What is Clay Tempering?
Clay tempering is an art form as well as a science. Pioneered by the Japanese, traditional clay tempering techniques are perhaps the pinnacle of Japanese sword design.
Today, clay tempering techniques are used by many modern smiths on swords as well as knives, both western and eastern.
The principle behind clay tempering is to achieve both hardness and flexibility, which equals strength. A hard cutting edge, and a flexible spine. A harder steel will sharpen better than softer, retain its sharpness longer, and resist denting. The flexible spine allows the blade to resist greater impact, without breaking.
These two things combined, hardness and flexibility, make for a great sword.
Clay tempering is called just that because the smith will use “clay” e.g. mud, to insulate the blades spine. Like a plastic sleeve around a wire. Where the clay is the plastic and the blade is the wire. Though it is just the spine of the sword that is coated in the clay. The edge is left exposed.
On application the smith is free to use their artistic licence. A pattern will be drawn in the clay, often appearing like an undulating wave flowing down the length of the blade
Once the clay is applied the blade is heated in the furnace. When it gets to its optimum temperature it will be glowing red. It is then plunged into a cool pool of quenching liquid (water, or oil). There is a rapid cooling of the blade. Due to the clay insulation the spine cools slightly slower than the edge which is exposed.
The differential tempering is now created, hard and flexible. It is this pattern that the Japanese call the “Hamon”. It is revered, studied and admired for its beauty.
A katana blade applied with clay, pulled from the furnace, just before it is about to be quenched.