I have a few questions about patina, or toning on coins. I have recently got my hands on some BU Cupronickel coins, and was wondering (they are slabbed but not air-tight) how they might develop? I could obviously wait a few years, but I'm still quite curious (I can always post a before/after if anyone is interested). Same question for copper coins. I've noticed, some Lincoln pennies from the 1960's (where it was 95% copper rather than today's plated zinc) seem to go two ways: either chocolate coloured, or dark red, sometimes with verdigris. What determines which colour it goes? Specific chemicals, or is it just random?
The brown (and sometimes blackish) toning is caused by cupric oxide (cuprous oxide), while the verdigris is from exposure to copper sulfate and a green/blue by copper carbonate. How a coin is stored (e.g., cardboard flips, folders, etc.), how it may be handled, and exposure to light, humidity, and the chemicals given off by nearby objects, air sprays, etc. are all determining factors.
Coins begin toning the instant they are struck because they are exposed to the air. So in simplest terms all toning depends on what is in the air. A lot of folks don't think about it but that simple phrase, what is in the air, is extremely complicated. I say it is complicated because literally everything puts something into the air. From the carpet on your floors to the paint on your ceiling and everything in between, it all matters and contributes to the toning of coins. And it not just in your home, it's everyplace else as well because air from one place mixes with other air, and that in turn mixes with other air, etc etc. So wherever the coin is and wherever it has been over the course of its life - all of it affects and determines how that specific coin will tone.
And to add what Doug just stated.... have you ever wondered why in your safe some of your coins show signs of toning as others remain blast white? They all are in the same storage place yet one or two are toned. So many things can effect the process some more than others. Another thing I find certain years mint products or sets seem to tone more than others. Who knows how or why. Other then the items they been exposed to.
As long as there's oxygen, things are gonna react. That's why they call it oxidizing. Chemical reactions don't need a chemist to happen, and the atmosphere - more or less in different decades and different places - can have a wonderfully odd mixture of chemical content. You ever notice how Seated coinage is so often found darkly toned? I've the feeling that was to an extent due to the very industrial, sulfur-laden atmosphere in which many circulated back when we were a coal-burning country. Yet, Morgan Dollars kept locked away in bags - even though held in polluted cities - developed magnificent rainbow toning simply because the reaction was slowed down.
