If you heat nickle very high in vinager 550 + it starts to turn copper. It even has marks on it that look like it was layed on a hot surface say a baking pan.
I know I put some nickles in vinager heated to 550 and for got about um , well 20 minutes later my whole house stunk !
took out the pan slide the nickles in water this is what I got red powerdery nickles scrubbed a few off
copper nickles ! some all copper some blotchey some half mainly the side that was facing on the metal pan I suspect that pan got hot and the metal on metal made the copper start to bleed from the other metals