Not that great intellectual question, but I have speculated for a long time: what exactly is the black materiel on these coins? They are certainly not dirt/earth on my Miliareson. They are a sort of black glue. I have marked them with red and white marks: The latter one is not mine, but another user’s. Source here: https://www.cointalk.com/threads/valentinianus-ii-solidus-387ad-constantinople-mint.276191/ So what is it? and why does it happen? Bad silver/gold at certain places that eventually got a little bit corroded?
Silver corrodes and the common black tarnish on silver is silver sulphide. Gold is a noble metal and doesn't corrode. Dirt will gather on gold however
As the owner of the gold above, I got it out and looked under a microscope. Prodding with a pin dislodged a bit of very dark brown and very dry dirt. I suspect I could clean it all off with soap and water but why???
IMO, you should never use anything metal to remove debris from a coin. A sharpened round toothpick would have been better. The microscope was a good choice! If you got lucky, there will not be any shiny scratches where the steel pin went through the dirt and scratched the soft gold. Since the coin is already cleaned, it might look better with out the dirt.
