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<p>[QUOTE=&quot;desertgem, post: 1631250, member: 15199&quot;]I hate to have to do it, but the terms &quot;reactants, environment, and &#039;relatively&#039; come into play.&nbsp; The surface of a coin forms a thin chemical layer ( generally called patina) which begins forming immediately after striking. If in a dry, low air sulfur reactants condition, and low acidity such as from atmosphere or environmental ( like woods, acid rain, pvc plasticizers, etc) could stay at that point for a long long time. Some ancient coins ( as you know) can develop such a stable patina, that it may even silica replace on the surface metal ( desert patina). That is one extreme.</p><p><br /></p><p>The other extreme in this modern world is that for a coin so exposed, the reactants in the industrialized world tend to be high ( especially high sulfur oil and coal burning areas) , and moisture quite variable, that&nbsp; toning/tarnish/corrosion (whatever it is called) can continue deep into the surface from the dark gray-brown/black stage with visible pitting. I have done experiments exposing silver coins directly to H2S gas in a sealed container and&nbsp; 0.1 M equiv. will take it to black in a few days. But hopefully this level doesn&#039;t occur in someone&#039;s living area. Since the gas was generated by a method where water was present, moisture level was high.</p><p><br /></p><p>I guess the real answer is , It can happen and continue,&nbsp; but it isn&#039;t necessary or a sure thing to occur. <img src="styles/default/xenforo/clear.png" class="mceSmilieSprite mceSmilie3" alt=":(" unselectable="on" unselectable="on" /> sorry.[/QUOTE]</p><p><br /></p>

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