Don't use ammonia.
It does an excellent job of aging brass.
If left too long it will CRACK your brass. Especially brass that has been cold rolled or formed.
The Brits first learned this whilst bringing the Light of Western Civilization to the Indian subcontinent. With their .577 Snyders. By golly, the brass cartridge cases developed cracks during the monsoon season. So they called it "season cracking"
Monsoon means a lot of rain.
Gotta store those .577 cartridges under a roof somewhere.
Not the officers' quarters!
Ah, so they kept them in the horse barn.
Horse urine (like cat urine, mouse urine &c) has, or breaks down into, nitrogen compounds.
That crack brass.
A friend of mine who is now a retired gunmaker decided to age some brass thimbles, hanging them above some ammonia in a container. They cracked. He told me then he recalled that Kelly had said something about ammonia and brass.
Engineers have to re-learn this every 50 years or so. I have a nice old British book that warns against keeping your auto in the horse barn. Autos used to have nickel plated brass trim.
The National Association of Corrosion Engineers published an article on brass valve parts that cracked. Because of mice. This was maybe 10 or 20 years ago, mislaid article.
Black gunpowder fouling contains sulphur compounds that blacken brass very nicely. There are also about one Zillion commercial products and recipes for your own stuff.
Just don't use ammonia.
