I just tumbled a bunch of brass and it came out looking terrible. See the pic below. The black residue is very well stuck to the brass. In some cases, it is caked on the spent primers, but none of it stuck to any nickel plated cases.
I recently switched to Frankford Arsenal brass polish, and have run a number of tumbler batches that came out looking great. On the other hand, from the first batch of FA, there is a basically solid layer of black crud on the inside of the tumbler. That, however, is not sticky at all, and has been there even when the brass came out fine. I do shake the FA polish very thoroughly before adding.
The media has been used a few of times, but I know I have gotten a lot more use out of media in the past. In fact, as I said, the previous batch of brass came out great, so the media was not really showing any sign of getting too dirty.
I replaced the media anyway and ran for, oh, another hour, and none of the black stuff seems to be coming of yet.
Any clues, folks?
Sorry for the cruddy cell phone pics; it's what I have available. First pic shows a close up of the residue. Second shows a shiny nickel next to the ugly brass...