Hugo_YIts a patina, it does not affect the structural integrity of the metal, it is only surface deep, but otherwise, it occurs in the same manner as rust, but a patina wont leave any residue on your hands. You can also artificially induce a patina, and give it some cool pattern, such as the hard boiled egg method, which uses the acids and ammonia in an egg to produce a cool patterned patina.