What has happened here?

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" 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.

Here is a classic example of the effect.


Here is another one, even more extreme.


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"

Here is a conceptual diagram showing what I think is happening:


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.

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.

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.

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.

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.

If anybody is interested enough and would like an example from this experiment, just send me a stamped self-addressed envelope an I'd be happy to send you one.

 

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