How does clay tempering affect the blade in these katanas?

 Updated Mar 2026

Clay tempering, or tsuchioki, is a traditional heat treatment method in which a clay mixture is applied unevenly along the blade before the final quench. The coated spine cools slowly, remaining relatively soft and resilient, while the exposed edge cools rapidly, developing a harder crystalline structure called martensite. The boundary between these two zones produces the hamon - the visible temper line that runs along the blade's length. In manganese steel blades especially, this process yields a hamon with a distinctive, natural appearance that cannot be replicated by simple acid etching. It is one of the most reliable indicators of authentic traditional construction in a collectible katana.

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