green patches on statute

Firstly, statues are usually made of bronze, which is an alloy of copper and tin. The copper in the bronze would be corroded to form patina (銅錄), which is a green compound with formula CuCO3•Cu(OH)2.

Secondly, if the statue is made of iron or steel, rust (Fe2O3•xH2O)­ would be formed which is a brown substance, but not green.

Thirdly, iron(III) ions in aqueous solution and its oxide are brown, but not green.
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