A list of ways to clean copper for enameling.
Rinse the piece of copper under your sink, any areas where the water is repelled or the water beads up, is not clean.
Use emery paper or a clean green kitchen scrub sponge to scrub the surface while holding the copper under your sink.
Do not use steel wool since some of the fibers may contaminate the enamel later and cause pitting.
Scrub the copper with pumice powder or a powdered detergent using a toothbrush, scrub pad or sponge. The detergent must not contain a hand softener which will unfortunately put grease on the copper and contaminate it. Rinse well under running water to clean.
You can use a pickle solution like sodium bisulphate or diluted sulphuric acid in a well ventilated area. Dilute approximately four parts water to one part acid. Heat the copper surface with a torch or in a kiln so that the surface looks slightly green, then place in pickle solution for a few minutes until clean, and rinse under running water.
Copper can be dipped into a dilute solution of nitric acid, one part acid and six parts water. Rinse the copper in baking soda and water then rinse in clean running water. Do not do this with soldered pieces since nitric acid will weaken the solder joints.
Sprinkle table salt on copper, wet with vinegar and polish with a clean kitchen scrub pad. Or soak copper in one part table salt to eight parts vinegar.
Use commercial copper cleaner.