Patina on silver medals

I'm trying to find out the possible cause(s) of an "orangy-gold"
patina on medals made of sterling silver (92.5% silver, 7.25%
copper). The question came up on a collectors discussion list
recently.

Like most medal (and coin) collectors we are aware of the usual
causes of the blackish or greyish tint on toned medals. It's the
"orangy-gold" *colour* of the patina which has us puzzled. Both
medals concerned were awarded in the late 1800s and had the patina
when acquired (within the last 30 years). The owner has no idea of
environmental factors affecting them prior to that.

Any references to a more appropriate discussion list (I doubt it, it
took 2 months to find this list), scientific journals, books,
indeed, any source which has some solid scientific data on this (to
us) extremely obscure point will be very much appreciated indeed.

We have also been advised that "There are now sophisticated
techniques for surface analysis. I believe scanning electron
microscopy could be used for examining these medals
(nondestructively) and giving an elemental analysis of the
surface". Any ideas on how much this might cost ?

Graeme Marfleet
Sydney, Australia (Medal collector, not a conservator)


                                  ***
                  Conservation DistList Instance 13:46
                  Distributed: Tuesday, March 7, 2000
                       Message Id: cdl-13-46-033
                                  ***