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<p>[QUOTE=&quot;-jeffB, post: 2649140, member: 27832&quot;]After lots of discussion here, my understanding is that it&#039;s the <i>work hardening</i> done on copper-nickel that makes the date more resistant to etching. Metal flows toward the voids in the die, hardening as it&#039;s deformed. (You&#039;re not going to compress most metals to a higher <i>density</i> with ordinary pressures, and even if you did, that should make the compressed areas <i>more</i> resistant to attack.)</p><p><br /></p><p>Copper should be subject to the same effect, but I&#039;m not aware of many copper coins that lose their dates in such a way that they&#039;re amenable to restoration. You&#039;d need an oxidizing acid like nitric, or an acid mixed with another oxidant like hydrogen peroxide, to make it happen.</p><p><br /></p><p>Steel is <i>most definitely</i> subject to work-hardening, and so you&#039;d expect restoration to work there, too.</p><p><br /></p><p>I don&#039;t think silver work-hardens as much, and I&#039;m quite sure gold doesn&#039;t -- it&#039;s one of the most malleable and ductile of metals.</p><p><br /></p><p>One member here has <a href="https://www.cointalk.com/threads/psa-vinegar-dating-is-the-best.282307/page-4#post-2517017" class="internalLink ProxyLink" data-proxy-href="https://www.cointalk.com/threads/psa-vinegar-dating-is-the-best.282307/page-4#post-2517017">posted photos of date-restored SLQs</a>, but I still don&#039;t think we&#039;ve got an answer about what was in the product that did it. And more to the point, while venues like eBay still offer a steady stream of dateless SLQs, I see no &quot;restored date&quot; keys there or anywhere else. Why the difference?</p><p><br /></p><p>And what other coins are commonly found &quot;dateless&quot;? I see Buffalo nickels, a <i>few</i> Liberty/V and Shield nickels, SLQs, and occasionally Walkers. Maybe an occasional cent, although they&#039;re usually trashed in other ways as well.</p><p><br /></p><p>It&#039;s <i>possible</i> that I&#039;m completely wrong here, and that you can restore dates on silver coins just as reliably as on copper-nickel, except that the chemicals are too hazardous to be generally available. But if that were true, given the financial rewards available, I&#039;d expect <i>someone</i> would be doing it, and we&#039;d be seeing the products on eBay and/or in Details slabs. I&#039;m not seeing them, and that makes me think it&#039;s because nobody can produce them.[/QUOTE]</p><p><br /></p>

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