Cleaning and Preservation of Coins - A Complete Guide, page 12
CHEMICAL COIN CLEANING with HYDROGEN PEROXIDE
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Coin's Surface After Being Immersed for 2 Hours
Coin's Surface After Being Immersed for 6 Hours
This treatment also has a chemical effect on coins: not only the hydrogen peroxide react with the metal, but also accelerates the action of other impurities that can attack the coin metal.
This is how the coin looks like after the treatment; the upper neon-green area on the coin is the area that was not immersed into the solution.
Coin's Surface After Being Immersed for 12 Hours
Coin's Appearance After Mineral Oil Was Applied to the Surface
Coin Before and After Cleaning