or how else would you go about making copper oxide?
Letting copper oxidize on its own is slow, and while the nitric acid method will work, nitric acid is harder to obtain than cupric oxide (I assume this is the oxide that you want, CuO, a black powder, rather than Cu2O, which is red/brown and called cuprous oxide). Furthermore, copper patinas are not copper oxides; they are hydroxides/sulfates/carbonates/chlorides (as pisgahchemist's website mentions). Also, copper(II) hydroxide will not decompose to the oxide on its own; it must be heated: Cu(OH)2 ----> CuO + H2O The water will be driven off as a gas and the light blue hydroxide will turn to the black oxide. The easiest method is to buy it: check pottery supply stores, as they stock both CuO and Cu2O for coloring glazes. The copper(II) hydroxide can be made by electrolysis: use copper as the anode so that it will be oxidized. Salt (NaCl) would make a good electrolyte, as the chlorine that is released at the anode will speed the oxidation of the copper. Let the electrolysis run for a few hours, collect the copper hydroxide, and clean the copper electrode. To produce copper(II) oxide, heat the copper hydroxide until it turns black as above.
Or you can produce copper oxide from copper metal. Copper (II) oxide is one of the products in the copper cycle series of reactions. Dissolve copper metal in nitric acid, then neutralize with sodium hydroxide. If you're not careful it will form copper oxide on the spot if it gets too hot. Allowing the copper (II) hydroxide to sit overnite will allow it to decompose to black copper oxide.
