Hardening A Small Knife

Heat the blade until critical temperature is reached (approx 780 - 820 degrees Celsius), this is the temperature where the steel loses it magnetism and quench it in a fluid medium. I heat the blade using a small furnace and test it now and again on an old speaker magnet if it is still magnetic or not. When it looses magnetism I insert it back in the furnace for a few seconds more to take it just over critical temperature and then quickly quench the edge side of the blade in oil. This cools down the steel very quickly and hardens the steel to ensure the blade will keep a sharp edge.