Rick, coming in a bit late on this I see lots of folk have laid the
blame on sulphur, from various possible locations. Now, I’m not
going to argue with any of that, but I’d like to throw another idea
into the ring. You say:
My process is that once the buffing is complete I clean the
piece with a tooth brush and dish soap. Then into the ultrasonic.
Then another scrub with tooth brush and water. I then let the
piece dry out overnight then I put each piece individually into a
zip lock bag.
Now, if your water is tap-water it probably has some chlorine in it,
and if your piece isn’t fully dry when it goes into the bag then it
could react with the silver to form silver chloride. Actually the
"could" refers just to the uncertainty of whether or not chlorine is
there, if it is then it’ll certainly react. Silver chlorise is
actually white, but quickly changes in light, eventually becoming
black (the basis of photography in fact).
Kevin (NW England, UK)