Suspend the pieces in a jar above an inch or so of vinegar, not in the vinegar, it is the gas off that will turn the brass. I make a few air holes in the lid, suspension wire through lid and let the piece hang, check every couple days, Ammonia is faster, but if you let it go too long, it won't be pleasant.... Lightly wipe the piece with 3x steel wool and let it go again. It will look more natural (and it is) than staining. This works, I have used it for many years. Just be sure to remove any "finish" that may have been applied to the brass.