Our club uses a vibratory tumbler with mixed steel shot and Barrel Brite to polish silver, gold, copper and gold filled jewellery and possibly brass. Is there any reason that these metals need to be tumbled separately other than not putting large flat items with sharp spikey items? Can cross contamination of different metals occur in your experience?
I've never used Barrelbright, so I don't know how it affects the metal being tumbled. I never ever use steel shot wet as a tumbling medium because it rusts when wet which will cause a problem with metals added to the tumbler. Even though Barrelbright has a rust inhibitor, I have to think that you're still going to get some rust which would be a problem. I use only stainless steel shot because it does not rust and lasts forever.
As to tumbling different metals together, I do tumble silver, copper and brass or bronze together and have also tumbled gold and silver together. I have never had a problem with cross contamination of the metals, as long as the surfaces of the metals are smooth and not pitted.
Sometimes after tumbling, I've noticed a fine residue of metal on the lid or bottom of the rubber barrel. Items with porosity could get tiny particles of another metal lodged in those tiny holes. If tumbling large quantities of items, separating them is probably a good idea, thus avoiding any potential problems. And tumble like with like. No wiry light stuff with heavy items, etc.
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