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<p>[QUOTE=&quot;Conder101, post: 1308475, member: 66&quot;]As the coin is heated, a bubble of gas trapped between the copper and clad layer tries to expand but can&#039;t because the clad layer is still too rigid.&nbsp; As the heat continues to increase the metal reaches a &quot;plastic&quot; stage.&nbsp; It isn&#039;t molten or liquid yet but the increasing pressure from the trapped bubble finally allows it to blow up a raised bubble on the surface of the coin.&nbsp; The coin image details remain on the surface of the bubble.&nbsp; As the coin cools one of two things can happen.&nbsp; If the coin cools rapidly the metal solidifies and holds it&#039;s domed shape. If it cools slowly the gas bubble contracts and creates a partial vacuum causing the bubble to collapse.&nbsp; In the case of this coin I suspect it was still domed and something pressed down flattening the surface of the bubble and causing the light damage on the reverse of the coin.&nbsp; The reason there was no real damage to the reverse of the coin was because there was no trapped gas bubble between the copper core and the reverse clad layer.&nbsp; But I have seen clad coins with similar bubbles on both sides of the coin.[/QUOTE]</p><p><br /></p>

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