First of all, I absolutely recommend that you do NOT try this. If you do try it, only do so in a well ventilated area, and wear face, eye and hand protection. Ammonia is nasty stuff.

....but it so happened that I had a bottle of it lying around, and I'd read somewhere that you can age brass by exposing it to the vapours. I was restoring a Swan/Hecton engine, and I needed to make a new boiler band. The shiny brass stood out like a sore thumb compared to the nice mellow boiler. So I suspended it over a little ammonia in a sealed takeaway container. After a couple of hours it had darkened nicely, but was slightly greenish. Gently heating it with a gas torch turned the greenish tinge into a beautiful rich brown.

I'll say it again: ammonia is nasty stuff. It's corrosive, poisonous and it stinks. The fumes will burn your lungs. Handle with care. But it does age brass very nicely!