Rio Grande tells me the copper atoms migrate through the plating
metal and alloy it causing the discoloration. The nickel plate
prevents this migration of the copper. Sterling needs to be
nickle plated prior to any other plating, whether gold or
rhodium. By the way, there is now a cold nickel plate available.
I have inspected ferrous metals and brass shell casings which
have been through this. I was impressed. Just recently
purchased a small plating setup and look forward to using it and
plan to try the cold nickel as well. Will be next year before
the "Silver Fox Gallery" is open though.