What is Yakiire in Japanese Katana terminology?

"""Yakiire"" (焼き入れ) is a process in sword-making where the entire sword is heated and then rapidly cooled in water. During this process, the blade edge is cooled quickly due to the varying thickness of the clay coating (Yakibatsuchi, 焼刃土), resulting in a hard structure known as Martensite. This forms the blade pattern (Hamon, 刃文) and also causes the sword to curve, a characteristic feature of Japanese swords.

In addition, during a process called ""Yakimodoshi"", the sword is exposed to flame to raise its temperature to about 150 degrees Celsius. This gives the sword its resilience, a process also referred to as ""Aito"" (合いをとる).

This process is crucial in sword-making as it determines the sword's cutting ability, its unique curve, and its resilience."