I’ve had an odd occurrence and I’d like to understand what made it
happen and how to fix it.
I have a set of castings from organic items (not that I think the
source has any impact, but figured I’d mention it). The castings were
cleaned and pickled, cut from the sprue and then tossed into my
rotary tumbler with stainless steel shot, water, and a bit of dish
soap – the same way I handle all my organic castings as the first
Usually, the castings turn out nice and bright, I can clearly see
where I need to do cleanup and hardened nicely to begin more
fabrication work. The tumbler water is usually a bit gray - sometimes
even dark gray - but the pieces are nice and bright.
This time, I left it tumbling longer than usual, because I forgot to
turn it off before going to bed. It ended up tumbling about 24 hours
instead of my usual 8 - 12.
The tumbler barrel was still sealed and no leakage had occurred.
But when I opened the barrel, the water had turned to a thick black
"sludge" that coated the inside of the barrel with a layer about
1/2mm thick. It seemed impervious to water. The castings themselves
are very dark grey, and my steel shot has darkened significantly.
Washing them thorougly had no noticeable effect.
I’m now tumbling my shot in coca cola, which has yielded wonderful
results in the past.
Here are my questions…
What IS this black sludge? (Nothing was black when it went in
to the tumbler.) Why does it form?
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
Are there additives that I can put in the tumbling water to keep
this from recurring?