Copper’s outer surface begins to oxidize as soon as it meets air; it first becomes dull, then turns brown and dark. In a damp environment, the process can continue until a greenish layer forms. Polish it with acid rather than abrasion: lemon and salt, or a paste of vinegar, flour, and salt, dissolves the dark layer. Rinsing and drying are essential afterward.
Copper is a reactive metal that reacts with oxygen in the air, and this reaction never stops. A new pot’s rosy shine becomes dull in the first few days; a thin layer of copper oxide forms on the surface and no longer reflects light as it once did. As the process continues, the color turns brown and then almost black. Everything up to this point is superficial and does not damage the metal. A tarnished copper pot retains its function completely; only its appearance changes, so polishing is a matter of preference rather than maintenance.
Moisture changes the color of the process. On copper left wet or stored in an airtight cabinet, oxidation advances another step and a bluish-green, powdery layer appears on the surface. Unlike the layers before it, this one is not harmless; it adheres loosely to the surface, leaves a mark on your fingers when wiped by hand, and should not be present anywhere that can touch food. Clean it as soon as it appears and reconsider where the pot is stored. The least expensive way to slow tarnishing is to dry the pot completely after every wash. That is why tarnishing is not a lack of care, but the metal’s own behavior.
The idea behind polishing is to dissolve, not abrade. Copper oxide dissolves in acid, so two ingredients already found in the kitchen are enough. Sprinkle coarse salt on the cut side of half a lemon and rub the outside of the pot with the lemon; the second method is to mix vinegar, flour, and salt into a thick paste, spread it on the surface, and let it sit for a few minutes. The flour helps the paste stay on the surface, the salt adds mild abrasion, and the acid does the work. The color lightens before your eyes and the rosy copper returns.
The step most often skipped comes at the end. Acid and salt left on the surface tarnish the cleaned copper again, and faster; that is why the pot is rinsed thoroughly with plenty of water and dried immediately. A newly polished pot left damp returns to its old state within a few days, and the user may think the method did not work. Commercial copper polishes work on the same principle; if you use one, do not skip the rinsing directions on its label. Polish a frequently used pot infrequently, because each round removes some metal.
One distinction remains: decorative copper and cookware are not the same thing. On cookware, polish only the outside surface; the inside is lined with tin, and neither polish nor an abrasive should go there. Tarnishing on the inside is a sign of a different problem—the tin wearing away—and is unrelated to polishing. Decorative copper may have a protective varnish; rubbing an acid onto a varnished surface stains the varnish, and that stain will not come out.