Acid wash with nitric acid, a 50% solution works well, and let it stand in
a hood, in an inert container or the acid will eat a metal sink.  I clean
up silver deposits this way, and this should work.  Sulfuric acid/potassium
dichromate solutions are forbidden now, since you can't drain dump the
chromate solution.  You can reuse the nitric, and neutralize to get rid of it.

Gayle Callis
Gayle Callis
Veterinary Molecular Biology
Montana State University
Bozeman MT 59717-3610
406 994-4705
406 994-4303