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<p>[QUOTE=&quot;rmpsrpms, post: 2779037, member: 31773&quot;]Yes, water can be sealed, but not with plastic no matter what PCGS tells you. It takes a &quot;glass&quot; material to make a hermetic seal. </p><p><br /></p><p>Perhaps you&#039;re thinking of &quot;water&quot; in its liquid state, and that if you dunk a slab into a glass of liquid water, it won&#039;t fill up. If the plastic is &quot;sealed&quot;, then this is true over short time spans. This is generally called &quot;water resistant&quot;. But over a longer time span, water will eventually seep-in through imperfect seals or directly through the plastic via diffusion. This happens even if it&#039;s not dunked into liquid, but through diffusion from the moisture in the atmosphere. </p><p><br /></p><p>It&#039;s generally not good practice to seal plastic containers that are meant to stay as dry as possible inside. Eventually, the inside atmosphere will reach equilibrium with the humidity level in the outside environment. If the container is then brought to a colder environment, the moisture inside the container can condense on the inside container surfaces and onto the contents (ie coin). For this reason, &quot;sealing&quot; a slab is actually a bad idea. It&#039;s better to have a small leak hole to allow the atmosphere to equalize when subject to changes in temperature and humidity levels. </p><p><br /></p><p>No matter whether the slab is sealed or not, the only way to keep it moisture-free inside is to keep it dry outside. Putting slabs into an environment that is kept dry (typically with dessicants) will keep their insides dry. In fact if you put a &quot;wet&quot; slab into such an environment, it will gradually dry out as moisture diffuses through the plastic! </p><p><br /></p><p>Keeping oxygen and sulphur from your copper and silver coins is a bit tougher. In the semiconductor industry, we have &quot;dry boxes&quot; that are constantly pumped with dry nitrogen to ensure no moisture or reactive air molecules (Nitrogen has low reactivity at room temp) are present that might cause problems. Most folks don&#039;t have the resources to contemplate such a method, but some dealers or deep-pocket collectors with safes might consider it. Pumping dry nitrogen into a safe would not be prohibitively expensive, and would help maintain the coins inside in an excellent long-term storage condition.[/QUOTE]</p><p><br /></p>

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