Turning Copper Green
Q: How can I turn copper and brass items green?
A: Usually this is done with chemicals which oxidize the copper. First of all, the metal must be very clean. That means no traces of grease on the surface. To remove it, sandblasting is best, but detergent and pumice powder will work with lots of scrubbing—rinse well afterwards, and don’t touch the surface with your hands. Remove residual oxides with acid dips or a substitute such as Sparex (sodium bisulphate) in a hot solution, which is less caustic.It is necessary to get the copper surface chemically clean, with any pre-existing oxides scrubbed off.
The patina chemicals can be sprayed on or applied with a brush, but be careful to use one that doesn’t have a steel ferrule, because this will contaminate the patina. Never get any iron into this solution—use copper tongs to remove small pieces, or attach a copper wire to big ones. Working in a very well-ventilated area—use an electric fan or two to move air past the piece and not into your face, get the piece hot with a soft flame, so that water splashed on will sizzle off (but don’t get it too hot, or you will burn your green to brown.) When it is hot, spray on cupric nitrate solution (or a proprietary mixture of chemicals) with an airbrush or spray bottle. You can use a bristle brush, but they don’t like the hot metal and the hairs tend to curl up at the ends.
Build up layers of green with repeated applications. In a half hour, it should be very green. Other methods involve making a tent with plastic to enclose the piece along with open containers of chemicals (the fume patina technique) or burying the piece in chemically treated sawdust or earth (a technique devised to imitate the patina found on long-buried bronze artifacts.) All these chemicals are toxic to a greater or lesser degree, so make sure you have some positive ventilation going when you apply the chemicals, especially if you are heating the piece previous to application, which accelerates the effect. There are proprietary formulations of patina chemicals available, or you can mix your own.
For green, I use an aqueous solution of 5% cupric nitrate (remember to use rubber gloves, chemical-splash goggles, and respirators with acid-gases cartridges as well as the ventilation) but there are other chemicals which give more or less the same effect—just be careful to check on the toxicity of any chemical you plan to use, and treat it with appropriate respect. If this seems like too much to deal with, you can usually take things to your local art foundry for professional patination.
Depending on your atmospheric conditions, your green patina may or may not last. To increase its longevity, treat it after patination with a good-quality paste wax, Renaissance wax, or Incralac. For more in-depth information, I highly recommend Ron Young’s patina book—it’s got all the recipes, safety info, directions for techniques like fuming and burying, and color plates which give an idea of the colors to expect. It is available from Sculpt-Nouveau: 21 Redwood Drive, San Rafael, CA 94901, 800-728-5787. Ron Young’s Methods for Modern Sculptors is the best book I’ve found on shell-casting, and Contemporary Patination stands similarly alone in its subject area, with invaluable color plates illustrating the effects of various patina formulations across different samples of metal.
by Andrew Werby