What does "calx" mean in oxidation reduction reactions?
It is a term commonly used in reactions where you are roasting a metal in a place with excess oxygen (I did this in an Inorganic Chemistry lab in a hood).
Basically you can put a metal in a crucible on a wire mesh (or clay triangle, like in the diagram) on a ring clamp on a ring stand over a bunsen burner, and heat it until it forms a purer substance. Calx is the remaining ashy residue.
You have to keep watch over it though, and if it goes for too long, the pure ore would be burned too and you'll just have soot.