What causes a coin to "turn" in a PCGS slab

Recently I bought a blast white 100 year old silver coin. In my safe deposit box a "ugly" toning spot developed. I showed it to several dealers who commented the coin had been improperly dipped. Fortunately, the selling dealer agreed and made me whole. I'm noticing other coins "tone" in my safe deposit box. What other factors would cause a coin to "tone" further in a slab?
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Answers
Without photos it would be hard to tell for sure.
It may be just what several dealers told you, it had been improperly dipped.
It's not soo much it was improperly dipped but it was improperly rinsed.
Any left over dip on the coin could cause discoloration.
Typically the humidity and atmospheric contaminants can do it. Exposure to surfaces or cleaners that emit gases would be a key thing to investigate. A lot of people will invest in intercept shield containers to help reduce the impact of those things and also use dessicants to reduce the humidity in the storage areas.
For some coins the holder itself can be a factor. Older holders, especially green label PCI ones, didn’t use chemically stable paper and the paper causes toning from within the slab over time.
You might benefit from placing a desicant bag or device in your SDB. Perhaps even rotating or removing your coins upon occasion. I have never had a PCGS or NGC or ANACS turn in my standing safe. I use a rechargeable dessicant in my safe, twice what it calls for which maintains 42 to 46% moisture. Note, they do not last forever. About 4-5 years and I replace them.
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@DisneyFan you should definitely get some desiccant for your SDB. I had one in mine for 30 years. No additional toning on the coins. Raw or slabbed.
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First, the slabs are not airtight. They also have inserts and some, especially PCI inserts, can cause toning. Moisture and heat are accelerants for what are essentially chemical reactions, so do your best to keep moisture as low as you can---I use cannisters loaded with indicating calcium sulfate (these can also be regenerated by baking the absorbed water out) to draw water vapor out of my safety deposit boxes. Second, keep things like unnecessary packaging (wood, felt/velour/velvet liners, paper products) out of the safety deposit box--organic dyes, sulfur-containing organics and trace amounts of sulfuric acid can volatilize out of these and cause toning or deposit on coin surfaces (deposited dye molecules will rinse off with acetone). Absolutely NO rubber bands! Third, add sacrificial coins (I use recent-vintage RED cents) to trap molecules in the atmosphere of the box. There are also special holders and boxes for slabs, with coatings that trap moisture and trace amounts of oxidants--I use these for some of my more valuable coppers.
RMR: 'Wer, wenn ich schriee, hörte mich denn aus der Engel Ordnungen?'
CJ: 'No one!' [Ain't no angels in the coin biz]
Heat, humidity, salt air or just about any exposure to the atmosphere. Better flip them quickly lol.
Unless coin in a vacuum it will tarnish over time (Coin Preservation Handbook). Biological attack can be another problem besides tarnish / spots / ugly muddy toning / dark tarnish - gets worse over time.
Many collector / investors ignorant of this, coin tarnishes over time, then get burned down the road. Buy coins that are PQ, bright and brilliant with super cartwheel luster.
I remember in HS that if u bought an unc roll at this guys shop he (had special setup) as a free service would dip it for u in one whack, nice!
"I remember in HS that if u bought an unc roll at this guys shop he (had special setup) as a free service would dip it for u in one whack, nice!"
Fifty years ago, I had a summer job at a local B&M, sorting and cleaning random coins. I saw dip-while-u-wait more times than I can remember. Also recoloring copper coins. I won't buy any copper coin that has a uniform, dark chocolate color on both sides, because I can guess how it got that way.
RMR: 'Wer, wenn ich schriee, hörte mich denn aus der Engel Ordnungen?'
CJ: 'No one!' [Ain't no angels in the coin biz]
Description of Intercept.
Some Intercept products.
Buy a safe - then you can keep track of and can control the environment you keep your coins in -
"When they can't find anything wrong with you, they create it!"
The downside of that suggestion is the 1967 du Pont coin robbery whereby " Two robbers stayed in the bedroom, forcing Miren to open the small bedroom safe. At one point, Miren panicked and couldn’t remember the combination, only to have a gun shoved in her face."
I have long suspected that the chemical content of some slab labels actual "gas" the coins.
Repetition of ignorance is ignorance raised to the power two.
Please be realistic. In reality the robbers would be having a gun put in their face.
"When they can't find anything wrong with you, they create it!"
I keep my coins at home. I have layers of security. I keep desiccants in my safe. I have no problem with tarnish developing over the many years and different states I have lived in with my coins/safe. (AZ, TX, CA, Fl, WA, NY, ME) Cheers, RickO
Great that the selling dealer made you whole. Dealers like this are vital to the hobby. Kudos.
Did the Dealer slab the coin?
An untold number of coins are ruined every year by the dippers. Sad situation.
BTW, very few 100 year old coins should be blast white to begin with.
Dave
That's interesting. The question becomes. "Is CAC stickering coins that have been dipped?"
Given that this was a very high end dealer, I doubt it.
Dipping.
It's the improper rinsing after the dipping that causes the coin to turn in the slab.
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"Paper money eventually returns to its intrinsic value---zero."----Voltaire
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I think they are making the holders tighter...

https://news.coinupdate.com/new-pcgs-holder-highlights-security/
My Saint Set
Of course they are, when they agree with the grade.
A 100 year old coin sounds impressive until you realize it could be a 1921. There are a lot of blast white silver dollars, commemorative half dollars and Liberty Standing quarters.
@DisneyFan - Through better chemistry, yes.
Dave
There might be something on the coin that later makes it turn that no one knows about until it actually does turn. This is a real hazard with pre 1815 copper, and can happen to anyone.
Years ago, I bought an MS 65 BN 1804 1/2 Cent (think it was the variety with stems, the type coin) from one of the most reputable dealers in the business, and put into my safe deposit box, enclosed in an intercept shield box. 8 years later, it developed a PVC spot on the date.
A copper specialist helped me unload it. I now will never buy an expensive pre 1815 copper unless it is in a PCGS OGH or older.
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