What is the coating inside this brass compote/goblet?
I purchased these two compotes (or goblets, if I want to stretch some boundaries) at local antique shops here in Savannah. The one on the left has an interior coating that mystifies me. What is it? The one on the right is clearly not coated. In case anyone is wondering, I purchased the one on the left for only $10 at one shop, and the non-darkened one on the right for $20 at another. The dealer who sold me the one on the left doesn't know what the coating is, and I can't find anyone else who does either. I am posting a second photo of a close-up of the area in a seperate message as this forum does not seem to allow more than one picture per post.