Gold at the End of the Rainbow: A Simple and Colorful Modification of the Golden Penny Demonstration

The "Golden Penny" demonstration is a popular experiment that involves treating copper coins with chemical reagents to form brass, an alloy of copper and zinc that has a golden color. Reported here is a very simple modification for forming golden color on copper coins that does not require the use of chemical reagents. Instead, golden colored surfaces can be generated by simply heating copper coins on a hot plate to form nanoscale films of copper oxide. In addition to gold, such oxide layers display a range of other colors including orange, magenta, violet, and silver.