: "Why does this sword have this particular curve?" and "Why does this sword have this cross-sectional shape?"
Summary — Hi-zukuri is the Process That Determines a Sword's "Character"
Hi-zukuri is not merely a shaping operation but a creative process in which the sword-maker's sensibility and experience are concentrated. The eye that reads temperature, the rhythm of hammer strikes, the manipulation of material calculated for its curve—it is only when these integrate that the "character distinctive of this particular sword-maker" is inscribed upon the blade.
When appreciating the differences in Japanese sword forms, keeping the hi-zukuri process in mind allows one to see what appears to be mere "visual difference" as "the crystallization of the sword-maker's judgment and technique." The depth of Japanese sword appreciation is inseparably bound to this kind of background knowledge of its manufacture.
