I use this on my copper bottomed cooking pots and pans. Works like a charm. And it's cheap to use.

Just smear the ketchup on the copper, let it sit a few minutes. Then use a scrubber sponge and rub ketchup into the copper. Wash with mild soapy water, then rinse clean with water. If pots are heavy tarnished, you might have to do this a couple of times, but it will work. I do my pots once a week or every 2 weeks as needed when I'm doing the dishes. Keeps my pots looking new.