What is Zuku in Japanese Katana terminology?

"銑" (Zuku) refers to iron that has a high carbon content of 1.7% or more. Due to its high carbon content, it is not directly used as a material for swords. Instead, it undergoes a process to remove the excess carbon before it can be used in sword making. This process is crucial to ensure the right balance of hardness and flexibility in the final product, as too much carbon can make the sword brittle and prone to breaking.