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Tsuchioki (Soil coating)

As preparation for 'Yaki-ire (quenching)' to rapidly cool the heated blade with water or other liquid, 'Tsuchioki (soil coating)' where three types of Yakiba-tsuchi soil (soil used for quenching) are applied to the Hirachi (blade), Hamon (blade pattern) and Shinogichi (ridge line) are done. Yakiba-tsuchi soil (soil used for quenching) is applied thinly and evenly over the Hirachi (blade side), then Hamon (blade pattern) is designed with a writing brush using Yakiba-tsuchi soil for quenching for Hamon. Lastly, a thicker coating of Yakiba-tsuchi soil (for quenching) is applied for the Shinogichi (ridge line) from the Hamon (blade pattern) to Mune (back). By using thicker concentration of Yakiba-tsuchi soil, for quenching, on the Shinogichi (ridge line), when cooling rapidly by Yaki-ire, the blade side is quickly cooled and quenched completely, and the Mune side is cooled relatively slowly and not fully quenched. Quenching makes a sword harder, the metal expands, and creates the distinct curve of a Katana. The Mune expands less, and takes on the property of tenacity rather than hardness, and this supports the blade side steel which is hard, but otherwise easily broken.