Here’s a couple thoughts.
I’ve seen this with my magnetic tumbler, and unless you clean the
pins after every use, you will see the black sludgy stuff. My guess
is that it is a chemical reaction with the amount of casting skin
and the polishing compound. However, there is a couple of quick fixes
Coca Cola. On a hot day, beyond Coke Cola’s refreshing
effervescent bubbly malted battery acid goodness, it is a superb
cleaner for your magnetic tumbling pins. No kidding. It must be
regular Coca Cola and only the Coca Cola formula, no substitutes, no
diet (diet doesn’t work, could be the lack of high fructose corn
syrup, LOL), but I always had a large 2 liter container at the
Dawn. It’s not just for dishes, it’s a superb degreaser and
lubricant for your silver stuff and stainless pins. Use about 3 drops
per one cup of water.
Polish in three steps not all at once. With casting, you have a
lot “skin” to remove and the amount of water/chemicals vs. cast
elements might be overloading the efficiency of the process. Break up
the polishing steps. I would clean the pins with coke first to remove
any residue from previous polishes, rinse everything well, add the
Dawn for two of the three steps. The last and final polish, use Rio’s
We forget sometimes that polishing, all polishing needs to be done
in steps. There isn’t a "one time it does it all for everything"
step. One kind of grit will remove the burs and this could be ceramic
media in a large vibrating tumbler. Then different polishing
compounds, whether it be a flex shaft, large motor, small tumbler or
magnetic pin polisher, needs finer and finer polishing compounds, be
it a waxy stick type, or Radial Bristle Brush, sand paper or tumbling
to achieve the level of brilliance as you require.
Hope this helps.