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Yaki-ire (Quenching)

Generally during Yaki-ire, Tosho dims the light of the workshop, and judges the temperature of steel by its glow. The blade coated with Tsuchioki is inserted deeply into the Hodo, and the whole blade from end to end is heated uniformly to about 800 degrees. The temperature is most important, and the optimal heat condition is checked with the greatest care, the body of blade is then plunged swiftly to a water tank and rapidly cooled. As mentioned above, the blade warps in the water, and it is pulled out after it is fully cooled, and is then ground with a rough polishing stone, and the Yakiba (焼刃, cutting edge) is checked. After that, the blade is reheated in a charcoal fire for 'Yaki-modoshi (tempering).' This work is called 'Aitori (neutralizing).' Since it also warps to the side a little, it is struck while on a wooden base with a Kozuchi (light hammer) to adjust straighten the blade. The Nakago (core) is also tempered and formed. After Yaki-ire (quenching), the surface of the blade is very hard and this is called Martensite. Depending on how the Martensite looks, the Hamon (blade pattern) that looks like round particles on the surface of the metal to the naked eye, is called Nie (literally "boiling"), and separates from the Nioi (literally "scent") that looks like fine lines because the individual particles cannot be distinguished. Other than water, some of other cutting tools are quenched in oil, and as were Japanese military swords during the war, but today, it seems to be reare that a Katana is quenched in oil. Although quenching in oil reduces failure, it is not suitable for modern swords that are meant to become a work of art because it cannot achieve a fine Hamon (blade pattern).