Answer:
The chemical phenomenon responsible for the coatings is corrosion. Copper and silver metals in presence of moist air gets oxidized forming protective layer over them. Silver turns black due to the formation of silver sulphide \[(A{{g}_{2}}S)\] while copper turns green due to the formation of a mixture of copper carbonate and copper hydroxide \[[CuC{{O}_{3}}.\text{ }Cu\text{ }{{(OH)}_{2}}]\].
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