Mizuheshi and Kowari — Preparing and Selecting Tamahagane Before Forging

Preparatory Processes Before Forging Begins

When one thinks of Japanese sword forging, the dramatic scene of red-hot steel being struck with a hammer—orikaeshi tanren (折り返し鍛錬)—comes to mind. But before entering that spectacular process, the swordsmith performs unglamorous preparatory work to make tamahagane (玉鋼) easier to forge and to ensure consistent quality. The two representative processes are mizuheshi (水減し) and kowari (小割り).

Mizuheshi—Flattening the Bloom

Tamahagane—high-quality steel nuggets produced by crushing the raw bloom (kera (鉧)) obtained from tatara smelting—exists initially as shapeless lumps. The swordsmith first heats these in a furnace and then strikes them with a hammer to extend them into thin, flat plates. This process is called mizuheshi (水減し).

Mizuheshi serves two purposes. First, when steel remains as lumps, heat cannot penetrate uniformly to the interior, and the interior cannot be examined by breaking it open. Flattening the steel into thin plates makes it easier to handle. Second, during heating and hammering, impurities contained in the steel are forced to the surface and removed to some extent. With each strike, sparks fly along with slag (kasu (滓)), and the steel gradually becomes purer.

The name 'mizuheshi' is often explained as deriving from the technique of rapidly cooling the heated steel in water before breaking it. Rapid cooling makes the steel brittle and easier to shatter during the subsequent kowari process.

Kowari—Breaking and Evaluating Quality

The swordsmith breaks the steel flattened through mizuheshi into small fragments. This is kowari (小割り).

The essence of kowari lies not merely in shattering the steel, but in selection. The swordsmith examines each fragment's fracture surface one by one, observing the color, crystal structure, and luster. From the fracture surface, they identify pieces with high carbon content, low carbon content, and areas with excessive impurities, then sort them according to their intended use.

An experienced swordsmith can judge the hardness of each fragment from the silvery-white gleam and texture of its fracture surface. This is tacit knowledge—something that cannot be fully conveyed in words and must be learned through the eyes and hands.

Sorting into Kawagane and Shingane

The tamahagane sorted through kowari is divided based on which part of the blade it will be used for. Most Japanese swords have a structure (tsukurikomi (造り込み)) in which hard, high-carbon kawagane (皮鉄—outer steel) envelops softer shingane (心鉄—core steel).

High-carbon, premium fragments are assigned to the kawagane, which provides sharpness; low-carbon fragments go to the shingane, which provides resistance to breakage. The precision of this sorting determines the success of subsequent forging and tempering, and ultimately the performance of the finished blade. It can be said that much of the blade's essential quality is already decided at the preparatory stage.

Moving Forward—Tsumi-wakashi and Orikaeshi Tanren

The fragments selected through kowari are stacked on a platform attached to the end of a pry bar, dusted with rice-straw ash, coated with clay slurry, and heated in a furnace. This process of heating them until the fragments weld together at high temperature is called tsumi-wakashi (積み沸かし). From here begins the well-known orikaeshi tanren. The swordsmith repeatedly strikes and extends the steel, then folds it back—through this process, the carbon content becomes uniform, impurities are further expelled, and a layered, resilient steel develops. Because mizuheshi and kowari have ensured consistent material quality, the welding in tsumi-wakashi remains stable and the forging achieves full effect. Preparation and production form an unbroken sequence.

The Unglamorous Processes That Determine the Blade

Mizuheshi and kowari are processes seldom discussed compared to the glamorous forging and tempering stages. Yet it is this initial selection—discerning the quality of materials and sorting them by intended use—that establishes the foundation of each blade. The accumulated judgments of choosing good material and discarding poor sections create differences that cannot be recovered in later stages.

Understanding the weight of this preparatory work, one can sense in the completed blade's jigane (地鉄—grain structure) and hamon (刃文—temper line) the countless fragments the swordsmith broke apart and the careful attention devoted to each one.

A Guide to Appreciation—Traces of Preparation in the Jigane

The surface of a finished blade displays hada (肌目—grain patterns) created through forging, such as itame (板目), masame (柾目), or mokume (杢目). Jigane that is well-consolidated and conveys a sense of moisture is said to result from material selection and forging working together at a high level. When viewing a sword in an exhibition or photograph, look not only at the hamon but also at the fineness and flow of the jigane. When you can imagine the soundness of the unseen mizuheshi and kowari processes from these features, your appreciation will deepen significantly.

Glossary of Terms

• Tamahagane (玉鋼): The high-quality portion of steel obtained from tatara smelting that is suitable as sword material.
• Kera (鉧): The raw bloom of iron that forms inside the tatara furnace. It is broken apart, sorted internally, and separated into tamahagane and other grades.
• Kawagane (皮鉄) / Shingane (心鉄): The harder steel that forms the outer wrapper and the softer steel placed at the core. This structure supports the simultaneous fulfillment of contradictory demands—cutting well while resisting breakage.
• Wakashi (沸かし): Heating steel to the high temperature at which it can be welded. Temperature is judged by experience, observing the color of the flame and steel.
• Orikaeshi tanren (折り返し鍛錬): The repeated process of striking and extending heated steel, cutting and folding it, then rewelding it. With each repetition, layers increase and the steel becomes more homogeneous.

Exploring Sword Craftsmanship

TOUKENZA (operated by M-Assets Co., Ltd.) is an online marketplace for selling Japanese swords. Understanding the techniques of sword forging provides a path to deeper appreciation of each blade. Swords currently listed can be viewed in our online catalog and purchased. If you have questions about a particular blade or need any clarification, please contact us through our inquiry form.