A recent example from the NCS conservation laboratory.
Proof coins are specially made for collecting. That special preparation doesn’t always translate to special storage. Over 100 years of improper storage took their toll on this proof 1887 Liberty Nickel. Unattractive brown streaky residues on the obverse and heavy hazing residues on the reverse obscured the beauty of the surfaces underneath.
Delicate conservation work was able to remove the tough brown residues on the obverse and the hazing residues on the reverse. Once again the original surface of this Liberty Nickel was revealed. This coin was able to grade well with NGC following the conservation work.