“During the early 1970s, the price of copper rose to a point where the penny almost contained more than one cent’s worth of copper. This led the Mint to test alternate metals, including aluminum and bronze-clad steel. Aluminum was chosen, and over 1.5 million of these were struck and ready for public release before ultimately being rejected. About a dozen aluminum cents are believed to still be in the hands of collectors, although they are now considered illegal, and are subject to seizure by the Secret Service. One aluminum cent was donated to the Smithsonian Institution.
The price of copper later returned to profitable levels. The Mint did not need to change the cent’s composition until 1982.”