I heard you should use vinegar but have never tried it. I just paint mine.

Vinegar was the first thing that came to mind when I read your question, same as PF. I would try it on the bottom first, just in case.

However, I googled it and have enclosed the site that says to use ammonia fumes. Hope it works. Several other sites in the lists also had ammonia in their short statements.

http://www.baynews9.com/content/24/2007/2/15/222101.html?title=Aging+Brass

Claudia