Posted 03/01/2023 10:02 pm
I have often seen this effect on high-grade Indian aluminium coins: streaks and lines of "bubbles" or "blisters". I believe it's actually a form of corrosion, not entirely unlike the "zinc bubbles" you can see on modern US cents. In this case, corrosion develops underneath a layer of passivated aluminium oxide on the surface of the coin. The corrosion then propagates along microscopic cracks and flaws in the metal that run in the direction the metal was originally rolled in.
This, at least, is what I have always assumed whenever I see it.
This, at least, is what I have always assumed whenever I see it.
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
