My question has to do with a method of testing pickle (I use Sparex) to determine when it's time to be replaced.
Suggestion one; change the pickle when it doesn’t seem to be doing it’s job
or removing flux and copper oxide.
Suggestion two: Change the pickle when the blur colour of copper sulphate
becomes quite pronounced.
However, even if you don’t change the pickle frequently, it won’t cause any
harm to either sterling or carat gold. (or for the pure metals either) It
just won’t do very much at all. Just don’t put any iron or steel in the
pickle along with your work; it will get an almost unmeasurable coating of
reddish copper. If it doesn’t wipe off, then polish it off. – John Burgess