The Hizukuri Process Explained: Steel Deformation and Microstructure Control from Sunobe to Rough Finishing

What is Hi-Zukuri? —The Core of the Forging Process

"Hi-zukuri" (fire-forging) is the process that follows "so-nobe" (rough drawing out) in Japanese sword forging, and it is the most critical process for refining the final external form of the blade—the edge thickness, the ridge shape, the point shape, and the finishing of the tang.

If so-nobe establishes "the rough skeleton of the sword (length, curvature direction, and point-to-handle taper)," then hi-zukuri is "the stage where fine details (edge, mune, kissaki, groove, machi, and nakago) are sculpted." This is the process where the swordsmith's skill is most tested, and any failures at this stage cannot be corrected in later processes.

Prerequisites for Hi-Zukuri—The Blade State After So-Nobe

At the point when hi-zukuri begins, the blade (in its post-so-nobe state) exists in the following condition:

• The edge length and point-to-handle taper are roughly established
• The blade thickness (kasane) is still uneven and coarse
• The kissaki (point), hasaki (edge), mune (ridge), and nakago (tang) are not clearly distinguished—it is a "flat, rod-like mass"
• The entire blade is covered with an oxide layer (black skin)

From this state, hi-zukuri creates the specific forms that determine the individual character of the blade.

Main Tasks of Hi-Zukuri

1. Drawing Out the Edge (Hasaki) Thinly—Creating Edge Thickness

One of the first tasks in hi-zukuri is precision hammering with the "ko-zuchi" (small hammer) to draw out the edge thinly.

Technical Points

• To thin the edge, adjust the hammer face angle so it strikes in the direction of the edge
• The heating temperature should be slightly below white-heat (1200–1300°C), around "red-heat" (900–1000°C). If too hot, the steel becomes too soft to maintain shape
• Keep the amount of deformation per strike small, thinning gradually over multiple strikes
• If the edge becomes extremely thin, it tends to crack during quenching, so the craftsman must develop a sense for the "limit of thinness"

Types of Hammers

• Oo-zuchi (large hammer): 1–2 kg. Used for rough, general deformation
• Ko-zuchi (small hammer): 200–400 g. Used for precision shaping of details
• Kaku-zuchi (square hammer): A hammer with a square cross-section. Used for shaping the mune and machi areas

2. Shaping the Mune (Ridge)—Creating the Blade's Back

The mune is the back side of the blade (opposite the edge) and is a critical area whose shape is determined during hi-zukuri.

Types of Mune and Shaping Methods

• Iori-mune (roof ridge): A ridge with a triangular cross-section (▲). A triangular peak is formed from the flat (hira) section through a ridge line. The most common ridge form
• Maru-mune (round ridge): A ridge with a round cross-section. Shaped while adding roundness with the hammer
• Kaku-mune (square ridge): A ridge with a square cross-section. Requires more complex shaping

Since ridge shaping directly affects the blade's center of gravity balance, the swordsmith must constantly be aware of the balance between edge thinness and ridge thickness.

3. Shaping the Kissaki (Point)—Creating the Blade's Tip

The kissaki is the blade's tip and is a critical area that determines both the thrusting capability and aesthetic beauty.

Kissaki Shaping Technique

• At the so-nobe stage, the tip is in a rounded, mass-like state
• During hi-zukuri, the ko-zuchi is used to shape the kissaki while creating the "yokote-suji" (the line between the kissaki and blade)
• Whether an inokubi-kissaki (small point) or oo-kissaki (large point) is decided in the design phase and finished to that shape during hi-zukuri
• Since the kissaki is the thinnest and most delicate area, temperature control and hammer force are especially challenging

4. Shaping the Machi (Guard Junction)—Creating Hama-Chi and Mune-Chi

The machi is the boundary between the blade and tang, with two locations: hama-chi (edge-side boundary) and mune-machi (ridge-side boundary).

