Ok. So I believe that the use of a special metal in crafting should incur a cost, to avoid easy access to superior weapons. I have a few suggestions:
1. Disjunction removes metal type.
2. Metal type can be added when crafting, but increases the potential required (though not the ML) (if you're bothered by the idea of adding/removing the metal, think of it as metal threaded like handwraps, and remember, the machine is magical)
3. The materials required to add metal types to weapons must come from metal items of the appropriate type. I.E must deconstruct several silver items to get "silver essences" to add silver to your weapon.