Scraps from the web on tarnishing brass, this is what I found with a quick search. Nothing here has been tested.

1. Choose a glass container with lid of sufficient size to contain the item requiring aging. Find some cotton wool and soak in ammonia and place in bottom of jar. Suspend brass item in jar and attach top. Brass item will tarnish fairly rapidly say 20 minutes depending on room temperature. Remove from time to time to check amount of aging required, then remove and neutralize ammonia with clean water.

2. You will think i am a freak for this, but bury it in the kitty litter box for a couple of days. Then wash well.(!) It will give a really interesting oxidized effect. (obviously i have way too much time on my hands)

3. You can repatina brass and bronze depending on the look you are after, simply, and at little expense. I use a closed container, bucket & lid, or whatever comes closest to fitting around the brass item. Polish and clean off any hand prints with paint thinner, wear gloves at this point. Locate a small jar and lid, place in this 1/8 cup of ammonia. Do this out doors with a breeze at your back. Masks and gogles are a bennefit with ammonia, but no mask will seperate amonia from the air you are breathing, so be warned. Next put the brass item in the container, put the small jar inside, remove the lid from the jar and cover the bucket. A clear container works well here, or else look inside from time to time to see how things are doing. Goggles help the peeking. Keep track of the time if you wish to repeat, and replace the ammonia with the same amount of fresh ammonia each batch. Next I take a polishing cloth, clean and very soft, buff the edges for highlights, spray with brass lacquer, or spray shellac. Some hobby shops are selling metal ageing chemicels that can give assorted colors, so if the ammonia isn’t the darkening you want, try them too.

4. Salt and Vinegar