Have you got an old copper kettle? You used it to warm tea, coffee or simply got dirty? You tried so far to clean it with usual methods? I have a tip. You can clean your copper kettle in 60 seconds using only 200-300 ml of white vinegar and a little salt, preferably iodine.
What do you have to do? Pour vinegar on the back of the pot then add salt to cover the vinegar layer, pour vinegar again to cover the salt layer and allow 10-20 seconds. Then pour vinegar on an ordinary sponge and start rubbing. The result is indeed formidable. You will have a new kettle.