Just a guess.....

You might go to your local pharmacy and purchase some powdered ascorbic acid (vitamin C, like oranges). Use this as a mild acid to clean the piece. You can start out very dilute and see how it goes... (you mix the powder with water to your desired strength). If you start with a very diluted solution (and considerable patience), you should not damage the metal at all. Then you can see what you might have.....perhaps.
GOOD LUCK,
Bruce (Junkmanme)

Thanks junkmanme; is this a scrub or a soaking solution?

You may be dealing with either a brass or a bronze. Either one can be cleaned by the method junkmanme suggested. If you want the metal to shine when done you can polish it with "Brasso" a commercial brass polish found at most hardware stores. I would recommend that you coat the polished metal with a clear gloss laquer to prevent the metal from tarnishing again.

Thanks