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

What is Fire-Forging — the process of giving sword form after repeated folding

The mass of iron that has had impurities removed and steel structure homogenized through repeated folding (ori kaeshi tanren) does not yet have the form of a sword. "Fire-forging (hi-zukuri)" is a collective term for the process of heating this steel mass in a furnace while striking it with a hammer to shape it into the form of a sword. If repeated folding is the process that determines the "quality" of the material, then fire-forging is the process that determines the "form (katachi)" of the sword.

Fire-forging is not merely a process of shaping. The direction of striking, the force applied, and control of heating temperature determine the curve (sori), width, thickness distribution, center of gravity, and deflection characteristics of the sword. Much of a single sword's cutting ability, ease of handling, and durability is determined by the skill in fire-forging. The fire-forging process is where the sword smith's skill is most directly tested.

Suno-be (rough drawing out) — drawing out the steel mass into sword form

The first stage of fire-forging is "suno-be (rough drawing out)". The steel mass, after completion of the initial forging, is reheated in the furnace, and while glowing red-hot, it is drawn out lengthwise using a large hammer (o-tsuachi). At this stage, while keeping the completed sword form in mind, the goal is to roughly adjust the length and cross-sectional shape of the material.

What is important in suno-be is the management of heating temperature. If temperature control is mishandled, the following risks occur.

• In the case of excessive heating ("overheating (yaki-sugii)"): the steel crystals coarsen, and brittleness (weakness) increases
• In the case of temperature that is too low: the deformation resistance of steel increases, and the risk of cracks forming from hammer strikes rises

The optimal striking temperature is roughly in the range of 900-1200°C, and craftspeople judge this empirically from the color of the flames (dark red ~ orange ~ yellow). Modern sword smiths also basically judge temperature by visual inspection, and the acquisition of this intuitive skill requires several years.

When performing suno-be, the sword smith does not simply draw out by striking, but performs hammer strikes with awareness of the cross-sectional shape of the steel. Japanese swords have a unique cross-sectional shape called "shinogi-zukuri" — a diamond-shaped cross-section with a ridge line (shinogi-suji) running through the center of the blade — and from this stage onward, deformation is applied with awareness of the thickness ratio between the edge side and the spine side.

Mune-uchi (spine striking) and ha-tori (edge drawing out) — precision shaping of cross-sectional form

Once the rough length is achieved through suno-be, the process moves to precisely adjusting the cross-sectional shape through "mune-uchi (spine striking)" and "ha-tori (edge drawing out)". Mune-uchi is the work of striking the spine (back) side of the blade body with a hammer to shape it into a square form, while ha-tori is the work of thinly drawing out the edge side to create the edge form.

These two processes proceed by alternating back and forth. When the spine is struck, steel flows toward the edge side; when the edge is struck, steel is pushed back toward the spine side. While maintaining the balance of this "push and pull", a uniform thickness distribution across the entire length is achieved. To give appropriate thickness and width to each part — from edge to spine, from kissaki to nakago — it is necessary to repeatedly re-strike each part while rotating the blade body.

Skilled sword smiths at this stage design the shape with foresight to the finishing after polishing and heat-treatment. In the heat-treatment process, the curve changes (the edge side contracts and the curve increases); in the polishing process, the edge becomes thinner. After calculating these changes in advance, the fire-forging stage requires the master technique of intentionally creating "reverse curve" or "thicker edge" to compensate.

Sori-dashi (curve forming) — hammer technique to control the blade's "curve"

The characteristic bowed curve of Japanese swords has both elements that occur naturally and elements that are intentionally created. First, as a matter of physical principle, because the amount of volume change differs between the edge side and the spine side during heat-treatment, steel tends to curve toward the edge side. This phenomenon can be controlled by pre-striking in the opposite direction during the suno-be stage to control the amount of curve in the finished sword.

The technique of curve-forming in fire-forging is called "uchi-zori (strike curve)", and is the operation of heating the blade in the furnace, striking the spine side upward in an arc while the edge side is placed against a supporting anvil (dai-gane). How much uchi-zori to apply differs depending on the type of steel used, the forging method, and the target final curve, and requires judgment unique to each craftsperson.

Curve is not merely an aesthetic feature, but has functional significance. Proper curve facilitates "pulling cuts" during strikes and optimizes the contact angle between the edge and the target to enhance cutting ability. Excessive curve leads to reduced blade strength and poor operability, and if the curve is too shallow, thrusting actions are limited. As the era, school, and fighting style change, so does the ideal amount of curve, and this has created the diversity of forms (sugata) in Japanese swords across each era.

Saki-marume (tip rounding) and formation of the kissaki (point)

Once the blade body shape is refined, the "kissaki (point)" at the tip is formed. The kissaki is not simply a sharp point, but has a unique form demarcated by a clear boundary line called "yokote (crossline)", with the kissaki-men (point face) and edge joining at a precise angle.

Formation of the kissaki is a particularly advanced technical part of fire-forging. The character of the sword changes greatly depending on the "size", "roundness", and "angle of the point" of the kissaki.

The bold large kissaki of the great tachi of the Nanboku-chō period and the delicate small kissaki of the small tachi in the peaceful Edo period clearly reflect the differences in historical background and demands as a weapon.

When striking the kissaki, particularly careful temperature management is necessary. Because the tip has a small amount of steel and low heat capacity, the temperature drops quickly after being removed from the furnace. If struck forcefully when the temperature has dropped too much, cracks will form, so the shape is gradually refined through repeated small-scale reheating. Some sword smiths describe the striking out of the kissaki as "the moment of creating the sword's face", which conveys its importance and difficulty.

The individuality of the sword determined by fire-forging — the trace of the craftsman's hand

The blade that has undergone fire-forging still has a rough surface, but its "skeleton" and "form" are already determined. At this stage, the blade is called "kiji (bare steel)" or "shiro-tanya (white forging)", and in the subsequent stages of clay application, heat-treatment, and polishing, the form and structure of this bare steel are not greatly changed.

Fire-forging is the process that determines the unique individuality of the sword. Countless elements such as the following combine to create a single blade that will never have the exact same form again.

• The craftsman's physique
• Strength
• Dominant hand habits
• The state of the furnace that day
• The weight of the hammer used

Much of the reason Japanese swords are said to have "individuality" and "character" derives from the subtle differences created in the fire-forging process. TOUKENZA's online catalog features swords with diverse forms that reflect the individuality of each sword smith's fire-forging. For more details, please contact us through our contact form.