Kokaji and Okaji: The Two-Stage Workshop Division in Japanese Swordmaking and Its Modern Inheritance

The Meaning of "Kokaji" and "Ookaji" Terms

While today it is common for a single sword smith or bladesmith to handle everything from forging the blade to finishing, historically there existed a functional division between "ookaji" (大鍛冶) and "kokaji" (小鍛冶).

The ookaji was responsible for refining and shaping tamahagane, preparing the steel material into a state that was easy for sword smiths to work with. In contrast, the kokaji was a craftsman who actually forged the blade using the refined material, and represents the direct ancestor of the craftsmen later called "tosho" (sword smiths).

This division of labor was an extension of the role division between "kanashi" and "toko" in government-run forges before the Nara period, and it persisted as functional specialization even in the Kamakura period and beyond during large-scale commissioned production phases.

Ookaji Techniques—The Science of Material Making

The core of the ookaji's work is "steel homogenization" and "carbon content control."

Tamahagane produced through tatara iron smelting has uneven carbon distribution depending on the furnace location and quantity charged. Areas with high carbon content are hard and brittle, while areas with low carbon content are soft. Using this uneven material directly for blade forging results in unstable hardness during quenching.

The ookaji heated tamahagane to an appropriate temperature and forged it to homogenize the carbon distribution, creating "refined material" that was easy for sword smiths to work with. The "oroshi-tetsu" or "refined forged steel" produced in this process served as preparatory material to make subsequent folding and forging more effective.

The technical challenge at the ookaji stage lies in "judging heating temperature." If too high, the carbon burns and scatters; if too low, adequate forging cannot be achieved. The craftsman's skill centers on visual judgment of flame color and iron color based on experience.

Kokaji Techniques—The Blade Forging Process

The kokaji's work proceeds through the following stages: "ground forging (shitaji-tanae) → folding and forging (oriakaeshi-tanren) → fire forming (hi-zukuri) → stretching (sunode) → rough finishing."

Folding and forging is the most distinctive process, involving heating the steel in the forge, flattening it with a hammer on an anvil, folding it from the edges, and repeatedly reheating and hammering. Through this folding, impurities (slag) in the steel are expelled and a laminated structure forms. The number of folds varies by school and craftsman, but typically 8–15 folds create a layered structure of approximately 2,000–30,000 layers. Excessive folding can cause embrittlement, so the judgment of the appropriate number of folds reflects the craftsman's expertise.

Fire forming is the process of creating the external shape of the blade, where heated steel is hammered with a special hammer to shape the blade's form—the curve of the edge, the width of the body, and variations in thickness. In this process, forming the "curve" (sori) is also important, and the technique of controlling uniform heating and cooling during forging determines the quality of the curve.

Historical Changes in Division of Labor—Why They Merged

It is believed that during the Kamakura period of high sword demand, the division of labor between ookaji and kokaji was functionally maintained. Research suggests that in the Osafune school of Bizen (Okayama), workshop organization developed and work from tamahagane refining to final finishing was systematically divided.

The division of labor dissolved due to multiple factors. Workshop organization was dismantled by warfare during the late Muromachi and Sengoku periods; during the peaceful Edo period, individual specialization of craftsmen advanced; and the evaluation of "solo smith" sword smiths who handled all work from material to completion increased—these combined to establish the modern image of "a single craftsman performing all processes."

Modern Inheritance—The Rehabilitation of Ookaji Processes

The tamahagane used by modern sword smiths is produced by tatara smelting (Okuizumo, Shimane Prefecture) managed by the NBTHK (Japan Art Sword Preservation Society), with some quality control in place. However, tamahagane varies in carbon content and hardness depending on the lot, and modern sword smiths rarely use it as-is; the general procedure is to first perform "ookaji-style refining processes" themselves to refine the material before beginning folding and forging.

In other words, modern sword smiths are craftsmen who "perform both ookaji and kokaji work by themselves" and inherit the techniques of both aspects of historical specialization. In this sense, it can be said that the technical demands on modern sword smiths are actually higher than those on historically specialized sword smiths.