few months after the euro entered circulation, Freek Sujver and Andres Mejerink of Utrecht University in the Netherlands irradiated some banknotes with ultraviolet light and recorded the color bands they emitted. The red light they saw was due to europium ions, a rare earth, bonded to acetone; the green to europium with strontium, gallium, and sulfur; and the blue to europium that had reacted with aluminum and barium oxides. Europium is the most reactive of the rare earths, so much so that it must be preserved in oil to prevent it from catching fire (but it is harmless when bound as in the compounds in banknotes). The EU has never confirmed or denied the presence of europium in banknotes.
