The traditional Japanese sword makers coated the sword in a clay mix on the back of the blade (the blunt side), and left the sharp side uncovered. They heated the sword and judged the right temperature by the colour, then quenched it. The sharp edge had no clay on it so it cooled immediately, the spine of the blade had clay on it so it cooled much slower. The end result was a sword with a tough flexible spine and a hard sharp edge.
