I use ammonia to age brass. I will admit to once, in the dim an distant past, leaving the brass exposed to the ammonia fumes for far too long, and ending up with rammer pipes that I could literally crumble just by squeezing them. Yes, it really happened! I am very careful now.
JMHO, but I personally believe that nothing will *realistically* age brass like ammonia fumes. Sure, lot's of different things will darken or otherwise change the color of brass. However, t's been my experience that genuinely old brass looks different, even if someone tries to clean it a bit. There seems to be a compositional change to the material itself over time and exposure to (I assume) atmospheric elements. I don't know, I'm not a metallurgist or a chemist. I just know that ammonia fumes will create a color and appearance that even when rubbed back, still maintains an 'old' color. Most applied chemicals will surely color the brass, but rub on it a little bit and it looks new again.
I will reinforce the wisdom that if you use the ammonia method, you better keep an eye on it and remove it about two shades lighter than what you want because it will actually darken a bit more once removed from the humid atmosphere. I use a variety of sealed plastic containers, and alternately apply gentle heat (with a drop light aimed at the container) or remove from heat to allow condensation.