Tamahagane (Jewel Steel) and Tatara Ironworking — How to Make the Iron Used in Japanese Swords

What is Tamahagane?

Tamahagane (jewel steel) is a special grade of steel produced exclusively as raw material for Japanese swords through tatara ironworking, a process using iron sand (tetsu) and charcoal as raw materials. This unique manufacturing process differs from conventional steelmaking (coke blast furnace smelting) and produces steel with a wide range of carbon content: 0.6–1.5%.

Tamahagane is an inhomogeneous mass containing both high-carbon portions (steel = hagane) and low-carbon portions (iron = tetsu). Master swordsmiths turn this mixture to their advantage, employing a blending technique (awase waza) that uses both skin iron (kawa-gane) and core iron (shin-gane) separately.

Basic Structure of Tatara Ironworking

Tatara refers to the collective term for Japan's distinctive ironworking furnaces in which iron sand and charcoal are alternately layered and burned. It consists of the following elements.

Structure of the Furnace (Kerajou)

• Furnace floor (toko-tsuri): The bottom of the furnace, with underground warming and drying structures
• Blast pipe (haguchi): Air inlet nozzle that receives air from the wooden bellows (fuigo)
• Charcoal and iron sand layers: Alternately stacked and ignited
• Furnace walls (rogawa): Constructed from clay, sand, and whetstone powder

Operational Scale

• Furnace size: approximately 3 meters long, 1 meter wide, 1.2 meters high
• Iron sand per operation: 10–15 tons
• Charcoal per operation: 10–12 tons
• Output: approximately 2.5–3 tons of kera (iron bloom), of which roughly 30% is tamahagane

72-Hour Continuous Operation

Tatara ironworking requires 72+ hours of continuous operation. Once the furnace is lit, it cannot be extinguished, and it operates 24 hours a day under shift management.

Operational Sequence

1. Ignition (hi-ire): The furnace is lit. Initially kept at low temperature to warm the furnace
2. Main operation: Iron sand and charcoal are alternately fed while temperature is raised to approximately 1400°C
3. Bloom removal (kera-dashi): After 72 hours, the iron bloom (kera) at the furnace bottom is extracted
4. Bloom separation (kera-wake): The extracted kera is split and sorted into tamahagane (first-grade), mid-grade material, and low-grade material (for core iron)

Note that cast iron (zuku) is a product of the zuku-oshi method and is not obtained from bloom-pushing (kera-oshi) method operations.

Temperature Control Technology

Tatara masters (murage) empirically adjust the air flow and the ratio of iron sand to charcoal input to manage furnace temperature. Excessive air causes the iron to over-melt, producing cast iron (excessive carbon), while insufficient air prevents the iron from solidifying. This delicate control is the essence of tatara technology.

Selection and Carbon Content of Tamahagane

After bloom removal, the kera is split and sorted into multiple grades according to carbon content.

Tamahagane refers specifically to first-grade material with approximately 1.0% or higher carbon content, while low-carbon material at 0.3–0.6% is distinguished as low-grade material for core iron use, not true tamahagane. Swordsmiths blend multiple materials with different carbon contents, creating hardness and flexibility suited to each part of the blade.

Iron sand quality can be assessed by color. Black-brown indicates magnetite (Fe₃O₄)-dominant high-quality iron sand, reddish-brown indicates hematite (Fe₂O₃)-dominant sand requiring additional reduction steps, and yellowish-brown indicates high impurity content unsuitable for ironworking. The Okuizumo region's abundance of magnetite-based iron sand, created by granite weathering in the Chugoku Mountains, is the fundamental reason it became a major tatara ironworking center.

Modern Production: Nittouhoutatsara

Currently, tamahagane is produced at only one location in the world: Nittouhoutatsara (Nittouh Tatara), operated by the Public Interest Incorporated Foundation Japan Sword and Ancient Art Preservation Society (NBTHK) in Okuizumo, Nita County, Shimane Prefecture.

Overview of Nittouhoutatsara

• Established: Began operations in 1977 (Showa 52) following restoration
• Operating period: Winters annually (January–February, when moisture-poor charcoal and iron sand conditions are optimal)
• Annual production: Approximately 3 tons of tamahagane (combined from three annual operations; distributed to all domestic recognized swordsmiths)
• Distribution: Distributed nationwide to recognized swordsmiths through the All Japan Swordsmith Association

Tamahagane supply is limited relative to demand, and each recognized swordsmith's annual allocation is restricted.

Following the Meiji period's adoption of Western blast furnaces, tatara ironworking rapidly declined and had nearly ceased by the early Showa period. The 1977 restoration of Nittouhoutatsara is a rare global example of "reviving an extinct traditional ironworking technique," carried out under the auspices of sword culture preservation. The tamahagane produced undergoes fold-hammering (ori-kaeshi tanren) approximately 15 times, creating a layered structure of 2¹⁵ = 32,768 layers. This layering is the source of Japanese swords' characteristic toughness and surface grain pattern (hada moyou).

Modern Rarity and Price of Tamahagane

The distribution price of tamahagane produced at Nittouhoutatsara is kept relatively low for association members, but market prices are extremely high, sometimes reaching tens of thousands to over 100,000 yen per kilogram.

Additionally, with increasing foreign sword enthusiasts and demand from overseas blacksmiths, the rarity of tamahagane is expected to grow in the coming years.

Summary

Tamahagane and tatara ironworking form the foundation of Japanese sword culture, and production continues through traditional methods even today. The existence of this special raw material is what enables the creation of Japanese swords' distinctive surface grain patterns and blade temper lines. Understanding the production process of tamahagane is essential foundational knowledge for deeply comprehending the artistic and craft essence of Japanese swords.