I guess this will...ahem...muddy the waters, but one of my best successes was to soak a very heavily crusted,
Probus ant, for over a year and it came up with a fragile coat of sllver that was very thin, but mostly
complete. I changed the DW once a week and about once a month did a gentle scrub with a nylon brush.
It was heavily encrusted and it would have been easy, about 6 months on, just before the silver started to show, to take a
brass brush and...well...have
had a nifty, cleaned coin that was pure basemetal with great detail, without the
silvering.
I cannot emphasize how gradual this whole process was and the patience involved. I'm sure I have taken the silver off of other similar coins without ever knowing it.
Bruce