Steel

[Hoofhurr]

It's the carbon metal ratio, the temperature, and the folding process that are the main contributors to strength/flexibility.

The carbon, of course, comes from the burning hot coals; so it stands to reason the type of coal you use will impact the strength mostly as a function of the molecular nature of the carbon in the wood you use and the heat that it generates, because carbon at the elemental level is the same as any other carbon atom. If it's not hot enough you risk getting carbon and other impurities from the iron in a non-elemental form which contaminates the steel and weakens the final product.

You bind the carbon into the steel by folding the steel and hammering it; the hammering actually strengthens the steel by aligning the molecules. Temperature is key as well. You have to get the metal extremely hot to work with it but then you douse it in water. I think it's the dousing that actually binds the carbon to the metal and on a molecular level you are locking the lattice that gives the steel its strength. If you don't cool metal several times during the process it doesn't matter if you get the carbon:metal ration dead on, the steel will be more brittle. I'd have to research more to get more specific but that's what I recall from memory.