How do I refurbish a plaster frame on a mirror?

I have a large mirror (100+ years old) that has a dull, dingy discolored plaster frame. I'd like to clean it and apply gold leaf. What is the best way to clean it so that the gold leaf will stick, but not damage the plaster which is otherwise in remarkably good shape? Also, what kind of adhesive would be best? Comments and suggestions are appreciated.

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