Make double, triple sure you have cleaned every bit of grit out of the barrel(s) before you put your metal in to be tumbled. Look at the rio catalog..there are all sorts of media: abrasive, polishing, burnishing, etc.
I prefer burnishing soap over dish soap, first it's been my experience dish soap is more expensive. You can use the burnishing soap over and over...till it gets black before you need to dump it, consquently a $10 bottle lasts a really long time...mine lasted 5 years.
Also, dish soap seems to take longer than burnishing soap, that's important because with every turn of the barrel the rubber barrel is abrading, which may lead to the dreaded 'black gunk'. My pieces stay in the tumbler for about 15 minutes, everything is clean and never had the black gunk.