I do not know if I am looking at (yet another) silver Antoninianus where the top layer of silver is peeling off or whether I am looking at a fourree. Can you help me please? (Ex J Eric Engstrom Collection, Ex CNG.)
I'm no expert but I think it may be a fouree. Silver don't usually turn green like that unless its plated. Maybe others who know more will chime in later.
I believe this is a little faulting in the outer surface where it was compressed in striking. The core metal was the same but not as compressed or the flan was pickled removing some base metal from the surface leaving a honeycomb that was pressed into a better silver layer on the outside. It looks nothing like a fourree to me.
So, you are saying it is like having shingles: you had chicken pox when you were younger and they did not pop out completely: (core metal was the same but not as compressed or the flan was pickled), so Shingles pops up later in your life! I get it!
I've had shingles and coins and miss the similarity. Oh, you mean when you get an itch for new coins???
Another way to check for a plated coin is to check the weight (really the SpGr, but weight is way easier). For RR denarius, this is a pretty good, but not absolute test. Find 10 +/- a few examples and calculate the average weight and standard deviation. If you are over a couple of standard deviations low, chances are you have a fouree.
Metal was the same, but an acidic treatment (lemon juice perhaps?) was aplied to the blank surface so it became "more silvered" when strike.
