I use the old antique shop trick I learned while working for a numismatic store (where polishing is a major NO-NO) Put it in a zip-lock bag with Heinz ketchup over night in the fridge. Next morning rinse and voila; done. To force patinate put in a zip-lock with two halved, hard boiled eggs over night. And rinse and... BOOM - Patinated... Ketchup and eggs works on copper, brass, bronze...
