I've always put the little packs that usually come in medicine bottles that absorb moisture in all my safes and I have never noticed any of my coins toning or turning colors.
Those things have reached saturation before you ever take them out of the bottles. They are also contaminated with whatever was in the bottles. Your best bet, is to buy the rechargeable packs. Oh yeah they'll work, until you take the coins out and condensation develops on them.
I would be worried about condensation as well. Water is never good to put on the surface of a coin. Everything bad that can happen to a coin gets accelerated with water.
It doesn't matter if the slab is air tight or not. The plastic that the slab is made of is semi-permeable to many chemicals. they pass right through the plastic itself. Sulfur is one of them. I have slabs coins that were 100% red as made and they have toned to a deep orange over the years (some even darker!) even though they were kept in humidity controlled areas and protected well against chemical contamination. I would never, personally pay a premium for an original red coin that hasn't a "skin" of some kind of toning on it.
most important is your ambient air composition. ozone,(0-3) and sulfur compounds are big offenders. when i was a kid i used to strip problem wheats with ketchup and retone them with PURE oxygen, which produced an absolutely undetectably natural brown in just a few minutes. i have a wayte-raymond cent album pages from '34 to '60 that have plastic slide covers and the coins were clearly full red gems when inserted. (i'll post a shot when i get it together.) anyway, they have incredible iridescent toning that could in no way be considered either red or brown. they're just stunning and many are flawless. (i believe they should remain in their album home of 80 years. i don't want to touch them.) does anyone know whether it's the plastic or the cardboard that contributes to the characteristic toning these albums produce? i scored them on ebay 15 years ago and don't know where they spent their time before that. the toning takes a long time, though, since some of the later ones ('50's) are still nearly full red. 2x2s work best to prevent toning for all coins. i have a '60 proof cent that's been in a 2x2 for 54 years and looks like it was minted yesterday. Derry
Does anyone here think something like the mosquito preserved in Jurassic Park amber could work for coins? Obviously the amber-like substance could not touch the coin directly. Instead have the coin in a slab and then preserve the entire thing.
Well they do stuff like this all the time - - but putting a coin in a slab, and then doing that to it ? Yeah, ya could. But plastic is still plastic and permeable. No doubt it would greatly slow down any toning far more than just the slab would, but what's the trade-off ? How would you store such things ? Even if you did something like this - - you still have the storage issues. You also have the scratching issues, not scratching the coin, but the plastic. It would be just like the issue with slabs getting scratched, scuffed up, thus making it hard to see the coin. I mean yeah you could this but why would ya want to with all of the other ramifications ?
Probably the only way to make the color change infinitely slow would be to put the coin in a glass chamber filled with something like pure argon or helium. It also wouldn't hurt to cool it down close to 0 Kelvin since I believe the reactions in question are less thermodynamically favorable at low temperatures.
Lacquer the coin. It worked for collectors 150 years ago in their sulfur-laden atmosphere, and it'll work today. And acetone removes lacquer without a trace. Good excuse to lose the slab anyway.
Now if you're gonna use a jar, at least use one of these They work much better than your regular mayonaise jar