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<p>[QUOTE=&quot;alurid, post: 7856608, member: 81991&quot;]The zinc core of the coin is oxidizing/ corroding underneath the copper plating.</p><p>&nbsp;Sometimes the zinc planchet can get and retain contamination before it is plated.</p><p>The plating seals the contamination in. The contamination will expand and make raised lines or bumps under the plating. They are referred to as Plating Blister when they are round and Linear Plating Blister when it forms an line.[/QUOTE]</p><p><br /></p>

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