Answers

2015-06-12T16:30:02+05:30
The copper when oxidises aquires a dull green coating.
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2015-06-12T16:31:21+05:30
Just like rust develops on iron, patina develops on copper when the copper sulphate on the surface interacts with oxygen in the environment.  Importantly, unlike rust which corrodes iron, patina actually protects and preserves the copper underneath.  
   Hope you understand the reason of coating of dull green of copper statue after a couple of month
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