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

The Meaning of "Kokaji" and "Ōkaji"

Today, it is common for a single swordsmith or blacksmith to handle the forging and finishing of a blade from start to finish, but historically there existed a functional division of labor between ōkaji (large-scale metalworking) and kokaji (small-scale forging).

The ōkaji specialized in refining and shaping tamahagane, preparing the steel as a material into a condition convenient for swordsmiths to use. By contrast, the kokaji were the craftspeople who actually forged the blade using the refined material, and they were the direct ancestors of what would later be called swordsmiths.

This division of labor was an extension of the role division between kanashi (iron workers) and tokoō (blade makers) in government-controlled metalworking before the Nara period, and it continued to exist as a functional specialization even after the Kamakura period in large-scale commissioned production.

The word "kaji" itself is composed of two kanji: "tan" (to forge/strengthen) and "ya" (to melt/work). The former refers to the work of striking to increase strength, while the latter refers to the work of melting metal and bringing it into a more workable state. In the very formation of this word, we can see evidence that "refining" and "forging" were originally distinct skills.

The Technique of Ōkaji—The Science of Material Making

The core of the ōkaji's work was "homogenizing the steel" and "controlling the carbon content."

Tamahagane produced by tatara iron-making has an uneven distribution of carbon content depending on the position in the furnace and the amount of input. Areas with high carbon content are hard and brittle, while areas with low carbon content are softer. If this uneven material is used directly for the blade, the strength during quenching becomes unstable.

The ōkaji would heat the tamahagane to an appropriate temperature and forge it to homogenize the distribution of carbon, creating "refined material" that was convenient for swordsmiths to use. The "oroshi-tetsu" or "refined forged material" produced in this process was a preparatory material to make the subsequent lamination forging more effective.

The technical challenge at the ōkaji stage was "judging the heating temperature." If it is too high, the carbon burns and scatters; if too low, sufficient forging cannot be achieved. Visual judgment of flame color and iron color based on experience became the key to craftsmanship.

In the ōkaji workshop, personnel were needed to constantly operate the bellows to maintain a constant temperature in the forge hearth, and roles dedicated to furnace management were often placed separately from the craftsperson handling the forging itself. The fact that material-making was not a single skill but depended on the coordination of multiple tasks—furnace management, air supply, and striking—was one of the reasons why ōkaji became an independent specialized field.

The Technique of Kokaji—The Process of Creating the Blade

The process handled by kokaji proceeded in the following sequence.

1. Base forging (shitaji-tanae)
2. Lamination forging (orikaeshi-tanren)
3. Fire-shaping (hizukuri)
4. Preliminary drawing (sunobe)
5. Rough finishing

Lamination forging is the most distinctive process, in which steel is heated in a furnace, struck with a hammer on an anvil to flatten and extend it, folded over from the edges, and then reheated and struck again in repeated operations. Through these foldings, impurities (slag) in the steel are expelled and a layered structure is formed. The number of folds varies depending on the school and the craftsperson, but typically around 8 to 15 folds produce a laminated structure of approximately 256 to 32,768 layers (8 folds yield 2^8 = 256 layers, 15 folds yield 2^15 = 32,768 layers). Excessive folding can cause brittleness, so the judgment of an appropriate number of folds becomes an expression of the craftsperson's discernment.

Fire-shaping is the process of creating the external form of the blade, striking heated steel with a special hammer (upper and lower anvils) to form the shape of the blade—the curve of the edge, the width of the body, and the variation in thickness. In this process, the formation of curvature (sori) is also important, and the technique of controlling uniform heating and cooling during forging determines the quality of the curve.

The work of kokaji was also not completed by a single person. A collaborative work called aizuchi (synchronized striking) was widely practiced, in which the master swordsmith (oyakata) would lightly tap the hammer to indicate the position and force of the strike, and apprentices and assistants would swing the heavy hammer in synchronization with this. As there is a theory that the phrase "striking in agreement" in conversation derives from this collaborative forging work, lamination forging was a process that was established through the coordinated effort of multiple people working in sync.

The Transition of Historical Division of Labor—Why Did They Become One?

It is thought that during the Kamakura period, when demand for swords was high, the division of labor between ōkaji and kokaji was functionally maintained. Research suggests that in the Osafune school of Bizen (Okayama), workshop organization developed and the work from refining tamahagane to finishing the completed product was systematically divided.

The division of labor was eliminated due to multiple factors.

• Workshop organizations were disbanded due to warfare from the late Muromachi period through the Sengoku period
• During the peaceful Edo period, individual craftspeople became more specialized
• The evaluation of the "single swordsmith" who handled everything from material to completion increased in value

These factors combined to establish the modern image of the "swordsmith who handles all processes alone."

Considering these transitions, it is not easy to determine from appearance alone whether most existing blades belong to the category of "blades made under a division of labor" or "blades in which a single swordsmith handled all processes." This is a matter that requires comprehensive examination in conjunction with transmission information and signatures regarding era, production site, and school, and cannot be determined by the appearance of the jihada or the hamon alone.

Inheritance into the Modern Era—The Revival of Ōkaji Processes

The tamahagane used by modern swordsmiths is produced at tatara iron-making (Okuizumo, Shimane Prefecture) managed by the NBTHK (Japan Sword and Art Preservation Society), and a certain level of quality control is maintained. However, tamahagane varies in carbon content and hardness from lot to lot, and modern swordsmiths also typically do not use the tamahagane as-is; rather, the standard procedure is to first conduct their own "ōkaji-style refining process" to prepare the material before proceeding to lamination forging.

In other words, modern swordsmiths are craftspeople who "perform both the ōkaji and kokaji tasks themselves," and they inherit the techniques of both historically divided roles. In this sense, it can be said that the technical requirements for modern swordsmiths are higher than those of specialized historical swordsmiths.

What You Should Know When Appreciating and Purchasing Swords

Knowing the history of the division of labor between ōkaji and kokaji broadens one's perspective in appreciating swords. The wood-grain and plate-like patterns that appear in the jihada are the layered structures created by lamination forging that have been brought to the surface through polishing. The fineness and uniformity of these patterns reflect how carefully the steel as a material was homogenized and how meticulously the forging was performed, and they demonstrate the accumulation of craftsmanship effort that derives from both the ōkaji and kokaji processes.

On the other hand, it is not possible to accurately determine from the appearance of the jihada alone whether the blade was made through division of labor or by a single person, or how many times it was folded. Even among blades from the same school and era, there are individual differences in the condition of the material and the forging procedure. When selecting a sword, it is desirable to make a comprehensive judgment that takes into account the impression of the jihada and hamon along with multiple pieces of information such as provenance, era, and evaluation by certification bodies.

In any case, before a single sword is completed, multiple specialized skills have been layered historically, from the stage of preparing the tamahagane material to the stage of forging the blade. The fact that modern swordsmiths bear both of these responsibilities alone is surely background knowledge worth knowing in order to understand the total amount of effort invested in a single sword.