Fold Count in Tamahagane Forging: How 15 vs 20 Folds Affect Jihada Texture and Hamon Quality

What Is Folding and Forging (Orkaeshi Tanren)?

In Japanese sword manufacturing, "folding and forging (orkaeshi tanren)" is one of the most important processes that determines the quality of steel. Tamahagane is heated and flattened, then folded and forged again—through this repetition, impurities are removed, the steel's structure is homogenized, and ultimately the beauty and strength of the blade's jihada are created.

While it may appear to be a simple operation at first glance, the number of folds has a decisive impact on the steel's properties. Too few folds leave impurities and result in coarse structure; too many cause excessive carbon loss, making the steel brittle. The sword maker discerns this balance while observing the material's condition and determines the number of folds accordingly.

The Mathematics of Folding and Forging—Calculating Layers

The relationship between the number of folds and the number of steel layers can be expressed as a simple sequence. One fold creates 2 layers. Two folds create 4 layers, three folds create 8 layers, four folds create 16 layers—for n folds, the number of layers is 2ⁿ.

Based on this calculation:

• 10 folds: 1,024 layers
• 15 folds: 32,768 layers
• 20 folds: 1,048,576 layers (approximately 1 million layers)

Looking at the numbers alone, 20 folds would seem to produce far more layers and result in much denser steel. However, the reality is not so simple. With each additional fold, the thickness of each layer is halved, and beyond a certain number of folds, individual layers become only a few atoms thick. Further layer formation becomes physically meaningless, and the process simply continues to lose carbon.

The Relationship Between Carbon Content and Folding

The greatest factor determining steel's properties is its carbon content. More carbon makes it harder but more brittle; less carbon increases toughness but reduces hardness. Japanese swords achieve a balance between hardness and toughness by adopting structures such as "koubuse," which combines high-carbon "kawagane" at the edge and low-carbon "shingane" at the core.

Folding and forging also affects carbon content. During forging, "decarburization" occurs as carbon escapes into the atmosphere from the steel surface exposed to high temperature. The more folds, the longer the total forging time and the greater the decarburization. From the initial carbon content of tamahagane (approximately 1.0–1.5%), the sword maker manages carbon content during forging toward the appropriate final level required for the blade (approximately 0.6–1.0% in the hardened edge).

There is a clear difference in decarburization between 15 and 20 folds. With 20 folds, there is a risk that the steel will lose too much carbon and approach low-carbon steel that is difficult to harden. For this reason, many contemporary sword makers choose 8 to 15 folds as their standard forging count.

Impact on Jihada

Jihada refers to the grain pattern (hadame) that appears on the blade's surface—essentially the steel's organizational pattern analogous to the grain of wood. The jihada pattern changes depending on the number and direction of folds.

Typical jihada patterns include the following:

• Itame: Wood-grain-like pattern. Tends to develop with relatively few folds or forging in one direction
• Masame: Linearly aligned pattern. Appears when extensive vertical forging is performed
• Mokume: Swirling, knot-like pattern. Emerges from multi-directional forging and folding

As the number of folds increases, jihada tends to become more uniform and dense overall. However, if there are too many folds, individual layers become indistinguishable and the jihada pattern itself disappears. From an aesthetic perspective, jihada with moderate grain is said to produce beauty in "utsuori" and skin quality, while overly uniform jihada can give a monotonous impression.

Impact on Hamon Quality

Folding and forging also indirectly affects hamon quality. The hamon created during hardening is determined by the blade's carbon distribution, structural uniformity, and residual stress distribution. Steel forged to uniformity exhibits predictable transformation behavior during hardening. This forms the foundation for producing distinct, complex hamon featuring chouji, midare, nioi, and nie.

Conversely, if forging is insufficient, structure is non-uniform, or carbon is unevenly distributed, unexpected transformation during hardening can cause the hamon to become distorted or, in the worst case, cause hardening cracks. As a prerequisite for creating hamon through "tsuchoki," uniform steel structure is essential. Folding and forging creates this uniformity, and proper fold count and process management ensure the hamon quality of a masterpiece blade.

Optimal Fold Count from the Contemporary Sword Maker's Perspective

When asked about the number of folds, many contemporary renowned sword makers answer "approximately 10 to 16 folds." This range is based on empirical knowledge that best balances three elements: impurity removal, structural homogenization, and carbon content management.

However, the optimal number of folds varies depending on the quality and carbon content of the tamahagane used, forging temperature, and target jihada pattern. Experienced sword makers make real-time judgments while observing the material's condition, and there is no fixed answer to "how many folds is correct." This flexible judgment is the essence of a sword maker's skill.

The exploration of folding and forging is a domain where steel metallurgy and artisan intuition intersect. In modern times, structural observation via electron microscopy and chemical analysis have become possible, and scientific elucidation of traditional techniques is advancing. However, ultimately, the sword maker's own eyes, hands, and experience hold the key to reading the steel's "condition" that cannot be expressed in equations and producing the finest blade.