The mixture of ferric and cupric nitrates was used back in the day for “browning” a gun. It is technically known as plum finish or rust finish. You can also use 2:1 ratio of NaOH and Ammonium nitrate fertilizer to make a blue/black finish. My recipe is 5lbs of NaOH and 2.5 pounds of Ammonium nitrate in one gallon of water. You heat it up to boiling and dip the piece in.