たたら製鉄と日本刀の素材
Tatara Ironmaking and the Material of the Japanese Sword
Tatara ironmaking—the uniquely Japanese smelting process that produces tamahagane (jewel steel) for Japanese swords—developed primarily in the Oki-Izumo region of Shimane Prefecture. This entry explores how the distinctive process using iron sand and charcoal created a sword-steel unmatched anywhere in the world.
Description
What Is Tatara Ironmaking?
Tatara ironmaking is Japan's unique smelting process using iron sand (satetsu) and charcoal. Iron sand and charcoal are fed alternately into a clay furnace (takadono), while enormous foot-operated bellows (tenbin-fuigo) maintain high temperatures over a two-to-three-day smelt, producing a bloom of iron called kera. The highest-quality steel within the kera is called tamahagane (jewel steel) and possesses the best properties for Japanese sword-making.
The Iron Culture of Oki-Izumo
The Oki-Izumo region of Shimane Prefecture (Nita-gun, Okuizumo Town) has been known since ancient times as a treasure-house of iron sand, with abundant mountain iron sand from the Chugoku Mountains. The combination of rich forest resources for charcoal and iron sand deposits made this area Japan's greatest tatara ironmaking region. Also known as the origin of Yasuki steel, the technical tradition of tatara ironmaking is carried on today at the Hitachi Metals (now Proterial) Yasugi plant.
Metallurgical Properties of Tamahagane
Tamahagane contains a wide range of carbon (roughly 0.6–1.5%); a single smelt produces both high-carbon steel (kawagane, for the cutting edge) and low-carbon steel (shingane, for the core). Swordsmiths combine these through the distinctive laminated-forging technique of tsukurikomi, achieving a blade with both hardness and toughness. Tamahagane has extremely low impurity content (especially sulfur and phosphorus), and is further refined through repeated folding and hammering, forming the distinctive surface pattern known as ji-hada.