Just FYI - that's hardening, rather than tempering. Hardening is done by heating the steel to above critical temperature, then a rapid quench where the microstructure is transformed, hopefully to a very fine grain. Tempering is a lower temperature process done after hardening to draw back some hardness and make the steel tougher. It is often slowly air cooled after tempering, rather than quenched, but it depends on the steel.
Hardening temperatures are in the area of 1400 F, where tempering is more like baking a cake typically - 350 F or so...
Not in this case, with Japanese swords this IS tempering.
"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.
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."