Clay Tempered Differentially Hardened Katana
The differentially hardened blade was a part of the katana's ability to hold a very sharp edge while maintaining blade flexibility. The sword smith packs the spine of the blade with a special clay during tempering. It acts as an insulation allowing the spine to cool slower then the edge. The result is flexibility in the spine and a hardened edge. This part of the process also creates the sori (curve). The different temperatures while cooling cause the straight blade to naturally bow before reaching the blade finishers who will apply a fine polish, creating a razor sharp edge.
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