A patina is a green or brown layer of film that appears on bronze or other metal after being exposed to air for long periods of time. I had to look this word up and did not know it.
I think I have not ever owned anything of a bronze value that would necessitate cleaning a layer of patina. All of the material items I have ever had probably broke way before this chemical reaction could take place.
Learning about patina made me realize that if I do ever own any precious metals, I better make sure to maintain them.