Removal of Artificial (dark/black) patina from antique belt buckle

Q. I have an antique Civil War belt buckle whose authenticity has been validated by some and questioned by others. The cause of the differing opinions has to do with the lead back on the buckle. It appears as though someone wanted to "enhance" the buckle by applying an artificial patina which darkened the lead on the buckle beyond the norm. I was told by one individual that he could smell phosphate on the buckle. My sense of smell is not all that good. I want to remove the artificial patina without damaging this antiquity. Any suggestions would be appreciated. I would need to avoid a total immersion process since the buckle face has been validated as proper.