The best kind of katana, or Japanese sword, is one that is forged by using traditional methods using the purest steel, which the Japanese call tamahagane, or jewel steel. During a period of three days and three nights, smelters using ancient techniques shovel about 25 tons of iron-bearing river charcoal and sand into a rectangular clay furnace called a tatara. This furnace is specially built to produce a single batch of tamahagane.
The traditional Japanese samurai sword is crafted during a period of three days that involves many precise steps. First, the iron-bearing river gravel must be fed into a furnace until enough raw steel is collected. Then, the steel is heated at high temperatures along with the charcoal. This allows just the right amount of carbon to dissolve into the steel. Next, the steel is heated, hammered and repeatedly folded in order to remove impurities. The next step is the actual forging of the sword, which involves the smith heating the carbon steel and shaping it into a U-shaped channel. The katana is then coated with clay and charcoal powder, and it is then shaped and polished. The last process involves a two-week process of honing and sharpening the blade.