If a sword is clay tempered, it means that the blade has undergone a specific heat treatment process known as clay tempering or differential hardening. This technique is commonly associated with traditional Japanese sword making.

During clay tempering, a layer of clay is applied to the blade before it is heated and then quenched in water or oil. The clay is typically thicker on the spine and thinner on the edge of the blade. As a result, the clay coating acts as an insulator, causing the blade to cool at different rates. The thicker clay layer cools more slowly, allowing the steel beneath it to remain relatively softer, while the thinner clay layer cools more rapidly, resulting in a harder edge.

This differential hardening creates a contrast in hardness between the hardened edge and the softer spine of the blade. This hardness differential is visually displayed as a distinct wavy line called the hamon, which is a sought-after characteristic in Japanese swords.

The benefits of clay tempering include:

1. Enhanced Cutting Ability: The hardened edge of a clay-tempered blade retains sharpness and provides excellent cutting performance.
2. Strength and Resilience: The softer spine of the blade increases its flexibility and resilience, reducing the risk of breakage during use.
3. Aesthetic Appeal: The hamon pattern created by the clay tempering process is visually appealing and adds to the overall beauty and uniqueness of the sword.

Clay tempering is considered a traditional and skilled technique in Japanese sword making, and swords that have undergone this process are highly valued for their performance and craftsmanship.