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<p>[QUOTE=&quot;desertgem, post: 1520652, member: 15199&quot;]Not only are plastics permeable to molecules such as H2O, they are permeable to other molecules. Generally molecular wt is a fair consideration of permeability. Water is 18, air mixture is considered to be 29, Hydrogen sulfide is 34, and at first glance it would seem that Hydrogen sulfide would be the less worrisome, except, that the diffusion mentioned by Doug, where the movement of air and water molecules go in or out determined by partial pressures, once H2S diffuses in and reacts with the metal surface, it can not leave, and partial pressure inside becomes less and enhances the migration of replacement H2S from the outside through the barrier. If water vapor becomes part of the reaction , it also can react similarly.&nbsp; However, the thicker any of the plastic barriers, the slower will be the process ( more molecules to run into). A coin sealed in a inch thick piece of acrylic plastic will probably never tone/corrode visibly in a person&#039;s life span.</p><p><br /></p><p>Practicality is needed as even glass or any molecular substance has permeability to some level. That is why dessication and sulfur entrapment such as use of Intercept foil or use of scrubbed sacrificial copper or silver coins, will intensify protection for coins inside of plastic ( mylar or hard) holders</p><p><br /></p><p>In gemstones, ugly colored sapphires and other stones are diffused with beryllium or other molecules to produce an intense coloration ( Gemology&#039;s&nbsp; AT problem, but it can be detected by testing). Heat loosen the gemstone&#039;s lattice ( Thad&#039;s cheesecloth illustration) so molecules which are also moving faster can infuse.</p><p><br /></p><p>Jim[/QUOTE]</p><p><br /></p>

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