How does clay tempering create the hamon on these blades?

 Updated Mar 2026

Clay tempering is a heat-treatment process in which the smith applies a thick layer of refractory clay along the spine of the blade before the final quench, leaving a thinner or absent layer near the edge. When the blade is heated and plunged into water, the clay-coated spine cools slowly and remains relatively soft, while the exposed edge cools rapidly and hardens into martensite. The boundary between these two crystalline structures is the hamon - a visible, three-dimensional line that appears as a misty, undulating band running the length of the blade. Because every application of clay is done by hand, no two hamon are identical. On T10 and 1095 high-carbon steel blades, the hamon is especially pronounced and can show activity patterns like nie (granular) and nioi (misty) along the transition zone.

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