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<p>[QUOTE=&quot;Pavlos, post: 7674832, member: 96635&quot;]Unless you put your electrum coin in a high concentrated nitric acid solution, that won&#039;t happen naturally <img src="styles/default/xenforo/clear.png" class="mceSmilieSprite mceSmilie1" alt=":)" unselectable="on" unselectable="on" /></p><p><br /></p><p>Just for extra info if you are interested. Corrosion of silver is induced by different causes. It is due to impurities such as copper, which is more chemically susceptible, and when they become excluded they cause &#039;gaps&#039; in the boundaries between silver atoms. This is what mostly happens first, alloys can &#039;strengthen&#039; eachother or &#039;weaken&#039; eachother, in this case the couple of percentage of copper (or more) is weakening the silver.</p><p>Also, the boundaries of crystalline grains and bounderies are in locally higher energy states, so silver oxide or silverchloride starts to form more easily, revealing the grain boundaries after diffusion or cleaning. This is often wat we call &quot;crystallization&quot;, eventhough it is a normal term in numismatics, it is very false in science, since the surface is just revealing what was already there, but people generally do not know that almost all metals are polycrystalline in the first place. Porosity is the same thing but on a larger scale and is mostly not due to impurities but corrosion, such as silver chloride aka horn silver that is formed and then removed.</p><p><br /></p><p>Anyway, back to the point, in modern times, noble metals, or alloys with a high content of noble metals, especially gold, are employed when strong corrosive conditions are needed. For example, alloys containing about 70% gold and 30% silver have been proposed for such purposes which, because of their high gold content posses substantial resistance to corrosion. Therefore an electrum coin is much more chemically resistant than silver and does not have these corrosion issues. Unless it is badly contaminated by Copper / Zinc, than you get the dissolution of the Zn and Cu species again and you get a similar story as silver, but in much less and slower amount though.</p><p><br /></p><p>Sorry if I bored anyone, I have done extensive research in a topic in Metallurgy at a university in the past, so I know a bit too much about it.</p><p><br /></p><p>Back to coins, this is my Archaic EL pieces:</p><p><img src="https://www.cointalk.com/attachments/316-3-jpg.1249078/" class="bbCodeImage wysiwygImage" alt="" unselectable="on" /></p><p><b>Islands off Caria, Kos. EL Forty-eighth Stater, Phokaic standard. Circa 625-600 B.C.</b></p><p><b><b>Obverse:</b></b> Crab.</p><p><b>Reverse:</b> Incuse square.</p><p><b><b>Reference:</b> </b>Stefanaki Series I, unlisted denomination; HN Online –; cf. HGC 6, 1295 (1/96th stater).</p><p>0.34g[/QUOTE]</p><p><br /></p>

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