My assumption would be "cast counterfeit", rather than "fire damage". Fire turns silver black. I also can't see fire making a silver coin lose an entire 1/3 of its weight.
It's entirely possible that it's a cast counterfeit, that someone tossed into a fire once they realised it was fake.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis