Tatara (furnace)

The is the traditional Japanese furnace used for smelting iron and steel. The word later also came to mean the entire building housing the furnace. The traditional steel in Japan comes from ironsand processed in a special way, called tatara system.
The steel, or , used to forge Japanese swords (, commonly known as ) by contemporary Japanese forge masters like Kihara Akira and Gassan Sadatoshi is still smelted in a "tatara". One of the few remaining "tatara" is the Nittoho "tatara" in Shimane Prefecture, Japan.
Etymology.
It is generally agreed that the word "tatara" is foreign to Japan, originating in India or Central Asia. There is a possibility that the word came from a place in ancient Korea, where the two dynasties would often meet. According to the "Kojiki" (one of the oldest Japanese historical texts), this meeting place was called Tatara-ba (たたら場) or Tatara-tsu (たたら津), and the word was possibly imported with iron-working technology. Japanese scholar Tokutaro Yasuda suggests that the word may be from the Sanskrit word "tātala", meaning "heat," noting that the Hindi word for steel is "sakeraa", which is very similar to the word "kera" (鉧) used in Japan for the steel bloom which the tatara produces. The two Chinese characters used when the word has the original meaning are 踏鞴 and, besides as "tatara", they can be also read as "fumifuigo", or foot bellows.
Process.
The smelting process used differs from that of the modern mass production of steel. A clay vessel about 1.1 meters (4 feet) tall, 3 meters (12 feet) long, and 1.1 meters (4 feet) wide is constructed. This is the "tatara". After the clay tub has dried, it is fired until dry. A charcoal fire is started from soft pine charcoal, then the smelter will wait for the fire to reach the correct temperature. At that point, he will direct the addition of ironsand, known as "satetsu". This will be layered in with more charcoal and more ironsand over the next 72 hours. Four or five people need to constantly work on this process. 
It takes about a week to build the "tatara" and complete the iron conversion to steel. When the process is done, the clay tub is broken and the steel bloom, known as a "kera", is removed. At the end of the process, the "tatara" will have consumed about 10 tons of "satetsu" and 12 tons of charcoal, leaving about 2.5 tons of "tamahagane".
The Nittoho "Tatara".
In 1977, the Japanese Society for Preservation of Japanese Art Swords (Nittoho), together with the Japanese government's Agency for Cultural Affairs and Hitachi Works, built in Shimane Prefecture the so-called Nittoho "Tatara" to provide the steel necessary for the production of Japanese swords. The Nittoho "Tatara" is managed jointly with Yasugi Works, a subsidiary company of Hitachi Metals, and is operational only during the winter.