It's either been melted by heat or by acid. I remember when I was first shown the difference between the old copper cents and the new zinc ones. A friend, who was an amateur jeweler, used her torch to heat a copper penny. It heated up, glowed red and nothing else. Then she did the same to a zinc penny. It began to heat up, then suddenly it melted into a round, white glob of zinc. The copper it was plated with just seemed to disappear. I was surprised and amazed.
Did you do that to those coins yourself? I've melted many a zinc cent since I first watched it happen. At a penny a pop, it's probably the cheapest entertainment I've ever engaged in.
Just a friendly reminder: zinc's boiling point (1665°F) is lower than copper's melting point (1984°F). If you're careful, you can pop one of the things like low-energy, corrosive popcorn -- boil the zinc inside the copper shell, bursting it. The shell's so thin, though, that it doesn't do anything very exciting when it lets go.
