I do not collect moderns but I share with you what may have contributed to the toning in this case.



Back in 2010 the Zumbro river experienced an once in '500 year' flood. The proof sets were submerged in 6' of water (sewage, farm run off, etc...) for hours. Are these coins bio-hazards? LOL. My parents gifted me the sets and I threw them in a box in my basement since then. The fine silt mud is still in every nook and cranny of some of the holders. Some of the proof sets were sealed well from the mint and survived (good/bad?).

All is well now with my parents, just wanted to show small extent of damage. This is what, environmental damage to the coin?