"Bronze disease" is one of the most serious hazards of bronze sculpture. This disease, caused when chlorides and oxygen combine in a damp environment, also attacks brass and pewter. The disease takes the form of a sudden outbreak of small patches of corrosion and is distinguished by rough, light green spots.
I have found that merely rubbing virgin olive oil on a bronze can cheaply restore patina and remove the green. Below is an example of a bronze I bought, before and after:
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bronze disease.jpg
And after a olive oil rub:
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bronze disease2.jpg