That's the end result. What was the coin exposed to that caused the copper sulfate to form? Was it sulfur?
When I was experimenting with toning/cleaning, I bought some raw sulphur fertilizer and placed it in a baggie with some Lincolns. It had the same outcome as yours, just not quite blue like yours.
I once left some metal detected coins in Blue dish soap and water. I forgot about it and a few weeks later after evaporation my coins looked like yours.
Well...if any one here finds blue coins while coin roll hunting in Southern California...you'll know where they came from!
And after all of the oil is pressed out, they grind the rest of the smurf into smurf powder. None of the smurf ever goes to waste.
Copper sulphate is blue, the most common sulphuric acid round the home is battery acid, possibly a bit of spilled battery acid. Copper carbonate is green,this accounts for the green tone of old copper roof coverings.
Let's see copper is green and green and yellow makes blue so something yellow would work. Or you could ask the Blue man group.
Whatever the color of toning may be, it's always caused by the same different chemicals. Chemical #1 may cause blue, #2 red, #3 green, etc etc. But all of the chemicals may originate from many different sources. And the most common source is the very air you breathe. And of course the air always, stress always, contains a widely varied mixture of different chemicals. So the answer to your question is - exposure to chemicals. And it doesn't matter if that exposure happens over a long time or it happens in an hour, the time element is merely determined by the concentration of the chemicals.
