I just received these from an eBay lot. Sorry about the lame photos, the iPad 2 camera sucks. They were pretty skanky upon arrival. After a soak in baking soda in a foil lined bowl and a scrub with a baking soda paste, they cleaned up nicely.
I use baking soda as well and scrub using an old toothbrush. It works because baking soda is also an abrasive.
it also works due to ion transfer in the reactive alkaline solution...foil vessel liner plays a role..Museums clean tarnish this way...it's so much more than just abrasion.
Read up on it a bit, I don't wanna help anyone wreck a finish....I just recognized his technique from the foil liner. It appears to possibly work on other "tarnishes". Experiment time This page is from U of WI and explains the idea. http://scifun.chem.wisc.edu/homeexpts/tarnish.html
I dig silver rings from the ocean. they are always black and crusty, if they have been there any time. I use a homemade electrolysis rig to bring them back to life again. a Google search will surely give you the details about how too make one. be warned plated jewelry does not fare well. but it does wonders on silver coins and rings.
