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Tsumi wakashi (stacked and heated)

This case-hardened piece is called Heshi gane (removed metal), which is broken into small metal pieces using a Tsuchi (hammer). These metal pieces are stacked on the tip of a forging tool called 'Teko', and wrapped in Japanese traditional paper. Straw ash is applied, and then coated with clay slurry, then it goes into the furnace (Hodo) to heat until the clay surface melts. The straw ash and clay prevents scaling loss of the steel during heating and oxidizing. It is then hammered with a Kozuchi (light hammer) to form 6x9cm block. If there are not enough iron pieces, more are stacked, heated, hammered with a Kozuchi, and formed into an ingot weighing 1.8kg to 2.0kg. This is the process known as 'Tsumi wakashi' (stacked and heated). Other than Tamahagane, pig iron (Sentetsu) which contains a lot of carbon, and pure iron called Hocho tetsu also undergo Tsumi wakashi and Shita-gitae processing mentioned below.