A green patina on copper is characteristic of antiques, old copper rooftops and statues. This long valued look takes years to naturally weather through a process called oxidation. Solutions can be prepared and applied to copper to greatly hasten the patina process and achieve this antique look within hours. When treating your own piece, take special consideration for how the solution is prepared and applied.
Clean the surface of the copper. Any dirt or grease that's on the copper will not patina correctly. A chemical reaction causes the greening of the metal and impurities will affect the process. Using a metal cleaner, the type that uses trisodium phosphate is recommended. A cleaner that leaves an oxide layer afterward is not recommended. Immediately after the cleaning, rinse the copper. Repeat this process until the water does not bead up on the surface of the metal.
Prepare your patina solution. There are many solutions to use when trying to speed up a patina process, some produce patina colours of brown, black or blue. To achieve green you'll need a solution with ammonium sulphate, ammonium chloride or hydrochloric acid. The Metalworking Compendium offers a green patina recipe that includes ammonium chloride, salt, ammonia and distilled water. All patina solutions should be prepared in a plastic container that is not used for anything else. Follow a recipe like the one from Metalworking Compendium below. You can find these ingredients through scientific laboratory manufacturers.
Metalworking Compendium Recipe
1 Tbsp ammonium chloride (Sal ammoniac)
1 Tbsp salt
1 qt. bottled or distilled water (warm or hot)
Spray the patina onto the copper and let the piece sit for two to three hours. Once it's dry, you can spray it again and again until you achieve the desired result. If this process must be done outside because you are treating a rooftop or a large statue, then consider the weather. If it rains within the first six to eight hours, the water may wash off the patina and not allow it to finish. When spraying, avoid large drops and try to maintain a fan like application. High humidity in the air will cause the reaction to occur quicker, it will need about six hours to complete, depending on conditions. After the process is complete and the solution has dried, the next rainfall should actually bring out the colour.
Stop the process. If at any point you want to freeze the process and no longer allow the copper to weather and patina, apply oil to stop further changes. Linseed oil or lemon oil are both acceptable. This technique can preserve a piece of copper for up to ten years. In harsher environments you may want to repeat the process every three to five years.