Truly Airtight Holders? One Would Expect In The Year Of 2024 That PCGS & NGC Holders Are Airtight!

PCGS says on their webs site that sonically sealed but not airtight: https://www.pcgs.com/faq#:~:text=It...0air%20tight.

Would an airtight holder guarantee that the coin will not oxidize/corrode (if it does not have any corrosion on it when holdering)?

What about airtight sealing as in a food vacuum sealer?

The very term "Airtight" is obviously just a silly advertising slogan for two pieces that simply snug together without a seal. Surely no one would take this literally.

Beyond the gaps at seals, no plastic prevents gas from seeping through.

There is a surprising amount of research because of the use of plastic packaging for food. Making things worse is that the plastic needs to be clear to use in a slab.


https://www.sciencedirect.com/scien...13549390202S
https://www.researchgate.net/figure...l1_318010903
https://www.sciencedirect.com/topic...ygen-barrier

In the real world, things can be "air resistant", but not "airtight". "Airtight" requires zero introduction of oxygen, and on this planet, "zero oxygen" in a confined space is really, really hard to do unless there's something in that confined space that's actively consuming the oxygen. That's the principle behind Intercept Shield products, but even these have a limited lifespan, as the supply of sacrificial metal runs out, but the Earth's supply of oxygen never runs out. As collectors of bright shiny metallic objects, oxygen is our enemy (which is kind of ironic, given how much we personally need the stuff to continue existing), but from a big picture perspective, the struggle is futile. If you declare war on oxygen, oxygen will win in the end, guaranteed.

Let's consider the hypothetical "perfectly airtight coin container": embed the coin in molten acrylic. You know those paperweights that were fashionable back in the 1960s and 70s, where coins were suspended in plastic? Those coins are all still perfectly preserved 50+ years later, so that seems like a pretty good way of assuring permanent air-tightness. That same acrylic is what they make slabs out of.

However, acrylic - or polymethylmethacrylate (PMMA), to give it its proper name - is not completely airtight either. It's oxygen permeability is really really low - but not zero. Oxygen does move through acrylic, but very slowly. Give those paperweights a few more centuries, and those coins will start to oxidize. Slabs are much thinner than those paperweights, and the heat-sealed joint between the slab half-shells is even thinner still. Oxygen will find a way through, and oxygen will win.

Yes. "Air resistant" might not seem as good as "airtight", but if "airtight" is essentially impossible, then the next-best thing is to reduce air flow as much as possible. That means air-resistant containers, that means silica gel and sacrificial copper placed in the storage box (because oxygen isn't the only enemy, you want to restrict atmospheric moisture and sulfur, too), and it means sealing the storage box up as best as practical. Because the slow seepage of oxygen into an environment is still better than the free and unfettered flow of air you'd get from an unsealed container.

On the subject of air-tightness, and specifically looking at in-the-slab coin doctoring, we see that the laws of physics make it impossible for a slab to be doctor-proof.

Gas permeability of a slab depends on several factors. There's the thickness of the plastic, for starters, and which plastic you're using (PMMA has low permeability, while polystyrene, for example, has such high permeability it might as well not be there). But temperature and pressure also play a part. They can make a slab nice and tight enough so that it ought to last 50 years before oxygen gets in, but a coin doctor can simply ramp up the temperature and pressure, and cause fifty years worth of gas to pump into the slab within weeks or days.