Tamahagane is the name used in Japan for steel made with their centuries-old process. There are several videos available online to see the process such as this one. It starts with “iron sand” which is a naturally occurring form of iron mostly composed of magnetite, Fe3O4, though can also contain some hematite (Fe2O3) and other constituents like quartz and titanium dioxide [1]. One nice thing about magnetite (as the name suggests) is that it is magnetic, so it can be relatively easily separated from other minerals with magnets. The iron sand is then “smelted,” which is the process of reducing the oxide ore to the base metal, in this case, iron. The smelting is performed in a traditional furnace called a tatara. A clay tub is used which is layered with charcoal and the iron sand [2]. The charcoal acts as a reducing agent to turn the magnetite into iron, and also is a carbon source to turn the iron into steel (steel is iron plus carbon). The charcoal reduces the steel because carbon monoxide and carbon dioxide forms when it burns in air. The carbon monoxide (CO) reduces the magnetite to iron [3]. At the end the clay vessel is broken and the tamahagane is removed, which is in the form of rough and porous sponge-like pieces that also contains slag (large undesirable inclusions). The swordsmith separates pieces by “grade” which corresponds to the approximate content [4]. The grade is determined by color and also the fracture appearance [4]. The swordsmith then forges the pieces gently, and then folds and forge-welds them multiple times to eliminate porosity, remove some slag, even out the carbon content, and break up inclusions and impurities.
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Testing Tamahagane – Traditional Japanese Steel
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