The Beadsmith XL Gel, Liver of Sulfur Extended Life Gel, 1 Fluid Ounce (29ml) Bottle, Apply a Patina Finish to Silver, Copper, and Other Metal for Oxidized Look on Art and Jewelry Projects

They also make Aluminum Black, which I've used on some steel business cards and aluminum license plates. Seemed to work better than the Brass Black solution. Also, if you're wanting to do stuff like embossed coins, you'll have better luck applying a patina with "Liver of Sulfur", such as this: https://www.amazon.com/dp/B004PLZ4SS