What is Tsumiwakashi in Japanese Katana terminology?

"""Tsumiwakashi"" (積み沸し) is a process in sword-making where the core iron (Shingane, 心鉄), blade steel (Hagane, 刃鉄), and skin steel (Kawagane, 皮鉄) parts of the sword are heated together with the small pieces of steel that have been broken down in a process called ""Kowari"" (小割り). This is done to forge them into a single mass.

The steps involved in this process are as follows:

1. The steel is heated until it turns red, then it is folded several times, extended thinly, and forge-welded to the end of a lever rod to create a lever base (テコ台).

2. The small pieces of steel that were broken down earlier are carefully stacked on top of the lever base.

3. The stacked steel is then wrapped in Japanese paper, doused in muddy water, dusted with straw ash, and then slowly and thoroughly heated in a furnace.

This process is crucial in the creation of a Japanese sword as it allows the different types of steel to be combined into a single piece, which is then further worked on to create the final sword. The careful control of heat and the meticulous stacking of the steel pieces are key to the success of this process."