The position and shape of the machi affect the blade's "balance point" and directly influence the feel of drawing and swinging the sword. During hi-zukuri, the machi's shape (angle and depth) must be precisely determined.

5. Shaping the Nakago (Tang)—Creating the Handle-Mounting Portion

The nakago is the part that fits into the tsuka (handle), and its thickness, length, and shape (rod-tang, sword-shaped tang, crow-neck tang, etc.) vary depending on the sword's purpose and the swordsmith's design intent.

During hi-zukuri, the nakago is drawn out thin and long, the position of the mekugi-ana (peg hole) is determined, and the nakago is finished to its rough form (the peg hole is drilled in a later process).

Temperature Control—The Most Critical Technical Element in Hi-Zukuri

Hi-zukuri is a process of repeated "heating in the forge→shaping→cooling (or reheating)," but temperature management at each stage has a decisive impact on the final result.

Dangers of Overheating (Burn/Overheating) Striking steel when overheated beyond white-heat causes the steel crystals to coarsen (overheated structure), drastically reducing toughness. This is a fatal defect that cannot be fully corrected by later quenching. Experienced smiths judge temperature by observing the color of the flame and the steel's surface color.

Striking at Optimal Temperature (Forging Temperature) Striking at the optimal forging temperature (typically 900–1150°C, depending on steel type) maximizes the steel's deformability. At this temperature range, the steel is soft enough to shape precisely.

Dangers of Striking at Low Temperature (Cracking) Striking when the temperature has become too low results in "cold working," causing the steel to crack or chip. The principle "if it turns black, return it to the forge" is a fundamental rule of forging.

The Impact of Hi-Zukuri on Steel Microstructure—Why Both "Form" and "Quality" Change

Hi-zukuri is not merely "shaping." During this process, the steel's microstructure also changes, affecting the beauty of the hamon that emerges during later quenching.

Microstructural Refinement Through Forging Repeated heating, deformation, and cooling refine the steel's crystal grains. The finer the grains, the finer and denser the jihada (steel surface) appears after polishing, earning the evaluation "well-consolidated ground."

Redistribution of Carbon As the steel is deformed during hi-zukuri, carbon (Fe and C compounds) is redistributed within it. This homogenization of carbon distribution affects the sharpness of the boundary between the "yakiba" (hardened edge) and "jihada" (ground surface) created during quenching.

Handling Scale (Oxide Layer) During hi-zukuri, an oxide layer (black skin/scale) forms on the steel surface. Each hammer strike scatters this scale. If scale is driven into the steel, it becomes a ground defect (jikizu), so work proceeds while removing scale by applying straw ash or water.

Ara-Shiage (Rough Finishing)—The Final Stage of Hi-Zukuri

The final step of hi-zukuri is "ara-shiage" (rough finishing). The swordsmith rechecks the overall blade shape and adjusts for left-right symmetry, uniform edge thinness, and ridge straightness.

Once ara-shiage is complete, the blade enters the final inspection state before proceeding to the next steps of "tsuchi-oki" (clay application) and "yaki-ire" (quenching). The swordsmith visually and tactilely reinspects the blade here, returning any areas needing correction to hi-zukuri.

Conclusion—Hi-Zukuri Determines the Blade's Lifetime

Hi-zukuri is one of the most critical processes, determining both the "form" and "quality" of a Japanese sword. The edge thinness, mune shape, kissaki beauty, and machi position—all are determined during hi-zukuri and cannot be altered in later processes.

Through the compound control of three elements—temperature management, hammer angle, and force application—the swordsmith freely manipulates the steel, creating the blade shape as designed. It is said that mastery of this process requires over a decade, not merely due to technical difficulty, but to internalize the sense of "dialogue with steel."

The traces of hi-zukuri are eternally inscribed in the finished blade's "sugata" (form). When appreciating a sword, feel in its graceful curves and precise kissaki the hours of hi-zukuri the swordsmith spent before the forge.