Junk Barber 1911 S Quarter. What is this gray stuff?

Picked up this as a junk silver piece. Not bad condition but what is this gray stuff on the obverse? Coin smells very musty, may drop it in some acetone. I'd give it an F12?
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Picked up this as a junk silver piece. Not bad condition but what is this gray stuff on the obverse? Coin smells very musty, may drop it in some acetone. I'd give it an F12?
Comments
I detect corrosion on the obverse.
Hit it with acetone just to be sure.
Technically fine 15 details.
Just another one of those many under appreciated dates in that series.
That was on my mind unfortunately. I will give acetone a shot and see!