If you have copper pots and they get brown oxidized you can easily clean them with copper cleaner, or use ketchup.

Here are the before and after Photos. Something like Coke-a-Cola which almost melts a penny but strips its surface of discoloration. Ketchup works the same. In fact there are numerous foods that can be used for cleansing; oats for dirty hands, or corn starch to clean grease spills on carpets. Pour cornstarch onto spots and let sit for 15 to 30 minutes before vacuuming. Then rubbing alcohol will help erase permanent-marker stains from finished wood floors or solid-surface counter tops.

But is Ketchup dangerous? I doubt it but too much of anything especially mass produced bottled sauces are not recommended. So what is it that cleans the copper? I guess the sodium and white vinegar that strips the tarnish.