Lately I was involved in a discussion about the hardening process of a Japanese sword and I did found it very inspiring to write a small piece about that just to give you a better understanding of that part of the forging process.
The hardening process of the blade is in one of the most important and perhaps the most difficult part of the sword making process. It is the correct hardening of the Japanese sword that gives the blade its ability to take and retain an amazing sharpness.
After the blade is forged and finished to it’s final (raw) shape, the differential hardening process can begin. To start with, the blade is being coated in a mixture (also know as yakibatsuch in Japanese ) of water, clay, ash, and other ingredients. Every smith has his own and in most cases, a ‘secret’ special recipe. The clay mixture is applied over the surface, thicker along the ‘mune’ (or spine) and thinner at the ‘ha’ (edge). In other words, the clay mixture will act as an insulating “blanket” during the quenching process. Allowing those areas that are covered by a thicker layer of clay to cool much more slowly.
Once this is done, “ashi” is applied to the clay coat. Ashi are the thin strips of clay you can see over the whole surface of the blade on this picture. These stripes are providing some insulating action in the quench as well, and will form little sections of softer material in the hardened edge. The Ashi are giving the blade the ability to prevent ‘cracks’ in the blade under hard use and lot of pressure. They also contribute to the formation of crystalline features within and around the hamon itself.