The natural or chemical oxidation of copper creates a variety of shades, from black to bright blue-green, on its surface. Savvy crafters can purposely oxidize their copper jewelry and projects to achieve a beautiful antique look.
Cause of Oxidation
Oxidation is a naturally occurring process wherein copper loses electrons to oxygen or other molecules. This changes the copper surface into verdigris, which is made up of a number of compounds, including copper oxide, copper carbonate, copper chloride and copper acetate. The surface layer eventually covers and protects the rest of the metal underneath. There are plenty of other oxidizing agents that can affect copper.
Materials for Oxidizing Copper
Bowls, towels, bags and utensils are some of the tools you need to oxidize copper. The ingredients for the process can vary, because there are many oxidizers that react with the metal. Egg, salt, vinegar, ammonia -- and commercial products, such as Liver of Sulphur. These ingredients are used in different ways to speed up the natural oxidation process that occurs when copper is exposed to the air and water.
Oxidizing Pennies with Vinegar
As a simple project to test out the process, use vinegar and salt to oxidize pennies. Mix up a solution of white vinegar and salt in a bowl. Place pennies in the bowl for several minutes. While in the bowl, the dull pennies will start to brighten up as their copper oxide layer is removed. Take the shiny pennies out of the salt and vinegar solution and place them, still wet, onto a paper towel. Leave them out overnight to get mild results -- they should develop a verdigris patina. Soak the paper towel in the salt and vinegar solution, and place the pennies on it for up to a week to get more intense results.
Protecting the Patina
Verdigris is not as durable as copper, so it may need some protecting to preserve the layer. Very thick layers of verdigris can flake off of the copper in spots when washed, so rinse copper gently. Pat the item dry, and allow it to air dry until it's completely bone-dry. Consider applying a light coating of clear polyurethane spray to help protect jewelry and other crafts. This coating may also help enhance some of the blue-green patina.