From Tamahagane to Finished Sword: The Complete Folded Forging Process and Its Metallurgical Science

What is Folding and Forging? — The Intersection of Tradition and Science

"Folding and forging (orikaeshi tanren)" is a core process in Japanese sword forging technique. Steel is heated and struck, then folded and struck again — through this repetition, impurities are removed and high-quality steel with a uniform crystalline structure is created.

At first glance it appears to be a simple repetitive task, but when analyzed from the perspective of modern metallurgy, it proves to be an extremely rational technique. In an era without thermometers or analytical equipment, swordsmiths judged the condition of steel by the color of the fire, sound, and the feel of the hammer blow alone, achieving optimal forging. The accumulation of this experience and sensibility represents the crystallization of craftsmanship that modern science has only recently come to explain.

Raw Materials — The Selection and Meaning of tamahagane

The starting point of a Japanese sword is "tamahagane." At the "Nitou-ho tatara" in the Okuizumo region of Shimane Prefecture, tamahagane continues to be produced today using the traditional tatara smelting method—reductive smelting using iron sand and charcoal.

The tamahagane obtained through tatara smelting is a non-uniform material with carbon content varying between approximately 0.5–1.5% depending on the location. The swordsmith first cleaves it through "kawawaari," then assesses the carbon content from the color, luster, and granular state of the cross-section to make a selection. This selectivity is said to be the first condition for a superior blade.

Kawagane contains a high carbon content and is a hard high-carbon steel used for the outer layers of the blade—the edge and surface. It bears the responsibility for wear resistance and cutting ability. On the other hand, shingane contains less carbon and is a tough low-carbon steel used for the core (interior) of the blade. By combining the hardness of the edge with the toughness of the core, contradictory properties are realized in a single blade—one that is both sharp and does not break.

From Basic Drawing to Folding and Forging — Impurity Removal and Microstructure Refinement

The chunk of selected steel is heated to approximately 1200–1300°C in the forge and struck with a hammer to stretch it. This work is called "sunobe." The purpose at this stage is threefold.

First, slag removal. When heated steel is struck forcefully, oxides and non-metallic inclusions (slag) remaining inside melt and fly out. Each strike produces showers of orange sparks for this reason. Next, uniformification of steel structure—the striking mechanically fractures and rearranges the metal's crystalline structure, refining coarse microstructure into something finer. And finally, shape formation—preparing the material in a plate-like form so that it will be easier to fold in subsequent processes.

The steel plate that has undergone sunobe becomes the material for folding and forging. This plate is reheated, folded in half, pressed together, and struck. By repeating this, the number of layers increases exponentially.

• 8 folds = 2⁸ = 256 layers
• 10 folds = 2¹⁰ = 1,024 layers
• 12 folds = 2¹² = 4,096 layers
• 15 folds = 2¹⁵ = 32,768 layers

Typical Japanese swords undergo 10–15 folds, giving rise to descriptions like "over a thousand layers" and "tens of thousands of layers." However, more folds are not necessarily better. Excessive folding causes carbon oxidation and decarburization through repeated heating and forging, which conversely degrades the steel quality. An experienced swordsmith judges the appropriate number of folds by feeling—reading the fire color, listening to the steel's sound, and sensing the hammer strike.

Metallurgical Effects of Folding and Forging

From the perspective of modern metallurgy, this process produces three important effects.

Uniform dispersion of impurities: Repeated striking fractures phosphorus, sulfur, and non-metallic inclusions within the steel into fine particles that disperse uniformly. Localized weak points disappear, resulting in uniform strength throughout the blade. This dispersed inclusion is also the origin of the "skin" (hada) that appears in the jigane—board-grain skin, wood-grain skin, straight-grain skin, and so forth.

Uniformification of carbon distribution: The non-uniform carbon distribution inherent in tamahagane becomes uniform through repeated diffusion and mixing. After quenching, hardness variation is eliminated, achieving stable cutting ability across the entire edge.

Formation of fine crystalline microstructure: Repeating the cycle of working and heat treatment refines the iron crystal grains, greatly improving toughness (jinsei). In coarse crystal structures, cracks propagate rapidly along grain boundaries, but in fine structures this propagation is suppressed. The characteristic toughness of Japanese swords largely depends on this fine microstructure.

Shaping (tsukurikomi) — Design Philosophy of Cross-Sectional Structure

The process of combining the kawagane and shingane that have undergone folding and forging to create the cross-sectional structure of the blade is called "tsukurikomi." Here the essential design philosophy of Japanese swords is concentrated.

Koubuse is the most common structure, with kawagane wrapping the shingane. This balance of hardness on the outside and toughness within realizes "the coexistence of sharpness and toughness." Honsanmai is an advanced structure combining three types of steel—edge, side, and spine—allowing optimal hardness to be positioned at each location, a technique used by master swordsmiths. Shihodzume is a structure where the shingane is wrapped by kawagane on all four sides, primarily used for short blades to maximize rigidity and keenness.

In modern composite materials engineering, the concept of "functionally graded materials (FGM)"—combining materials with different properties in layers to extract optimal performance—was intuitively practiced by Japanese swordsmiths centuries ago.

Toward Quenching — The Final Process That Defines Form

After shaping, the blade undergoes "hizukuri," rough shaping to approximate form, and "yasuri shiage," finishing the outline. Then it enters preparation for quenching.

In "tsuchioki," refractory clay is applied to the blade. Thinner application on the edge and thicker on the spine creates a difference in cooling rate during rapid cooling. The thin edge section is rapidly cooled into hard martensite structure, while the thick spine section cools slowly and retains a tough pearlite structure. This difference in cooling rate produces the hamon, a beautiful line, while simultaneously imparting the dual functionality of edge hardness and spine toughness in a single blade.

"Yaki-ire" (quenching) is the culmination of the swordsmith's technique. When the edge reaches a temperature around 750–800°C (the swordsmith judges by fire color), it is plunged into a water tank for rapid cooling. Even slight temperature variation affects the shape of the hamon; overheating causes cracking, low temperature results in insufficient hardening. A split-second judgment concentrates years of experience.

Conclusion — The Place Where Artisanal Sense and Science Converge

Japanese sword forging may appear "inefficient" to modern industrial technology. Yet a blade created through traditional hand-forging possesses unique characteristics not easily reproduced in industrial products. Selecting non-uniform tamahagane as the material, uniformifying it through folding and forging, designing function through shaping, and establishing performance through quenching—this entire sequence converged toward optimal solutions through swordsmith experience long before modern metallurgy systematized them.

The fine microstructure folded into countless layers, the temperature read by the skilled eye from the fire's color, the judgment at the moment of immersion in water—all these accumulate to give birth to the Japanese sword, both artwork and weapon. Even now that science has come to explain it, its profundity remains undiminished. By comparing photographs of jigane and hamon published in online catalogs, one can verify with one's own eyes how the traces of this forging appear.