The brass findings I buy in bulk generally come in raw brass form, so I've been wanting to oxidize them myself to give them an antiqued bronze look. I've searched in the Etsy Forums for instructions and figured I would try the ammonia fume method. This weekend was my weekend to experiment. It worked like a charm and is so easy! So here are the items and steps I used:
thread, Glad plastic container, clear ammonia
First I washed the findings with soap and water to get rid of any oily residue. Then, I strung the brass findings onto the thread.

Then I poured some ammonia into the plastic container and then held the threaded findings across the top. Making sure the thread is taut, I snapped the lid on. Remember, ammonia fumes are toxic, so be sure to do this in a well ventilated space.

After about 1/2 hour to an hour, I took the brass findings out of the container and let it air out a bit. The longer you leave it, the darker it turns. The bottom picture of the locket shows what happens when it touches the ammonia. It turns almost black with a green verdigris.

The only thing I have to do now is seal it. I read that I can just use spray lacquer which is available at your local hardware store. So after trying this, I think I put way too much ammonia in the container. It was a small one, so only a little bit is needed since those fumes are pretty powerful. So good luck and have fun!