The clay used in the process of creating a hamon (the distinct wavy line pattern) on a Japanese sword, such as a katana, is typically a mixture of clay and other materials. The exact composition of the clay mixture can vary depending on the preferences and techniques of the swordsmith. However, traditional clay mixtures for hamon typically consist of clay, water, and various additives.

The specific additives used in the clay mixture can include materials like ash, powdered charcoal, or clay from specific regions known for their unique properties. These additives are incorporated to achieve the desired heat treatment results during the sword forging process.

The purpose of the clay mixture is to create a thermal barrier during the quenching process. When the blade is heated and then rapidly cooled by quenching, the clay coating acts as an insulator. The thicker layer of clay on the spine of the blade slows down the cooling rate, allowing the steel beneath it to cool more slowly and remain relatively softer. The thinner clay layer on the cutting edge side allows for faster cooling, resulting in a harder edge.

This differential hardening process, aided by the clay mixture, creates the distinct hardness contrast along the blade, forming the hamon pattern.

It's important to note that the exact clay mixtures used by swordsmiths may be closely guarded secrets or can vary depending on individual techniques and traditions. The specific composition and proportions of the clay mixture can contribute to the final appearance and performance of the hamon pattern on the sword.