Before being quenched, a special clay mixture can be applied onto the blade to harden the edge and obtain different hardness on the blade. The clay mixture was a special recipe and considered a crucial trade secret, guarded protectively by sword making masters. It would contain such things as feathers, powdered bones, grass, etc. and would be applied to the edge of the blade before being quenched. During quenching, a chemical reaction between the clay mixture and the hot steel occurs during the sudden temperature drop and carbon is fed into the blade in high amounts, creating an extremely tough edge. A clay hardened blade can only be quenched in water, thus increasing the defect rate even more.    
Another way for clay tempering is to apply 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.