What IS this black sludge? (Nothing was black when it went in to the tumbler.) Why does it form?
Karen - The black sludge likely forms from abrading the inside of
your rubber barrel. Normally, any work you want to harden and
brighten only needs 45 minutes to an hour in a rotary tumbler. 8 to
12 is overkill and 24 hours is asking for trouble. The little
rotary tumblers that were designed to polish rocks may react to the
bit of soap over time.
How do I clean my castings and get them bright again? Can I tumble silver in Coke? Or would that eat away the surface of the silver?
Sure, run it in Coke - but for an hour, check it, and if it isn’t
clean in 90 minutes, rinse, add new can of stale coke and repeat.
I’d clean the silver in an ultrasonic.
Are there additives that I can put in the tumbling water to keep this from recurring?
Yes - and this is the lesson for all who just add a dab of soap -
use the stuff that is formulated for the steel. Each supplier has
something that has chemistry specific to the job. For stainless
steel, I like Rios Sunsheen Burnishing liquid. You dilute it a lot
so its not very expensive.
Judy Hoch, G.G.