Kera-oshi and Zuku-oshi: Two Operating Methods of Tatara Iron Smelting and the Birth of Tamahagane
* Image is for illustrative purposes only.Table of Contents
Tatara Has Two Types of "Operations"
Tamahagane, the material used in Japanese swords, is produced through "tatara steelmaking," a process in which sand iron (satetsu) and charcoal are alternately fed into a furnace to extract iron. However, this steelmaking method, commonly referred to simply as "tatara," actually encompasses two distinct operational methods with different purposes: "kera-oshi" (the pressing of kera) and "zuku-oshi" (the pressing of zuku).
While both methods use the same box-shaped furnace and sand iron and charcoal, they produce different primary products by varying the furnace temperature management and the method of feeding sand iron and charcoal. Understanding this difference reveals why tamahagane is such a precious and special material.
Kera-oshi — The Operation for Obtaining Tamahagane
Kera-oshi is an operational method aimed at generating a large mass of iron called "kera" at the bottom of the furnace. Over three days and three nights, sand iron and charcoal are continuously fed, and the iron grows as a semi-molten mass within the furnace.
When the operation concludes, the furnace is destroyed and the kera, weighing several tons, is extracted. This kera is crushed, and the fracture surface is examined for color and quality to classify it into grades, with the finest portions selected as tamahagane. The tamahagane used in Japanese swords is steel obtained through this kera-oshi method, with a carbon content held relatively low at approximately 1-1.5%. It is rather the zuku (pig iron) obtained from zuku-oshi that has high carbon content (approximately 3-4%), which is hard and brittle and unsuitable for forging as-is. Kera-oshi is operated at relatively low temperatures and is characterized by extracting iron without completely melting it, keeping it in a state close to solid.
Zuku-oshi — The Operation for Flowing Out Zuku
Zuku-oshi, on the other hand, primarily aims to maintain a higher temperature in the furnace and flow out iron containing high carbon content and having a lowered melting point as "zuku" (pig iron) in a liquid state. The flowing pig iron becomes material for casting, and it was also processed through decarburization treatment, described later, into iron suitable for forging.
Pig iron has high carbon content and, as-is, is hard and brittle, making it unsuitable for sword forging. Therefore, iron obtained from zuku-oshi was processed in a step called "o-kaji" (major forging), where carbon was removed to transform it into easily forgeable materials such as hocho-tetsu (knife iron) before use.
What the Difference in Products Means
The greatest difference between kera-oshi and zuku-oshi lies in whether iron is obtained as a solid mass or flowed out as a liquid. This difference is created by controlling the furnace temperature range, operational time, and the ratio of sand iron and charcoal. The differences between the two are summarized as follows.
| Item | Kera-oshi | Zuku-oshi |
|---|---|---|
| Primary Product | Kera | Zuku (pig iron) |
| State Obtained | Solid-like mass | Liquid |
| Carbon Content | Approximately 1-1.5% | Approximately 3-4% |
| Temperature Range | Relatively low | High |
| Subsequent Process | Crushed and graded to select tamahagane | Decarburized through o-kaji and processed into hocho-tetsu and similar materials |
Tamahagane optimal for Japanese swords is obtained from kera-oshi. Because the steel does not completely melt, "unevenness" remains in the distribution of impurities and carbon, and this heterogeneity becomes the basis for generating refined structure through repeated folding and forging and the grain pattern of the ji-gane (body steel). It differs from the outset from modern steel, which is created by uniform melting.
Until Tamahagane Becomes One Sword
Tamahagane obtained from kera-oshi does not become a sword as-is. Once tamahagane reaches the sword smith's hands, it transforms into one sword through several steps.
- Mizuherashi (water-quenching) and small breaking — Heating and thinly forging to make it easier to fracture upon rapid cooling, then dividing it into small fragments
- Fracture surface selection — The sword smith examines the fracture surface to select for high or low carbon content
- Tsumi-wakashi (stack heating) — Stacking the selected fragments and forge-welding them into one piece
- Repeated folding and forging — Repeatedly forging and folding steel to layer it, expelling impurities as sparks, evening out carbon content, and creating multiple layers of organized structure
It is precisely because tatara steel starts out as an inhomogeneous material that this careful refinement process has meaning.
Connection to the Grain of the Body Steel
The layers created through repeated folding and forging appear as "grain" on the surface of the polished blade. Grain patterns such as itame-hada (board-grain appearance), mokume-hada (annual ring-like pattern), and masame-hada (straight grain) are landscapes created by the combination of forging method and steel properties. Additionally, fine glittering particles within the body grain are called ji-nie, and black patterns running like lightning are called chi-kei, both considered highlights created by the quality of steel and the subtlety of forging.
When appreciating a sword, looking closely not only at the hamon but also at the grain of the body steel allows one to read the traces of how that particular sword was forged and from what steel it originated. The final point of the long process beginning with kera-oshi tatara is inscribed in the grain of the blade in one's hand.
Tatara in the Modern Era
Currently, tatara operations supplying tamahagane for Japanese swords are conducted during winter by "Nittouho Tatara," operated by the Japan Art Sword and Sword Preservation Society (a public interest incorporated association) in Okuizumo, Shimane Prefecture. What is performed here is kera-oshi, and it serves as a major source of tamahagane supplied to sword smiths throughout the country.
Understanding the two operational methods of kera-oshi and zuku-oshi reveals that tamahagane is not merely "iron," but rather a material carefully selected under a specific operational philosophy. When gazing upon the grain of a sword, take a moment to reflect on the steelmaking choices that lie behind it.
Experiencing the Craft of Sword-Making
TOUKENZA (operated by M-Assets Co., Ltd.) is a sword marketplace that sells Japanese swords online. Understanding the technique of sword forging becomes a key to appreciating each sword more deeply. Swords currently listed can be viewed in our online catalog and are available for purchase. If you have questions about a particular sword or any other inquiries, please feel free to contact us through our contact form.
