To make a nice patina mat finish to aluminum you can use what we call "Lye" or sodium hydroxide (NaOH, also known as 'caustic soda') or "Draino". Mix with a generous amount of water, clean the aluminum well and immerse in the solution. Rinse with lots of water.
BE VERY CAREFUL WITH THE STUFF. IT WILL BURN HOLES IN ALUMINUM, EVERYTHING ELSE AND YOU. USE RUBBER GLOVES, MASK ETC.
I don't know what will make the same patina on brass.