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<p>[QUOTE=&quot;-jeffB, post: 1720872, member: 27832&quot;]Nothing&#039;s leaking in very quickly.</p><p><br /></p><p>Once upon a time, the product called &quot;Saran Wrap&quot; was made of polyvinylidene chloride (PVdC). PVdC is <i>excellent</i> at blocking oxygen and moisture. Unfortunately, it&#039;s kind of messy to manufacture and dispose of, in ways that draw the attention of environmental regulators. So, today, at least in the US and Canada, it&#039;s made of low-density polyethylene -- cheap, clean to manufacture, and environmentally benign, but several THOUSAND times more permeable to oxygen.</p><p><br /></p><p>For food, this means that you can still knock over a Saran-sealed bowl without spilling the contents. But if you wrap a slab of meat and put it in the freezer, it&#039;ll go bad a <i>lot</i> faster in the new wrap than the old stuff. It&#039;s still &quot;air-tight&quot;, in the sense that it doesn&#039;t have holes. But if you covered a bowl with today&#039;s Saran, removed all the oxygen, and then let it sit for a few weeks, you&#039;d find that quite a bit of oxygen had diffused back into the bowl.</p><p><br /></p><p>Think of a latex balloon filled with helium. It&#039;s not &quot;leaking&quot;; it holds the helium. But latex is <i>permeable</i> to helium, and so after a day or so, the balloon sinks to the floor. Aluminized Mylar is a lot less permeable, so a shiny Mylar balloon can retain its helium for days or weeks.</p><p><br /></p><p>Oxygen, water vapor and hydrogen sulfide all diffuse through plastic more slowly than helium does -- but they still do make it through eventually. Vacuum-sealing your coins in bags <i>probably</i> protects them to some degree, but not completely, and not forever.</p><p><br /></p><p>Really, though, I&#039;m still talking in generalities. If anybody has actual information about the actual plastic used in these systems -- its thickness, its permeability, how long it really does &quot;hold a vacuum&quot; -- that would certainly trump my speculation. <img src="styles/default/xenforo/clear.png" class="mceSmilieSprite mceSmilie1" alt=":)" unselectable="on" unselectable="on" />[/QUOTE]</p><p><br /></p>

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