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<p>[QUOTE=&quot;Lehigh96, post: 659595, member: 15309&quot;]The planchet is a silver alloy.&nbsp; The striking process causes physical stresses on the planchet but there is no chemical reaction, therefore the composition of the planchet/coin can&#039;t change during the minting process.&nbsp; Work hardening has been used in many commerical applications on many alloys to increase corrosion resistance.&nbsp; However, in this case, it is purely coincidental.</p><p><br /></p><p>BTW, I certainly meant no condescension in my previous post.&nbsp; You have a broad knowledge of toning and coins in general.&nbsp; And you are right that the coin changes from the rest of the coin in some way, it is just not a composition change.&nbsp; The localized work hardening around the stars, date, and letters changes the physical properties of the metal inlcuding the resistance to corrosion.</p><p><br /></p><p>Here is an article about the negative shadow effect.&nbsp; Dick Johnson believes that every Morgan Dollar that exhibits the negative shadow effect has some tin in its composition.&nbsp; No way I am going to agree with that theory.&nbsp; Note that he skips the why part!</p><p><br /></p><p><a href="http://coinbooks.com/club_nbs_esylum_v12n09.html" target="_blank" class="externalLink ProxyLink" data-proxy-href="http://coinbooks.com/club_nbs_esylum_v12n09.html" rel="nofollow">http://coinbooks.com/club_nbs_esylum_v12n09.html</a></p><p><br /></p><p>The article is about half way down the list.[/QUOTE]</p><p><br /></p>

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