Coins currently take 1 bar of any metal.
I propose that instead the recipe should take 10 Nuggets of any metal.
However. The resulting coinage would reflect the types of metal nuggets used.
If you put 5 gold nuggets and 5 silver nuggets into your press to make the Pile of 100 Coins, the resulting 100 Coins would be 100-50% Gold, and 50% Silver.
Or you could have coins that use 3 copper, 2 silver, and 5 gold nuggets, resulting in a 100 coin pile of 30% Copper, 20% Silver and 50% gold. And same goes for any combination of 10 nuggets and types of metal.
See where I'm going with this? The coins can aways be melted back into their nugget forms, reversing the process.
We should also be able to NAME the coinage we make before its pressed. So we can call them Denari, Frank, etc.