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I would offer an exception to this definition. This states the three type process as having a hard center, and softer sides. The truth is that is a three point application of different hardness, and it is not done directly during the initial forging process. A san mai blade is made from taking a billet of hard steel, extruding and folding until nearly the proper length of the blade. Then the center is opened and a softer steel is pounded into the blade and the outer layer is folded over. After the combination is fully shaped it is hardened again by quenching. This is when a Japanese blade creates it's own curvature based on the different cooling speed between the harder and softer steel in the jacket and center. But this is just two of the three hardening steps. After finishing and filing the blade is painted with a clay base, then a water brush is used to remove most of the clay in a pattern (Hamon)from the edge. It is then heated and quenched again leaving a san mai, or three layer hardness, the center is softer to absorb blows without breaking and allowing some flexibility, the jacket or outer sword is folded and tempered steel, and the edge is again tempered almost to the point of being brittle. This makes a very sharp, very durable,and very beautiful blade.17:41, 10 December 2016 (UTC)71.121.146.122 (talk)
You are talking about forging sword blades from tamahagane, the article is about making knife blades (both forged and ground) from bar stock. San mai bar stock for knife blades already has three layers before shaping and heat treating. DrHenley (talk) 02:12, 24 February 2022 (UTC)[reply]
There's a similar process called Go Mai involving five layers, rather than three. It isn't mentioned here, and there doesn't seem to be a wiki page for it.
