Commercially prepared chemical metal darkeners are available, but you can also make metal blackener at home that will be effective on brass, bronze and copper. There are many variations on recipes to make metal patinas. Italian artists and craftsmen used a similar combination centuries ago to create works of art and for other decorative uses. Several coats of the patina will produce a near-black finish. An oil-rubbed look can also be achieved with this patina simply by applying less to the edges and seams of the metal.
Pour the ferric chloride and ferric nitrate into the bowl. Slowly pour in the water and mix well with a wooden or plastic spoon.
Dip a sponge into the mixture and apply evenly across the surface of the metal. The metal will begin to turn a reddish brown.
Rinse the metal with running water and wipe clean with a slightly damp newspaper. Burnish the metal with steelwool, making sure to wipe away any shavings or debris.
Allow the piece to sit for 12 to 24 hours. A brown patina will develop. Repeat the process until the desired darkness is achieved.