Hello Greg,
I would like to suggest adding "Dust Clusters" to the game as a precursor for combination smelting. Simply pour the correct combination of metal dusts into a mixing bowl, swirl around a bit, and pull out a Dust Cluster of the alloy you wish to create.
This will allow you to mix up your proportions before tossing them into the crucible, and allow you to keep track of it in your inventory.