Jack P. writes: The nickel in my sealed 1975 proof set has normal obverse, but the reverse is a flat dark grey with dark grey with slight brightness around 40% of the very edge. On the center, Monticello almost appears to have some coppery redness in the grey. I had sent the set to Tom DeLorey at Coin World in March of 1976 and he advised me not to break open the set. Any ideas?
Nickels are made of an alloy of 75% copper and 25% nickel. When the planchets (coin blanks) are made, sometimes the alloy isn’t mixed correctly. It wouldn’t be impossible for some of the copper to have come to the surface as the metal mixture cooled. This problem occurred quite often on U.S. gold coins. Copper and gold alloy are even harder to uniformly mix so that the copper color is hidden (U.S. gold coins after 1834 are 10% copper).
The grey toning you mention is oxidation caused by contact with some chemical agent. The 1975 sets are not very well sealed allowing air and other pollutants to come in contact with the proof surfaces of the coins.