The Katana is a traditional sword used by the Samurai warriors of feudal Japan. The Katana is distinguished by its distinct curved blade and elegant hilt and scabbard. The katana is made of high-carbon steel known as tamahagane that was heated in a clay furnace and hammered, folded, and welded multiple times to give it its layered impression, or hamon. It is this process of combining different types of steels that gives the katana its superior strength and sharpness.
During the final stages of making a katana, the swordsmith uses a clay slurry to coat the entire blade in a process called yaki-ire. The smith applies a thicker layer of clay to the body and spine of the sword, and a thinner coating to the edge. It is then heated and quickly cooled, a process known as quenching. As the metal cools, it contracts in the spine and core but expands in the blade side, giving it its distinctive curve.
After the hamon is formed, the smith strikes and elongates the Mune (blade) of the blade to reduce its thickness, then the Hirachi (edge) and Shinogichi (ridge line). The Nakago (tang) is then struck again to round it out. The smith then reheats the blade for tempering, or Yaki-modoshi. When it is cool, the smith shaves off the black taint of the Martensite with a plane for shaving metal called Namatogi and shaves off unevenness. The sword is then reheated for Yaki-modoshi again, and when it is cool the smith shaves off the unevenness again. click on this page