To better answer your question, it would help to know what kind of
tumbler - rotary? Barrel type - rubber or plastic, burnishing
solution - concentration and amount, media - stainless, ceramic,
plain steel, run time, cleaning process prior to tumbling.
Try these things in sequence
Use an ultra-sonic cleaner to make sure that you don’t have pickle
or contaminants in or on your work. Dipping work in water isn’t the
same as cleaning it.
Check your chains to see if there is steel in the clasp.
Make a run with only sterling - use just a couple. That should
separate out whether the tumbler is the problem or what you are
putting in it.
Have you used liver of sulfur on some pieces? Could you have some
iron contaminants - maybe rouge - that would start a plating process.
Do you have sufficient liquid in the tumbler? Are you using a fresh
solution of burnishing compound with each run? Do you clean the media
How much stuff do you put in the tumbler? Does the “gold” appear
evenly on the pieces or is it spotty?
To remove the “gold” make a peroxide pickle rather than try to
polish it off.
For what it’s worth - I regularly run mixed metals in my tumblers
with no problem. But - I am rigorous about cleaning the work before
processing. I clean/rinse media after each run. I use fresh solution
in closed batch processing. In flow-thru processing, I finish each
run with a full tumbler rinse of the media. Tumbling works very well
if you are meticulous about cleaning everything.
And if you really want to know how the tumbling stuff works - get
the book - “Tumble Finishing for Handmade Jewelry”. Rio Grande, Otto
Frei and Gesswein have it. Amazon does not.