This is what a small cut can do to a proof set.

ajaan

1959 Proof set. Small cut in the film in the dime slot. I assume the dime was right under the cut at one point.

privatecoin

Lucky number 7.

Sadly, it's unattractive black toning.

Maywood

Point taken, but many of those old sets have holes/cracks and the coins remain unblemished. I think it really depends on storage as much as anything. I used to store the sets inside baggies.

coastaljerseyguy

So why do some proof mint sets turn a beautiful blue/red/yellow toning while some turn black? Why does adding air or maybe contaminants in the air cause the set toning to interact differently.

LJenkins11

Partial sideburns designation for sure.

Baylor8670

E-Z-Est to the rescue!

291fifth

Ultra-rare "7" toning. Start the promotion!

coinbuf

Same thing happens with slabs.

PerryHall

Same thing with those 2"X2" cardboard fold and staple coin holders. Over a period of time, a small pinhole in the window frequently results in an unsightly black toning spot on the coin at that location.

MsMorrisine

@PerryHall said:
Same thing with those 2"X2" cardboard fold and staple coin holders. Over a period of time, a small pinhole in the window frequently results in an unsightly black toning spot on the coin at that location.

even on clad

Maywood

So why do some proof mint sets turn a beautiful blue/red/yellow toning while some turn black?

That is due to a combination of rinse residue left on the coins and the composition of the packaging.

Franklin_Fan

I opened a bunch of 1963 sealed envelope proof sets, and some of them had the glue or whatever seals the individual coins compartments wear off and the coins were laying together in the cellophane. So the cool thing is the penny being copper toned the other coins beautifully. So I have franklins, quarters, dimes and nickels nicely toned. it was about 5 to 10 sets that were like that, and they all came from a batch of 25 that I bought from the same seller.