It occurred to me, though I really know nothing about this, that putting metals under a microscope could lead to a better understanding of their structure, and therefore better metallurgy. It might even lead to the knowledge that carbon is the key to making steel. (At 1500x you obviously couldn't even get close to the individual atoms, but maybe something in the structure would suggest the addition of a new ingredient.)
I haven't yet decided what level of magnification my world has been able to achieve, but for this question assume something between 275x and 1500x. Also assume some clever scholars with motivation to observe metals of different types and even at different heats. I'm most interested in iron and bronze, and the carbon question, but I'm interested in any application to metallurgy.
Finally, I should mention that while the technology of this world doesn't have a straightforward analogue to our own, most everything besides some of the chemistry is pre-Renaissance. This question represents one step in determining the world's metallurgical knowledge.
Just a quick note, despite my past questions this particular one does not involve pyromancers, and of course does, as stated, involve iron.