TLDW #26 – Traditional Yaki-Ire, Hardening a Tanto

During the hardening process the clay layer causes a split second difference in cooling time which creates two different hardness areas in the same piece of steel. The edge cools faster and forms a very hard steel structure called martensite while the body cools slower and forms a very tough steel structure made of ferrite and pearlite. The boundary between these two areas is called hamon and is commonly seen as a frosted line down the length of a polished sword blade.
more about the process of yaki-ire

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