First, the clay has a twofold purpose. One, it keeps the quenching medium, oil or water, from contacting the main body of the blade, so that portion of the blade will not harden. Second, the clay helps hold some of the heat in the main body of the blade so that it cools slowly enough so that it will not be shocked into hardening.
Steel has a very discouraging tendency built into the process of selective hardening.
I don't mean warps, it bends, curving upwards from the cutting edge. You have to heat a carbon steel blade to around 1500 degrees F to make it harden The entire blade expands. This is one of those facts of life and elemental physics that one cannot escape. When steel is converted into martinsite, the hard stuff, it is frozen in the expanded size. The back of the blade does not harden, and slowly shrinks back to the original dimension.