There is a lightly-abrasive cleaning powder called Barkeeper's Friend sold in hardware shops in the UK. You use it damp, like Vim or Ajax (it's nowhere near as violent as those). It doesn't produce as shiny a finish as Brasso, but gets the gunge off much more efficiently. Makes sense to use it first and then go over the object with Brasso. I've used it for Indian and Middle Eastern copper and brass trays that have been stained by cat pee.