Did you use the buffer before tumbling? When I first bought my
tumbler, I was relatively new to jewelry work, and I had exactly the
same problem. I was sure the black rubber barrel was somehow
involved. After a bit of detective work, I learned that I was not
cleaning my jewelry sufficiently after using the buffer, assuming
that the tumbler would help to clean it. The tumbler was then taking
the small amount of grease left from the buffing compuinds, and
spreading it very efficiently throughout the shot and the inside of
the barrel. Grey goo everywhere. I used Bon Ami, Comet, or fine
jeweler’s pumice to scrub the pieces, and poured some in the tumbler
with the shot, water, and dish soap (no jewelry) as well. I ran it
for 10-15 min and rinsed, then repeated until all was scrubbed
clean. It only took 2-3 runs. Since then, I always wash my buffed
pieces with a soft brush and soapy water with ammonia. I have never
again had the problem.
I would suggest you clean out your barrel and shot well, then run it
again, exactly as before, but without the jewelry. If you get more
black goo, the problem is the barrel or an interaction between it
and something you are using. If you don’t, the problem is something
related to the silver/jewelry pieces, and contaminants left on it
that should not be there.
I have used all of those mild abrasives at one time or another in my
tumbler. They are gentle and effective when shot starts to get a
little dull. Incidentally, as for time, I have often run my tumbler
(an old Loretone, with steel shot) for 10+ hours/overnight, esp. if
I want to harden earwires or other fine wire work. It has never
caused a problem, and hardens and deburs the silver beautifully.
Time should not be the problem. Good Luck!