Since copper is not very air-resistant, copper is initially lightly coated with an initially very thin layer of copper oxide, which shimmers faintly in rainbow colors, but soon turns into a light brown.
Over time, especially in humid air, the color becomes darker and darker to a deep brown. The coloring occurs more quickly and becomes dark brown to almost black when the air contains a lot of hydrogen sulfide. But if there is a lot of carbon dioxide in the air, it combines with the copper oxide that has already formed and forms a green coating on the copper, which is called patina.
(cf. F.Michael; Source name: Metallniederschläge und Metallverfärbungen, year: 1927)
You may know it from the Statue of Liberty. The Statue of Liberty is a good example for natural oxidation of copper.
Patina is a covering layer on copper and copper alloys that forms in the atmosphere. It is weather-resistant, firmly adhesive and non-toxic.
The term verdigris is incorrect. Verdigris is only created through the chemical reaction of copper materials with acetic acid and is a mixture of basic copper(II) acetates. In contrast to copper salts, which form the natural patina protective layer, verdigris is water-soluble and has a piercing green color.
So you don’t need to be alarmed that it is poisonous verdigris as long as you don’t work with acetic acid.
(cf. Auto:Deutsches Kupferinstitut Auskunfts- und Beratungsstelle für die Verwendung von Kupfer und Kupferlegierungen; Source name: Chemische Färbungen von Kupfer und Kupferlegierungen; year: 2010)
If you notice unusual stains or discoloration as a first-time user, don’t worry—this is a natural process caused by the oxidation of copper. While you can clean copper to restore its shine, oxidation is inevitable and simply part of its natural beauty.
