I have been using the same media in my tumbler for maybe 700-800 rounds of brass. The media (corn cob) I purchased came with the polish in it and I never added anything to it. A couple of days ago, I went out and noticed that my media had suddenly turned black. I thought maybe a piece of lead had run through and discolored the media. I dumped it last night and found this on the bottom of the bowl. It was hard as rock and had to be chiseled out of the tumbler. Any ideas what would cause this?
http://i1145.photobucket.com/a..._20120828_192516.jpg

It's cryptonite , keep it way from Clark Kent.

media does have a lifespan. accumulated oils and dirt from the cases accumulate. Yours is a really bad case, but when media starts to turn color, discard it, otherwise you'll end up with that same black gunk adhered to the brass & you'll have to steel wook it off

Have you added any additives to the media? I've never had anything like that happen. And I use corn cobs.

switch to walnut and use inchlings made from fabric softener dryer sheets. Do it outside when possible with no lid on the tumbler.
The only additive I use is Bon-Ami. Only once in about twelve years have I replaced the walnut. I just keep adding to it. The walnut I now use is untreated and I get it at the pet store at a lower price.
I have noticed that in the past when I used cleaning or polishing liquids the media tends to get stuck in the cases. This has happened with some short,pretty wide mouth cases with not much shoulder.
So now I go dry. roger