Here is a photo tutorial of how to antique brass using ammonia.
I have used a small ingot of brass for demonstration purposes. The container is a drinking glass. The bottom is filled with ammonia.
I placed a pedestal with the glass and placed the brass ingot on top of the pedestal. To contain the ammonia fumes, the glass is covered in plastic wrap that is air tight. Here is a picture of the setup that I used.
Here is a closeup of the brass ingot on top of the pedestal:
Here is a photo of the brass after an hour in the ammonia fumes. It has taken on a dark appearance.
Here is another view.
Here are before and after photos of the brass after about six hours in the ammonia:
The brass original on top has taken a silvery tone.