Not a bad idea actually. Each item could be smelted down to a certain percentage of the materials used to create them.
For example, it takes 12 "units" of steel to make a sabre. We could let players smelt them back down to, say, 9 or 10 "units" of steel to account for bits that are lost during smithing, sharpening, and melting.
This could also be used to solve the sword handle "problem" (I'm sure many don't see it as a problem but I do). If players lose 10-20% of their sword handle material every time they reheat and re-pound the metal, they'll maybe be less inclined to stand still and pound sword handles for 12 hours at a time. At the very least it'll force them to actually gather some more material.