Clay Hardening：It applies clay along the blade but let edge exposed. Thus, while quenching the blade into water, the uncovered edge will cool down suddenly, but the rest of blade will cool down slowly. Such differential temperature change results in the different hardness of the blade. So the edge is tough enough to cut, where the back of blade is soft /flexible enough to absorb the impact during cutting. Such quenching process usually will leave beautiful wavy tempered line on the blade, as known as "hamon" in Japanese swords term.