1839 Bust Half question - Delamination or ???

I'm curious about this coin and I see there is a lot of expertise here. It was this way when I purchased it at an auction a long time ago. What is this type of damage called, and how might it have happened? Thanks!

 

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It almost looks like corrosion. Silver can and does corrode

 

Yes, a good theory. Perhaps someone or something deposited a concentrated, corrosive substance on the coin at some point. I know that sulfur is silver's enemy but I thought it usually caused only surface tarnish.

Is it possible something attacked the copper in the alloy?

 

https://www.cointalk.com/threads/toning-is-corrosion.319522/

 

Thanks for the link. That sulfur test is a real eye-opener.