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Tamahagane (玉鋼) is a type of steel made in the Japanese tradition. The word tama means "round and precious", like a gem. The word hagane means "steel". Tamahagane is used to make Japanese swords, knives, and other kinds of tools.
Good tamahagane contains around 1% of carbon and should not go over 1.5%. However, historical tamahagane was frequently in the range of 3% to 4.5%, essentially making it cast iron. This resulted in objects made from it (especially swords) being extremely fragile.
Tamahagane is made of iron sand (satetsu). There are 2 main types of iron sands: akome satetsu (赤目砂鉄) and masa satetsu (真砂砂鉄). Akome is of lower quality and masa is of better quality. The person who decides the amount of the mixing parts is called the murage. Depending on the desired result, the murage mixes one or more types of sands.
The iron sand is put in a tatara, a clay tub furnace. The clay tub measures about 4 feet (1.2 m) tall, 12 feet (3.7 m) long and 4 feet (1.2 m) wide. The tub is then dried and heated. The clay tub is heated to a high temperature, about 1,000 °C (1800 °F). Then, it is mixed with charcoal to add carbon to the steel so it can be hardened.
The process of making tamahagane continues for 36 to 72 hours (a day and a half to 3 days), depending on how many people work and how much metal is to be obtained. The iron sand is added every 10 minutes and the mixture is frequently turned over.
When the tamahagane is finished, the clay tub is broken and the steel is removed. The best steel is on the edges of metal block, because this is where the oxidation process is stronger. The quality of tamahagane is determined by its color: bright silver pieces are very good for making blades.