It’s both a secret and the best shared tidbit among copper collectors and newbies to the metal: ketchup cleans copper.
I say this with a caveat. Ketchup doesn’t clean blackened copper, burned and oxidized dark with use over the fire for years, or scorched copper bottoms that haven’t been polished in decades. Ketchup polishes relatively clean copper, or copper with the early deep bronzing of use over a gas or wood stovetop. It removes that first bit of oxidation and removes the blush of the developing patina with the oxalic acid within the ketchup itself. Citric acid plays a part to some degree, but the oxalic acid does the real trick.
The property of oxalic acid in rhubarb working its magic on copper was also discovered by Bob (the master tinsmith) at a reenactment when he put a spoon covered in rhubarb sauce in a copper pan. A while later, he removed the spoon to find a bright oval of clean copper under the spoon. That oval stayed there a long time!
If you find that ketchup alone doesn’t completely do the trick, you can make a paste of ketchup with some vinegar and flour added to create a thicker polish. Leave it sit on the copper until it starts to get tacky, and then wash it off. Don’t use salt unless you are okay with the light etching that is likely to occur (unless you get the super fancy salts or super fine French salt…that’s less likely to leave a mark).
The beauty of ketchup, rhubarb, vinegar, flour, and even a potato, is that they are all-natural and even organic cleaning tools so you don’t have to worry about keeping these materials away from a tin lining. If you do decide to use a chemical cleaner, be careful to keep them away from the interior lining: it can etch, discolor or even strip a few layers off of the interior metal due to the abrasiveness and chemical compounds themselves.
What if you do have a vintage piece or something very old that is blackened by time and use? Well, invest in some power tools and acid and start cleaning! Actually, you can send to a restorer who can clean up the deep oxidation which have bonded to the copper. It all comes off, but it needs more than elbow grease at some point.
But if you have something moderately in good shape, and it’s obvious it’s a copper pot, you can probably make a good dent in shining it up with nothing but the ketchup in your refrigerator! Now you know the secret, too!