A clay-tempered katana is a type of Japanese sword that has undergone a traditional forging technique known as clay tempering or differential hardening. This technique is used to create a distinct hamon (temper line) on the blade and to achieve a balance of hardness and flexibility in different parts of the sword.

Here's an overview of the process and characteristics of a clay-tempered katana:

1. Blade Preparation: The swordsmith starts with a blade made of high carbon steel, typically a type like Tamahagane or modern steel such as T10. The blade is shaped and profiled, and the edge is typically left thicker compared to the rest of the blade.
2. Clay Application: A layer of special clay mixture is meticulously applied to the blade, typically using different thicknesses in specific areas. The clay acts as an insulator during the heat treatment process.
3. Heating and Quenching: The blade is heated and then rapidly quenched, usually in water or oil. The different clay thicknesses cause the blade to cool at varying rates. The edge, covered by a thinner layer of clay, cools faster and forms a harder martensitic structure, while the back, covered by a thicker clay layer, cools slower, resulting in a more flexible pearlite structure.
4. Resulting Characteristics: The clay tempering process imparts distinct features to the katana. The hardened edge is extremely sharp, providing excellent cutting performance. The softer back of the blade adds durability and resilience, allowing it to absorb shocks and resist breaking. The transition between the hardened and softer portions of the blade creates the visually striking hamon, which is the line that separates the two regions.
5. Aesthetic Considerations: Clay-tempered katanas are highly regarded for their visual appeal. The hamon, with its unique patterns and variations, is often considered an art form in itself. The specific appearance of the hamon depends on factors such as the clay composition, application technique, and heat treatment process.

Clay-tempered katanas are appreciated not only for their functional qualities but also for their cultural and historical significance. They are often regarded as works of art and are sought after by collectors, practitioners of Japanese martial arts, and enthusiasts interested in traditional Japanese sword craftsmanship.