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Squirt a dollop of ketchup onto a soft cloth. Rub the surface of the brass with the ketchup. Wipe off the residue. Buff until dry and shiny.

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A lot can go a long way, so you only need about 1 to 2 tsp (5 to10 ml) of ketchup, at most.  Use a soft cotton or terrycloth rag for best results. Tomato juice is a mild acid, so the use of tomato-based products can help dissolve debris causing tarnish and dullness on your brass. Ketchup is your best option since it is so thick, but in the absence of ketchup, you could also try tomato paste or tomato juice. Wipe the sides of the brass object down with the ketchup-covered cloth, coating all sides in the ketchup. For best results, rub the ketchup on in a single direction instead of rubbing it on in a back-and-forth motion or a circular motion. Use a damp cloth to wipe off the ketchup after allowing the tomato substance to sit on the surface for a couple of minutes. You could also rinse the brass under running water to get the ketchup off, but using a damp cloth provides the added benefit of a little extra buffing. Use a dry, soft cloth to dry off any remaining moisture while giving the brass a final shine. Thoroughly buff the entire surface by passing over it in small, circular motions.