Copper is a popular metal for high-end costume jewelry, pots and of course, pennies. Because copper can sometimes take on a yellow or orange patina after years of use, it can be easy to mistake it for brass or another metal. But copper has a few unique characteristics that sets it apart from other metals, and that can help you determine if something is copper or some other metal. If all else fails, a chemical test can be done to tell you once and for all what type of metal a particular piece is.
Attempt to place a magnet on the surface of the item. Copper is not magnetic, so if the magnet adheres, you can be sure that it is not copper.
Examine the color of the cleaned piece. Copper has a natural pink tone that can darken to look red, yellow or orange over time. Have the piece professionally cleaned so as not to alter the finish of the piece, and then view it to detect the pink tone. You may want to compare the color to a newly minted penny; it should be roughly the same color.
Look for areas on the piece that have turned green. When exposed to excess water or oxygen when unfinished, copper can turn green or black in places where it has been excessively handled. Those dark spots can help you determine if you are holding a true copper piece or not.
Test the strength and the sound of the piece. Look for divots and wrinkles in the surface of the piece. Copper is notoriously soft, so it can be difficult to keep a piece perfectly smooth when working with the copper. If the copper piece is thin enough, you may even be able to bend it with your bare hands. You may also knock on the piece and listen to the sound that it makes. Real copper will have a deep and mellow sound, as opposed to brass, which can be high-pitched and tinny.
Examine the piece for a maker's mark. If you find one, you can trace it back to determine if the artist primarily worked with copper pieces. Also, some metals are regulated by the Unified Numbering System. Copper is not. If your piece has a letter 'C' stamped on it, followed by numbers, it is likely brass and not copper.