Japanese metalsmiths would add carbon in the form of charcoal powder, and they would beat this into the steel and fold it into layers. High carbon steel is very high strength and holds a very strong edge, however it is brittle.

Japanese metalsmiths wood alternate different types of steel in the composition of a sword, including softer mild steel that would form the main spline of the blade, allowing it to deflect because it was more malleable and less brittle than the folded deal used on the blade.