Fold-Forging and Quenching — 32,768 Layers and Moonlight on a Dark Night

Fold-Forging — Why Fold So Many Times?

The tamahagane obtained from the tatara as covered in [Tamahagane and Tatara Smelting] cannot become a sword as it is. Heated steel is hammered flat, folded in half, reheated, hammered again — repeated 10-15 times. This is orikaeshi-tanren (fold-forging). Why?

Two reasons:

1. Removing impurities — Residual phosphorus, sulfur, and oxides in the tamahagane are scattered as sparks with each fold
2. Homogenizing carbon content and refining the microstructure — Smoothing out the unevenness of the tamahagane

The physical meaning is the creation of a layered structure. One fold gives 2 layers, two folds 4, 10 folds 1,024 layers, 15 folds 32,768 layers. Each layer becomes only a few microns thick, and impurity and carbon distribution become homogeneous.

That said, “more folds is better” is not true. Each heating drives carbon out of the surface. To maintain the 0.6-0.8% needed for kawagane, the smith must stop at the right count. This too is a matter of judgment.

Shita-gitae and Age-gitae — Two Stages

Fold-forging usually proceeds in two stages:

Shita-gitae (foundation forging)

• Consolidated steel is folded 5-6 times
• The main purpose is removal of impurities (phosphorus, sulfur, oxides)
• Each fold produces sparks — the impurities burning off
• After shita-gitae, the steel has lost more than half its original mass

Age-gitae (finishing forging)

• The shita-gitae steel is cut, restacked and welded, and folded 5-6 more times
• The main purpose is refinement of microstructure and homogenization of carbon
• Finishing the steel to a state usable as kawagane

10-12 folds total is common. Earlier eras used 7-8; later smiths sometimes went as high as 15. Choice varies by school and by smith.

Jihada — The “Landscape” Left by Folding

The traces of folding appear on the surface of the finished blade as jihada. When the polisher brings up the ji with jito (ground stones), the layers become a visible “landscape” to the naked eye. Major types:

• Itame-hada — Wood-grain figure. The most common
• Masame-hada — Straight linear grain. Common in the Yamato tradition (Hoshō, Senjuin)
• Mokume-hada — Whorled figure. Ko-Bizen, Ko-Aoe, late Heian to early Kamakura
• Ayasugi-hada — Regular wave pattern. Characteristic of the Gassan school in Dewa
• Nashiji-hada — Fine granular figure. The refined jihada of the Awataguchi school
• Ko-itame-hada — Fine itame. Common in the Shintō era

The direction of folding (tateori, yokoori, or slanted) changes how the jihada appears. Vertical folding tends to itame or mokume; horizontal folding to masame.

Ji Effects — Chikei, Yubashiri, Ji-utsuri

Within the jihada, still finer “workings” appear:

• Chikei — Black linear lines glowing within the jihada
• Yubashiri — White spots of quench that appear outside the hamon
• Ji-utsuri — A whitish shadow that runs parallel to the hamon within the jihada. A hallmark of Bizen swords.
• Shirake-utsuri — Similar to ji-utsuri but paler and more diffuse

These arise from a combination of the smith’s deliberate technique and the incidental behavior of the steel. The most delicate region of connoisseurship.

Yaki-ire — The Highest Peril, in Seconds

After fold-forging, tsukurikomi, sunobe (rough drawing), and hizukuri (fire-shaping), the final step is yaki-ire (quenching). The blade built up over weeks is heated in the forge and plunged into water — its fate decided in a matter of seconds. Failure means cracks, warping, or no hamon. Success means a masterwork.

The chemistry of quenching — martensitic transformation

• Heating steel above about 750 °C produces austenite
• Rapid cooling then fixes it as martensite — hard, but brittle
• Slow cooling produces pearlite/ferrite — soft

In a Japanese sword, only the edge is rapidly cooled to martensite, while the ji is kept slow-cooling. The medium of control is the yakiba-tsuchi.

Yakiba-tsuchi and Tsuchi-oki — The Moment That Decides the Hamon

The composition of yakiba-tsuchi is a secret handed down within each school. It is generally clay-based, mixed with charcoal powder and stone powder from polishing. The proportions determine its insulation and adhesion, and directly affect the resulting hamon.

The tsuchi is applied after the blade has been degreased: thin along the edge (tens of microns), thick over the ji (a few millimeters). The line where thin meets thick decides the shape of the hamon:

• Suguha — A straight edge to the tsuchi
• Midareba — A wavy or irregular edge
• Gunome — A regular succession of round arches
• Chōji — In the shape of clove flowers. The signature of Bizen.

Some smiths spend hours on the tsuchi-oki. The fineness of the particles, the modulation of thickness — everything connects to the hamon that will appear.

Moonlight on a Dark Night — Judging the Heat

After the tsuchi has dried, the blade is placed in the forge. The forge is a traditional fire pit fed by wood and pine charcoal — there is no thermometer. The smith reads the color of the incandescent blade in the fire.

The traditional dictum is “judge the color by moonlight on a dark night.” The austenitizing temperature of 750-800 °C must be picked out from the subtle color range from dim red to a red closer to orange. It cannot be judged under electric light, and even modern smiths dim the lights at the moment of quench.

At the decided instant, the blade is pulled out, held vertically, and plunged into a water tank. Water temperature is usually around 13-15 °C. Some schools (Hōki, for instance) use warmer water. Too fast, and the blade cracks; too slow, and the edge lacks hardness.

The Instant of Quench — Where the Curve Is Born

In the water, a great burst of steam rises with a hiss. In a few seconds the blade cools and martensitic transformation completes. At the same moment, the volume expansion of the edge curves the blade toward the spine.

The added curvature is generally 5-10 mm. Anticipating this, at the hizukuri stage the smith deliberately leaves the blade “reverse-curved” or nearly straight, so that after quench it settles into just the right sori. The curve is not a directly intended shape — it is a natural consequence of a fully mastered quench technique.

Hamon — The “Painting” of a Glowing Boundary

The hamon is not mere ornament: it is the boundary of hardness difference between the edge (ha) and the ji. Differences in how light is reflected make the boundary appear white. Major categories:

Broad categories

• Suguha — Straight hamon. Archaic, dignified. Common in Yamashiro-tradition Awataguchi and Rai
• Midareba — Wavy hamon

Subcategories of midareba

• Gunome — Successive round arches
• Chōji — In the shape of clove flowers. Ko-Bizen through the Osafune school
• Notare — Broad, gentle waves. The Sōshū signature.
• Hako-midare — Square-shaped succession
• Sanbon-sugi — Cedars in groups of three. Signature of Mino / Seki no Magoroku (Kanemoto).
• Hitatsura — All-over quench, reaching into the ji

Bōshi — The Hamon in the Kissaki

The hamon in the kissaki (tip) is called the bōshi. The shape of the bōshi is intensely individual — school by school, smith by smith — and one of the most important elements in authentication:

• Ko-maru — Small round return in the kissaki
• Ō-maru — Large round return
• Midare-komi — The hamon flows into the kissaki as-is
• Yaki-zume — The yaki stops in the kissaki, without a return

From This Article to the Next

From material through construction, forging, and quench — we have followed the process by which a blade comes into being. From the next article on, we enter the schools that systematized these techniques by region and period — the Gokaden. The five great producing regions of Yamato, Yamashiro, Bizen, Sōshū, and Mino, each with its own jihada, hamon, and form. We start with an overview of the Gokaden as the framework that still underpins connoisseurship today.