I wonder if this was done intentionally or was just the result of the minting process.
From my page on Metallurgy
http://www.constantinethegreatcoins.com/metallurgy/"For flans with more than 5% silver, cold hammering followed by annealing resulted in lead and silver being forced to the surface. A dilute acid bath would give the flan a silvery surface. For flans that contained less than 5% silver, a bath in molten silver chloride displaced silver and deposited it on the surface, which gave the flan a silvery wash. Hot working and blanching prior to hot striking also enriched the surface silver content."
photo from Beast Coins
