Patina (also known as verdigris) is the visible result of chemical oxidation/reduction that takes place on the surface of copper (and bronze). A combination of moisture, air and salt or acid causes the reaction. Unlike rust with iron, patina does not occur throughout the metal, but coats the surface only, forming copper sulfate or copper chloride. It actually protects the metal underneath from oxidizing further. This is why copper surfaces outdoors can last for many decades, while iron eventually degrades completely.
Brass is an alloy of approximately 75% copper and 25% tin. Because it's not pure copper, brass doesn't acquire patina over time, but rather simply darkens to a rich golden brown color. Aluminum doesn't darken at all, but instead acquires white oxidation spots.
Patina can develop quickly or take many years, depending on environmental conditions. For example, on the beach in Hawaii, patina may develop in a few months, while in Southern California, only a little green may show after three years or more. Copper metal, exposed to the elements, darkens and dulls first, before any green begins to appear. To keep copper shiny and "new" looking outdoors, it must be coated with lacquer or other protective coating. Copper can be cleaned by dipping it into vinegar; children often try this with a penny as a classroom experiment to see a chemical cleaning effect.
There are commercial patina chemicals, in a variety of colors, that can be applied to copper to achieve a patina effect quickly. This can provide a more uniform, and sometimes thicker, effect that natural aging. Colors may vary depending on ambient humidity and temperature.
We think letting nature take its course and watching your copper product age gracefully over time is best, but if you'd like to do your own patina, CLICK HERE to see our tutorial!
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