I’ve written a couple of blog posts on the subject of medieval
All sorts of acids can be used, including lime juice and orange
juice - the problem is time. Many of the acids I tested took 1-3
days to do their work. I haven’t tried vinegar and salt together,
but I’d expect it to take a day. You might be able to speed this up
by keeping the acids warm, and acid-quenching hot metal (take
precautions if you do). Anything that works really fast (within
minutes) is going to be more harmful - there isn’t one that’s fast
and safe, as far as I know.
Another options is abrasion - if there are no concave shapes, you
can easily rub off the oxidation with files or abrasive paper (not
so easy to remove glassy flux residues that way).
Yet another option is to work in fine silver, fine gold, or forging
platinum, which shouldn’t oxidise at all, and therefore don’t need
fluxing, abrading or pickling (although it’s often a good idea to
pickle and clean stuff between processes, if you have time). But
that’s a very expensive option.