Clay-tempered steel refers to a specific heat treatment technique used in traditional Japanese sword making to create a distinct hardness pattern along the blade. This technique is commonly associated with the production of Japanese swords such as katanas.

During the clay tempering process, a layer of clay is applied to the blade before it is heated and quenched. The clay acts as an insulator, causing differential cooling rates during the quenching process. The thicker clay coating on the spine of the blade slows down the cooling, resulting in a softer and more resilient spine. Conversely, the thinner clay coating on the cutting edge side allows for faster cooling, resulting in a harder edge.

This differential hardening creates a contrast in hardness along the blade, with a harder edge and a softer spine. The distinct boundary between these hardened and softer areas forms the hamon, the wavy line pattern visible on the blade. The hamon is highly valued for its aesthetic appeal and serves as a visible indication of the blade's differential hardness.

The clay tempering process is a skillful and complex technique that requires precise control of the clay thickness, heat treatment, and quenching methods. It is believed to enhance the cutting ability, durability, and overall performance of the sword by providing a combination of a sharp edge and a resilient spine.

Clay-tempered steel is considered a hallmark of traditional Japanese sword craftsmanship and is highly regarded by enthusiasts and collectors for its historical significance, aesthetic beauty, and functional benefits.