How does clay tempering affect a katana's hamon compared to oil quenching?

 Updated Mar 2026

Oil quenching produces a uniform hardness along the entire blade, which may result in a faint or absent hamon. Clay tempering - applying a thick clay layer to the spine before quenching - insulates that section from the rapid cooling, leaving it softer and more flexible. The exposed edge hardens quickly, and the transition zone between these two states is where the hamon forms. On a T10 clay-tempered blade, this line often shows complex activity: misty clouds of nioi, bright sparkling nie crystals, and subtle undulations that vary from sword to sword. Because each clay application is done by hand, no two hamon are identical, which is a major part of what makes individual pieces collectible rather than interchangeable.

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