Whenever a recipe calls for a Plain Metal Ingot, there is no link within the recipe to create the Plain Metal Ingot.
This has come up before, but I don't think it's been addressed: http://www.glitch.com/forum/ideas/11756/
RELATED:
When using the smelter, it would be nice to know how many chunks of metal rock are needed to create an ingot, instead of having to guess.