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<p>[QUOTE=&quot;-jeffB, post: 8298268, member: 27832&quot;]Metal forced into the devices is <i>work hardened</i> as it flows. This makes it both physically harder and more resistant to chemical attack. At least, that&#039;s the explanation I&#039;ve received from metallurgists (I&#039;m not one myself).</p><p><br /></p><p>This is also why date restorers work on nickels, and why they don&#039;t work on silver and gold coins (silver and gold don&#039;t work harden nearly as readily as copper and its alloys).[/QUOTE]</p><p><br /></p>

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