Capped Bust Dime With Amazing Rusted Dies...

Agreed fire damage. Coin was heated to the plastic stage just below the melting point. Tiny gas bubbles trapped inside began expanding creating the lumps on the rev and some in the fields on the obv. The coin was laying on the obverse and that side stayed cooler. Between the slight coolness and the weight of the coin laying on the face the bubbles stayed smaller on that side and basically non-existant on the surfaces actually in contact with the surface below. Then the coin colled and the metal regained its solidity which is why the bubbles are "hard". It isn't a thin film of metal like a plating bubble.