The traditional katana sword is fashioned only from the purest steel, which the Japanese call tamahagane ("jewel steel"). Over three days and three nights, smelters using ancient techniques shovel roughly 25 tons of iron-bearing river sand and charcoal into the mouth of a tatara, a rectangular clay furnace built specifically to produce a single batch of tamahagane. Composed of carbon, the charcoal is as much a key ingredient in steel as a source of fuel for the furnace. The tatara will reach temperatures of up to 2,500°F, reducing the iron ore to steel and yielding about two tons of tamahagane. The highest quality tamahagane can cost up to 50 times more than ordinary steel made using modern methods.