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There are a number of youtube videos where someone heats a US quarter (which is copper sandwiched between layers of 75% copper, 25% nickel) to red heat and cooling it in methanol. This results in a coin that appears to be "copper plated". I've replicated this effect at home with rubbing alcohol and a US nickel, which is a solid piece of 75% copper, 25% nickel. The apparent copper coating is somewhat fragile, and there is a layer of black underneath.

What reaction is occurring here? Is the outer coating copper, and what could the black layer be? I've looked for an explanation online, but I've only found wildly inaccurate speculation.

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