First of all - I made a suggestion (as a standalone a while back, thinking I'd get this written up sooner) that bars should be a smaller amount of metal - i.e. smelt ore for a certain number of bars rather than one bar; richer ore gives more bars. (maybe/instead you could include fractional tracking)
Also - instead of designating an object for melting, it should be able to be marked as scrap metal. It will be stored in a scrap metal stockpile rather than an armor/weapons/trinkets stockpile, and treated for some purposes like a certain number of bars (the same number that went into making it). It would remain the same item (and recoverable, just like you can reclaim a dumped item) until it was used - if partially used, it would break down into a stack of generic (by "generic" i mean "no quality modifiers, etc" - particularly important if we want to enable fractional tracking - could just have a bin with [42.31] iron scrap) "scrap" - which is kind of like a bar, except it can't be built as floor/wall bars, it's only usable for melting down / casting (see other suggestion thread) / making alloy.
(Also, a "reforge" option to repair worn metal items; this could perhaps be done directly at the forge without melting the item down and making a new one)
Second part - a different amount of material should be required for armor of different metals; since a stronger metal means that armor of the same effectiveness could be made of thinner plate/wire. This would depend on material strength. Or maybe you could choose how effective your armor would be - so either have similar weight steel armor to lesser metal for greater strength, or have a lower weight for equal strength. (maybe even have a sliding scale that allows more choice than just "max strength" and "minimum weight")