Have you ever seen shiny new brass on an old piece of furniture? It just doesn’t look right. What I do is tarnish the new brass to give it an antique look that better suits the furniture it’s attached to. Pour a little ammonia in the bottom of a glass casserole dish. Place a brick, or similar object, in the bottom of the dish to elevate the brass above the ammonia. Put the brass piece on top of the brick and cover the container to keep it airtight. Ammonia vapors are harmful so do this in a well-ventilated room. The process takes only a few minutes; the amount of tarnish on the brass is regulated by the amount of time the brass is exposed to the vapors.
– Frank Neimeyer
Gun blueing (the cold type like Birchwood Casey) does the same thing and works instantly. It’s also not too dangerous to handle. Small bottles are available at gun stores.