What is Mizuheshi in Japanese Katana terminology?

"Mizuheshi" (水減し) is a process in sword-making where the Tamahagane (a type of steel used in traditional Japanese sword-making) is heated and then hammered to a certain thickness. The steel is then broken apart to select the appropriate material for the sword. Initially, the steel is heated at a low temperature and lightly hammered. As the steel starts to blend, the temperature is increased and the hammering becomes more intense. This process is crucial in the sword-making process as it helps in the selection of the right material for the sword.