I wonder, Duo accepts clean as an alternative to pure. So Καθαρός can be translated by either clean or pure but these words are not synonymous in English. Clean iron might mean the iron is free from rust but it does not mean it is pure, not adulterated. I think in this case the possibility of using clean in this sentence is too generous!
Καθαρός is an alternative for "pure"=αγνός, so even St. Catherine's name reminds us of her purity. It's a standard phrase for metals and more: καθαρός σίδηρος, καθαρός χρυσός (ατόφιο χρυσάφι), καθαρή ληστεία=pure robbery!
A friend has pointed it out to me, and indeed "clean iron" would mean something else, different from "pure."
I focused on some Greek collocations that you might find useful.
I agree with you, since there is an actual context issue here. We'll look into this one, thank you for your comment. ^.^
pure iron/ καθαρός σίδηρος, is rarely found in nature since it tends to oxidize
clean iron/ καθαρό σίδερo, with time the iron bottom needs cleaning
you might try and add context, so that the "pure" translation is the obvious (right) one? e.g. "Chemistry deals with pure iron" or "this is not an alloy, but pure iron"?