A little while ago, I was in the garage, working on another knife, sort of in the style of a skinning knife, but designed for use as a general-duty pocket knife. Anyway, I had finished the grind of the knife, and so I was ready to harden and temper the blade. Before I did so, I remembered the Japanese way of hardening a blade, that is, to cover the spine of the blade with a special paste so as to block much of the heat. That way, the grind and edge of the blade are left exposed to the heat, and so when quenched, lock in the hardness. The parts of the blade covered with the paste do not receive as much heat, and so does not harden. Anyway, I tried looking up on Wikipedia what exactly the paste is made of, but all they said is that the base ingredients were clay and ash, and the other ingredients are trade secrets. Darn Samurai.
So, what I did, was I went out in the backyard, where we dumped the winter ash from the fireplace, scooped up a bit, and went into the garage. There, I grabbed a bit of Plaster of Paris. I mixed up the ash in a little container first, and put in a bit of water to make it into a paste. Then I put in a bit of plaster, about 20% plaster of paris to 80% ash, not including the water.
I then spread a good thick layer of this paste over the spine of the blade, and those other places of the blade that were to remain soft. Then I cranked up the Acetylene torch.
I applied even heat to the blade, avoiding the pasted areas, until the blade was an even orange-red, where a magnet would no longer stick. At that point, I quenched the blade, locking the atoms in their places. The sudden cooling knocked off the dried paste, and when I drew out the blade, it the black color it usually is after hardening, though a bit of the paste remained to show the outline of where the rest of it had been.
I then chose a hardness pick, a Mohs Hardness of five, and started drawing it in the area that was previously covered with the paste. It scratched easily. I then re-directed the pick into the area not previously covered with paste, and in that area the pick did not make a single scratch! It had worked! I then tested the hardened area, and it turned out to be a hardness of about 7-7.5!
Perhaps later, when, or if, I polish the blade, the signature wave of a katana will appear, and then, I will truly be happy!