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<p>[QUOTE=&quot;Tom Maringer, post: 2740266, member: 7033&quot;]Well... I stand corrected. This is circulation damage after all and I have proven it experimentally. I took about 400 near-mint post 1982 zinc pennies, put them in a 6&quot; diameter rubber tumbler barrel, and spun them at 60rpm for four days. The results were that about 25% of the coins showed exactly this type of damage... some of it very extreme. I believe I can now explain how this happens and why I had never seen it before.</p><p>[ATTACH=full]625401[/ATTACH]</p><p>Here is a classic example of the effect.</p><p><br /></p><p>[ATTACH=full]625403[/ATTACH]</p><p>Here is another one, even more extreme.</p><p><br /></p><p>[ATTACH=full]625404[/ATTACH]</p><p>And here is one where I plucked at the edge of the overlap with an exacto blade and was able to show that it is a thin piece of the copper plating that has delaminated from the zinc core, and has been forced over the underlying copper plate. As measured with a micrometer, the flap of loose plating looks to have a thickness of about 0.0025&quot;</p><p><br /></p><p>Here is a conceptual diagram showing what I think is happening:</p><p>[ATTACH=full]625405[/ATTACH]</p><p><br /></p><p>The reason I have never seen this effect is that in my minting experience I have worked exclusively with solid metal coins. The phenomenon appears to be confined to coins struck on plated blanks, specifically the post-1982 US cents.</p><p><br /></p><p>I think that the upsetting (compression) blows on the rim of the coin during agitation/circulation cause the plating to shear and delaminate from the core right at the crease in the rim (where the plating is thinnest) and override a portion of the field, including parts of the design if the effect is strong enough.</p><p><br /></p><p>It is interesting to note that, while the effect was common, about 75% of the sample did NOT show this effect, suggesting that variations in the plating thickness or adhesion may exist from batch to batch on the blanks.</p><p><br /></p><p>The small flap of copper plate that I peeled up and examined had white zinc metal on the underside, suggesting that it had not delaminated at the metal-to-metal contact, but rather that the stronger copper layer sheared off of the weaker zinc core. The key here is the zinc, which is much more susceptible to shear failure.</p><p><br /></p><p>The remaining question now is... why did some of them NOT fail in this same way. I may select out those and put them back in the tumbler to see if they will eventually fail in the same way, or not.</p><p><br /></p><p>If anybody is interested enough and would like an example from this experiment, just send me a stamped self-addressed envelope an I&#039;d be happy to send you one.[/QUOTE]</p><p><br /></p>

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