I know this would be extremely rare, but I don't know what else would cause this. I have a coin with three identical raised bumps on the obverse. All I can think of is a Rockwell Hardness Test on the die. http://conecaonline.org/content/RockwellHardnessTest.html Nothing found on the reverse What's everyone's thoughts? Any idea what caused this?
I don't see from the linked article how we'd get three dots on one test piece. On the other hand, I do see lots of plating bubbles on Zincolns. It's hard to be sure, but my inclination here is "if you hear hoofbeats on the track, look for horses, not zebras."
I read somewhere that to get an accurate test they should do at least two places. At least two means it could be three if they felt they needed to do another one.
A Rockwell test would have been performed on the Planchet, therefore the test pieces would show a dimple not a plating blister. http://www.error-ref.com/?s=Rockwell It says there are no known examples on us coins. I would bet that if you pushed in the dome shaped bubbles they would collapse, and not be solid underneath.
It's very likely it is bubbles, and as jeffB said, I'm looking for zebra's. But I guess if people didn't look for zebra's a lot of coins would never be found. According to the link I provided, it say's that Rockwell test are also performed on dies and master hubs.
Agreed The reference you linked was actually reffering to the same work by @mikediamond as I referenced.
They are unusual looking, but the size and resolution of your photos is inadequate to make a diagnosis. Usually solid corrosion domes are surrounded by other signs of subsurface corrosion. Usually hollow plating blisters are surrounded by other blisters or areas of plating disturbance. However, a Rockwell test mark in a die will be made in the field, not within a design recess. You end up with artificially high values if you place the tip of a tester in a die recess. If you'd like to send it to me for a closer examination, I will oblige.
Thanks for the offer Mike. I looked at it again using a 20x loupe. It does show some signs of mild corrosion around the dome.
