Jersey - My first guess is that you are using an inappropriate
liquid for cleaning in your tumbler. But - exactly what are you
doing? Did it work before? Are you new to the process? Do you
neutralize your pieces with baking soda in solution after pickling
and then rinse? In which step in tumbling is this occurring?
Abrasive or steel? Do you rinse between steps? Is the deposit
inside the seam or on the exterior? Are these hollow pieces that
didn't get neutralized and rinsed? Are you using a flow thru system
and changing liquids between abrasive and burnishing steps?
Do you wash your pieces after tumbling? How long does it take to
show the discoloration?
Judy Hoch, G.G.