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<p>[QUOTE=&quot;Lehigh96, post: 482153, member: 15309&quot;]It certainly is possible that a coin in a slab can tone if exposed to high enough levels of hydrogen sulfide gas for a long enough period of time.&nbsp; As Doug has pointed out, no holder is air tight.</p><p><br /></p><p>However, Weimar&#039;s experiment with regards to this only proved that the slabs were not air tight.&nbsp; He does not do an analysis of the concentration of hydrogen sulfide gas in the experiment compared to concentrations in normal atmospheric conditions.&nbsp; Therefore, the time that it took the slabbed coins to tone in his experiment are completely meaningless.&nbsp; If he had completed an actual scientific experiment and concluded that a slabbed coin runs the risk of toning after 100 years of exposure under normal atmospheric conditions, nobody would have cared that the slabs were not air tight.</p><p><br /></p><p>As you know, my entire collection is predominantly rainbow toned.&nbsp; I have never seen any of my toned coins turn in their holders.&nbsp; If I do see one turn, it will lead me to believe that the coin was AT to start.&nbsp; Most rainbow toned coins are not a result of exposure to atmospheric hydrogen sulfide gas.&nbsp; They tone because they had direct contact with a sulfur rich toning source (eg canvas bag, album, etc).&nbsp; Once the coin is removed from the source of the toning, the toning stops.&nbsp; The silver sulfide layer on the surface of the coin can&#039;t get thicker if the source of sulfur is removed.&nbsp; The sulfur in the silver sulfide layer can&#039;t make babies and grow like a virus.&nbsp; Unless a slabbed coin is stored improperly in an atmosphere that is rich in hydrogen sulfide gas, you will not be able to see any discernable change in appearance of the coin for many many years.</p><p><br /></p><p>Your question about the slabbed coins that are undergoing an AT process is extremely important.&nbsp; This can and does happen.&nbsp; It is also much harder to spot since the coin is in a slab which is supposed to indicate market acceptability of the toning&#039;s originality.&nbsp; Even if you see one of these coins and suspect the coin is AT, your first though will be that the TPG considers that type of toning to be market acceptable.&nbsp; My only advice in this regard is to become familiar with the traditional toning patterns and colors for a particular series before purchasing rainbow toned coins.&nbsp; Even then, if a slabbed coin doesn&#039;t look right, don&#039;t buy it.&nbsp; I say this only for the collector&#039;s that find it of the utmost importance to avoid AT coins.&nbsp; I am not one of those collectors and will knowingly buy a slabbed AT coin if I find the eye appeal of the coin such that it overcomes the AT stigma.</p><p><br /></p><p>In closing, there are no real telltale signs that a coin has been toned after it has been slabbed.[/QUOTE]</p><p><br /></p>

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