Silver coming out with black grease/scum
I'm using a Lortone tumbler (relatively new barrel) with mixed jeweler's shot, small pinch of burnishing compound, and enough water to cover shot and jewelry plus about half an inch. Usually it works great, but recently I'm having some problems with sterling silver pieces coming out with a thin film of dark grease. I tumble for an hour to two. Any suggestions or hints? It would be very much appreciated :)
It sounds like your shot and barrel may have been contaminated by what's been put in it. Burnishing compound is great for some items, like shiny components that need smoothing and polishing, but not for oxidized or etched items. There's nothing to hold the "gunk" being removed from the metal in suspension, so your tumbler is just depositing it right on the metal...including the shot! After a just a few uses, you can acquire quite a build-up!
If you have put etched items in the tumbler without making sure that the etching solution was neutralized and thoroughly cleaned off the metal first. Traces of the etchant can have disastrous results on the items tumbling, including the shot. First clean your rubber barrel with warm/hot water and dish-soap using a sponge or paper-toweling - not an abrasive scouring-style pad. Next, clean your shot. You may use either a shot cleaning compound, or flat coke with a squirt of dish-soap.
Once your shot and barrel are clean, you are ready to add items to it. Place them in the tumbler along with the shot and add water to about two inches over the items. Normally, I add water to only one inch above the jewelry or items I'm tumbling. Then add a healthy squirt of dish-soap. Smaller batches of jewelry will clean more evenly allowing the shot to make contact with all the surfaces. Tumble for only about 2 hours, then rinse and refresh and repeat if necessary.
I hope this works for you and that you'll soon be getting great results from your tumbler!