Color, light exposure effects

Also, statues with copper skins are particularly prone to the effects of oxidation-reduction processes, and good examples are the Statue of Liberty or roofs of old buildings. While these would have had a characteristic rich copper-brown coloration, enviromnental exposure caused oxidation of the copper and other constituent metals, changing their appearance to a light-green coloration with additional corrosion. 

Stability. In order to have maximum effectiveness over long periods of time, an antioxidant should be stable upon exposure to heat, light, oxygen, water, etc. Many antioxidants, especially in the presence of an impurity when exposed to light and oxygen, are subject to oxidation reactions with the development of colored species. Alkylated diphenyl amines are least susceptible and the -phenylenediamine derivatives the most susceptible to direct oxidation. 

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