Hi Rob,
I have been doing some research for my new book, 'Badges and Insignia of the King's Own Scottish Borderers from 1689-2006 also Associated Volunteer and Militia Units' from the Regimental Museums Collection and Others'.
one of the belt plates has a gilt frosted ground, on the plate the Cross of St Andrew in burnished silver plate, surmounting the cross the Castle of Edinburgh in silver plate the whole surrounded by a Thistle Wreath also in silver plate. However, on one of the plates only the castle remains in silver, the cross and wreath are completely copper with no sign of any silver. I might point out that there is no polishing damage to the detail of the thistle wreath.
This made me think as the cross, castle and wreath can be removed from the plate for cleaning, and after some research, if silver plate on copper is submerged in a cleaning solution there may be a chemical reaction which would remove the silver plate or blacken and tarnish the silver. I have included a link which explains the effects of damage to silver items.
Also a picture of the belt buckle in question.
Just a thought
Cheers
Hiram
https://www.canada.ca/en/conservatio...-products.html