There have been suggestions that touched upon this, so I thought I'd give it a shot.
Picture different technology levels of of workshops. A smelter is a good example.
A really low level smelter consists of a simple camp fire or similar fire, with some sort of vessel for separating metal from slag. This is what the earliest ones looked like and is quite sufficient for some ores. I imagine copper and tin would work well.
The higher up in technology level, the more advanced the workshop, the more materials are needed for it, and eventually you'll get to the point where you actually need materials produced in a lower level workshop to build a higher level one. (Or you could buy them from someone else, of course.)
Different civilizations would have different tech levels, which are of course moderated by the materials available to them, as well as other circumstances. (A completely isolated settlement in a peaceful valley isn't going to spend a lot of resources on advanced weaponry.)
In the above example, ratmen might have been able to learn how to extract bronze and tin, and maybe even smelt them together to bronze, but maybe not more.
The dwarves, on the other hand, could have advanced blast furnaces for steel production, etc.