Fundamentals of Fire-Forging: How Temperature Control and Hammer Timing in the Forge Determine Blade Shape

What is Hi-zukuri: Which Stage of the Process?

The manufacturing process of Japanese swords generally follows the sequence: "sunobi (rough drawing) → hi-zukuri (forging) → tsuchиoki (clay application) → yaki-ire (quenching) → kenma (polishing)." Among these, "hi-zukuri" refers to the stage in which steel that has undergone repeated folding and forging (tanren) and been shaped into a basic rod form is then shaped into the actual form of a sword—the blade length, curvature, blade width, and position of the shinogi.

In the "sunobi" stage that precedes hi-zukuri, the steel mass that has completed the repeated folding and forging process is drawn out to a long length, and the overall cross-sectional shape is refined. Hi-zukuri is the work of using hammers (large and small) and chisels (tagane) to bring the sunobi material closer to the final shape of the sword.

Whether this process is executed with precision greatly influences the final quality of the blade's form, making hi-zukuri one of the stages where a sword smith's skill is most directly tested.

Temperature Management: Reading Flame Color and Steel Condition

Temperature control in hi-zukuri is not performed with precision thermometers like modern industrial products. The sword smith observes the color of the flame heating the steel and the brightness and color of the steel with the naked eye, judging the optimal striking temperature.

Kuroaka (dark red): Approximately 400–600°C. The steel is hard and difficult to deform even when struck with a hammer. Striking at this temperature range risks creating internal cracks, so it is considered a temperature at which striking should not be done.

Aka (red) to daidaiaka (orange-red): Approximately 700–900°C. The steel begins to soften and can be reshaped with a hammer. This temperature range is sometimes used in the sunobi stage.

Kiaka (yellow-red) to ki (yellow): Approximately 900–1100°C. This is the temperature range most commonly used in hi-zukuri of Japanese swords. It is considered the optimal forging temperature for tamahagane, and the steel softens sufficiently to respond obediently to hammer strikes.

Shiro (white) to hakuko (bright white): 1200°C and above. The steel becomes overheated (burnt), and there is danger that carbon will combust, making the steel brittle. Striking at this temperature should not be done.

The sword smith discerns this color of temperature in a dimly lit, naturally dark forge. For this reason, traditional forges are designed to minimize natural light, with intentionally few windows.

The "Ma-ai" of Hammer Striking: Timing and Force Distribution System

Alongside temperature management, the "ma-ai" (interval or timing) of hammer striking forms the foundation of hi-zukuri. The window of time when steel is at striking temperature is extremely short (a few seconds to roughly ten seconds), and necessary deformation must be applied within this limited window.

Division of roles between the large hammer (oduchi) and small hammer (koduchi): In traditional forges, the sword smith (blacksmith) held the small hammer, and an apprentice served as the "uchiко" (striker), wielding the large hammer—a two-person team system was the norm. The sword smith used the small hammer to give "instruction strikes," indicating where and with what force to strike, and the striker followed these instructions to swing the large hammer. In modern times, many sword smiths work alone, sometimes using electric hammers as an auxiliary tool.

Relationship between striking force and deformation: Strong strikes produce large deformations, but the precision of the form decreases. Light strikes make the form easier to refine, but work efficiency is low. The sword smith adjusts the force of the strikes in real time according to the deformation required for each section of the blade.

Positioning of the shinogi (ridgeline): The cross-sectional shape of the "shinogizukuri" (ridged construction) unique to Japanese swords—the structure where the angles of the faces differ on the blade side and spine side around the shinogi (the ridgeline of the blade)—is determined by how the chisel and hammer are used during the hi-zukuri stage. Even a slight deviation in shinogi position becomes difficult to correct in the subsequent polishing process.

Formation of Curvature (Sori): Utilizing Cooling and Stress

The beautiful curve of a Japanese sword is formed by the operations in the hi-zukuri stage and the stress changes caused by cooling during quenching. In the hi-zukuri stage, the sword smith shapes the blade to a form somewhat close to a straight blade, but during quenching the blade side contracts sharply due to rapid cooling, and a curve naturally forms.

However, the curve does not form from quenching alone. By intentionally performing "uchizori" (inner curve)—forging with the blade side subtly turned inward—during the hi-zukuri stage, the amount and shape of the curve after quenching are controlled.

The difference between koshizori (where the apex of the curve is near the base) and sakizori (where the apex is near the point) arises from differences in material shape control during the hi-zukuri stage and differences in cooling direction and cooling speed during quenching.

Forge Equipment: Fire Bed, Bellows, and Anvils

Understanding the basic equipment of the forge that makes hi-zukuri possible is also necessary.

The hodo (fire bed) is the furnace for heating steel and burns charcoal (primarily pine charcoal) as fuel. The quantity and placement of charcoal directly affect the heating temperature and uniformity. Confirmation of "whether the entire steel is being heated uniformly" is performed by the sword smith carefully observing inside the furnace and adjusting the amount of charcoal.

The fuigo (bellows) is the air supply device for the furnace; historically, hand-operated box-type bellows were used. Increasing air supply raises the temperature, and reducing it lowers the temperature. In modern times, electric fans are often used, but fine temperature adjustment still depends on the skilled intuition of an experienced craftsman.

The kinuta (anvil block) and kanatoko (iron bench) are the bases that receive hammer strikes, and their hardness and shape affect how the steel deforms.

Experiencing Hi-zukuri: Modern Sword Smith Training and Viewing Opportunities

In modern times, "sword-forging technique preservation training" based on the system of the Agency for Cultural Affairs is conducted, and apprentice sword smiths in training learn by experiencing actual hi-zukuri in their master's forge. Some facilities offer forging experiences for the general public, conducted in regions deeply connected to sword craftsmanship and tatara iron production, such as Seki in Gifu Prefecture and Okikumo in Shimane Prefecture.

For collectors and enthusiasts, witnessing the actual site of hi-zukuri is extremely valuable for deepening understanding of swords and blades. By directly observing the sword smith's hand movements and the color of the steel rising from the fire bed, an intuitive understanding can be gained of questions such as "why does this sword have this particular curve?" and "why does this cross-section have this 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 smith's sensibility and experience are concentrated. The eye that reads temperature, the rhythm of swinging the hammer, the manipulation of materials calculated to achieve the desired curve—when these are integrated, the individuality that is "characteristic of this particular sword smith" is carved into the blade.

When appreciating the differences in the form of Japanese swords, keeping the hi-zukuri process in mind transforms what appears as a mere "difference in appearance" into "the crystallization of the sword smith's judgment and skill." The depth of Japanese sword appreciation is inseparably linked to such background knowledge of the manufacturing process.