I found this 1972-D half with a very unusual edge. At first glance I thought it was just grove worn into the edge but it's still reeded. I thought maybe a partial collar error but the photos I've seen of those are not reeded where there was no collar contact. I'd really like to know what would cause this ???
You coin was exposed to an acidic bath. The copper core, being much softer than the cupronickel outer layers, dissolves into the acid much faster than those outer layers.
Kind of reminds me of something known as "railroading". I'm not saying this is, but there is a general appearance of it. The outer silver edge would have to be on the other side as well.
I thought it might have been caused by acid too, the core layer looks pitted and filled with a black residue between the reeds, but I got stuck on why the obverse edge is so nicely rounded where it meets the core. ?? I also have another coin that seems to have been exposed to acid but it was the outer layers not the core that was affected:
Try searching railroad rim. Otherwise go to: Jim's Coins and Stamps in Madison Wisconsin. Click on the heading: Collecting "Mistakes". In the glossary see "Partial Collar".
