What causes silver discoloration on Lincolns?

dpoole

When swatches of silver discoloration appear across the face of an MS Lincoln, is that the precursor to the coin turning brown? Any information on causes/thoughts on preventing it or slowing it down?

Coulport

Never seen "swatches" of silver on Lincolns. However, I've seen proofs cents with a silver sheen where someone has dipped it in Jeweluster that had been used for silver coins. It leaves a thin plating of silver over the entire surface of the coin.

dpoole

An example is this 1938 Lincoln proof , in a green PCGS holder. The discoloration has occurred only recently. I've seen examples on other Lincolns as well.

The discoloration occurs across Lincoln's face and into the fileds, from 8 o'clock to 2 o'clock, on the shoulder/coat and right at the crown.