These base-metal late Romano-Egyptian tetradrachms are very prone to corrosion. They aren't made of regular coinage bronze, there's usually high levels of lead, iron, sand or whatever else the mint happened to have lying around. They don't always respond favourably to treatment for bronze disease.
My advice would be to watch it and see if it actually gets worse, and only treat it if it does.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis
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