What is Tanren in Japanese Katana terminology?

"Tanren" (鍛錬) refers to the process of heating and hammering steel to forge it. The purpose of this process is to fold and forge the steel multiple times to increase its strength and resilience, remove impurities, and equalize the carbon content. This task is performed by the swordsmith and the most skilled apprentices, who work together in a process known as "Mukouzuchi". In Mukouzuchi, the apprentice hammers in response to the swordsmith's cues, a process referred to as "Aizuchi o utsu". This is a critical step in the sword-making process, as it directly affects the quality and performance of the final product.