2007 Silver Proof Set Lincoln Confusion ?

Not zinc rot...yet. Split copper plating.
John1

Split plating, yes. Should be able to mitigate potential zinc issues by keeping the coin in a low humidity, air-tight environment. Mint packaging isn't necessarily air-tight. Is that a discoloration 'spot' on the memorial building?

Agree, spilt plating, zinc rot soon to follow (there is no stopping it, sorry).

Jin put the point. Those slab even air tide do not mean vacuum. Mean when they close, they close with air inside. The air contain H, O, C, and in California iodine and sulph. Inside of the slab different temperature will produce ionization and oxidation, mean any small default, microscopic also will be the seat for those irreversible oxidations.

The solution is a vacuum slab, but because of the cost the mint will never introduce. They sell perfect product. So you buy, you are responsible for the future. Like you buy a car: After you are in charge of the maintenance.

The spots on the coins, I presume to be an reaction of the iodine. Without invasive cleaning is no way to take out.

Sorry to complete this. Here in Canada we have with silver from mint the problem of yellow. Here I proved already is caused by the fact that they plating the dies with Ir (iridium), and his oxide is yellow. They will not change. Before they strike 120 K with a die, now go till 220K.