In traditional Japanese swordsmithing, the specific clay mixture used for creating the hamon (the wavy line pattern) on a blade can vary depending on the swordsmith's preferences, techniques, and the desired results. However, there are no strict rules or universally defined clay mixtures for hamon.

The clay mixture typically consists of clay, water, and additional additives to achieve the desired thermal barrier during the heat treatment process. Some swordsmiths may incorporate materials such as ash, powdered charcoal, or specific clay types known for their unique properties into their clay mixtures.

The clay mixture's composition and consistency are important factors as they influence the cooling rate of the blade during quenching. The thickness and application technique of the clay layer also play a role in creating the desired heat differentials that result in the hamon pattern.

The exact clay mixture used by individual swordsmiths is often considered their closely guarded secret or a part of their unique craft. Each swordsmith may have their preferred recipe or variation that they have developed through their experience and experimentation.

It's worth noting that there are modern alternatives and variations of clay mixtures used for creating hamon patterns, including synthetic refractory materials that can withstand high temperatures during the heat treatment process. These materials may offer more consistency and controllability compared to traditional clay mixtures.

Ultimately, the specific clay mixture used for creating a hamon can vary, and the choice of clay and additives is determined by the swordsmith's experience, preferences, and desired results.