How does this damage occur?

I found both of these in two different boxes of rolled cents. Just wondering how damage like this occurs?

 

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Someone filed down the reverse, and a little bit of the obverse, leaving just the zinc and no copper plating on the high points. Then they used some sort of acid to etch the zinc. When zinc reacts with an acid it turns black resulting in the black etched in design.

 

Ohhhh. Would have never thought that. Lots of creative minds out there. Thanks

 

Not creative, destructive.

 

And lots of boredom as well....

It's good you're thinking about such things though as this, along with the approach you're developing, will only benefit you in the long run.

 

Unfortunately, you're completely correct (although rickmp is too ).

 

I got bored and filed down a zinc cent and poured lemon juice on it. The lemon juice did that exact same affect as the one you have That's how I know lol! I am always going to be a 13 year old on the inside!

 

Tounge in cheek humor-mindless creativity actually, and very confusing for newbies. Send them to the corner with a dunce cap.