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Tamahagane (玉鋼) is a low quality steel made in the Japanese tradition, also known as pig iron, Tamahagane steel was used in ancient japan due to their lack of international trade and lack of refined steel production methods. The main issue with Tamahagane is the inconsistent levels of carbon distribution so to make blades it had to be folded then forge welded multiple times, anywhere from 6 to 12 times depending on the quality of steel required and sometimes as many as 20 times. 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 an average 1% of carbon and should not go over 1.5% though individual spots may greatly exceed this number.
Tamahagane is made of iron sand (satetsu). There are 2 main types of iron sands: acome and masa. Acome 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 tall(121.92cm), 12 feet long(365.76), and 4 feet wide(121.92cm). The tub is then dried and heated. The clay tub is heated to a high temperature, about 1000 °C (1800 °F). Then, it is mixed with charcoal to give tamahagane hardness.
The process of making tamahagane continues for 36 to 72 hours, 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.