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Originally Posted by Timo Nieminen
For low-tech metalwork (i.e., unknown carbon content, no thermometers or temperature controlled ovens), differential hardening can be easier than trying to achieve a uniformly non-brittle hard blade.
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Hi Timo,
This blade is no low-tech metalwork, the blade is thin, tough, very sharp and absolutely flawless forged. This blade was forged by a master blacksmith. It is made from unknown type of crucible steel with very low contrast structure, possibly wootz. The high resistance against abrasion is an indicator.
A skillfull differential hardened blade has a very uniform hardness along the cutting edge. Differential hardening with clay on the blade is always more difficult than uniform hardening, especially if hardened in water. Many things are important like the temperature of water, the hardening time, the consistance and mixture of the clay and so on. The volume of hardened steel is bigger than unhardened steel, this causes tensions in the material. Even the best Japanese swordsmiths have lost blades during the hardening process because of hardening cracks (Hagire).
The temperature of hot steel can be controlled by its color. Well trained eyes do this with a precision of +- 10°C.
Roland