How does clay tempering affect the appearance of a T10 tanto blade?

 Updated Mar 2026

Clay tempering — known as tsuchioki in Japanese smithing — involves coating the blade spine with a clay mixture before the quenching process. The insulated spine cools more slowly than the exposed edge, resulting in a harder edge and a softer, more flexible spine. The boundary between these two zones becomes visible as the hamon, a distinct temper line running along the blade's length. On T10 carbon steel, this line often appears as a misty, undulating pattern called nie or nioi, depending on the grain structure. Because the clay is applied by hand and no two quenches are identical, every hamon is a one-of-a-kind feature — one of the most valued visual characteristics among Japanese blade collectors.

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