Money Trouble

Euro banknotes contain rare earth elements to foil counterfeiters.

Isaac Newton was undoubtedly the greatest scientist ever called upon to protect his nation’s currency. In the 1690s, decades after he invented calculus and developed his monumental theories of gravity and color, Newton was named head of the Royal Mint in London. Previous heads had treated the job as a lucrative sinecure, but as with everything he did, Newton threw himself and his fearsome mind into the work. England was overrun with counterfeiters at the time, including “clippers” who shaved the edges off gold and silver coins and melted the scraps to make new money. Clipping was considered treasonous because it debased the nation’s economy, and in pursuing counterfeiters the great Newton found himself entangled with spies, lowlifes, drunkards, and thieves—an entanglement he enjoyed thoroughly. In fact, Newton prosecuted wrongdoers with relish, often refusing pleas for clemency. He even allowed one notorious and slippery “coiner,” William Chaloner—who’d goaded Newton for years with accusations of fraud at the Mint—to be hanged and disemboweled.

Circumstances have changed since Sir Isaac’s time. No one bothers counterfeiting coins anymore, just paper bills: coins are too cumbersome and worth too little. Scientists fighting counterfeiters today also tend to take less draconian measures, focusing more on prevention than messy punishment. But counterfeiting is no less of a problem 300 years on, and nations now more than ever call upon their scientists to defeat forgery.

Perhaps most notably, anticounterfeiting chemists brew up the fancy dyes used to print paper money, including dyes that fluoresce under special light. Each atom absorbs certain characteristic colors of incoming light, and the light’s energy can rocket its electrons up to higher orbits around the nucleus. Electrons cannot stay in this excited state indefinitely, though. They eventually crash back down, which causes them to re-emit light of the same color that they absorbed before. It’s all very symmetrical: green in, green out; red in, red out.

Fluorescence also involves an exchange of light but with some differences. Instead of single atoms, fluorescing requires large molecules with long chains of atoms and lots of bonds (and double bonds) between them. When light strikes such a molecule, atoms absorb light as explained above. But instead of the molecule spitting identical photons of light right back out, some of the light’s energy gets dissipated, either in twanging the bonds and letting them vibrate or in rotating atoms. When the molecule settles down and emits light again, there’s less energy available. As a result fluorescing molecules can take higher-energy light and alchemize it into lower-energy light: ultraviolet in, visible light out.

Molecules fluoresce especially well when coupled with certain lanthanide, or rare-earth, elements. The row of lanthanides (elements 57 to 71, lanthanum to lutetium) has a reputation as one of the more monotonous stretches on the periodic table. And true to their dull reputation lanthanides often cannot absorb or re-emit light efficiently. But if you attach a lanthanide atom to a larger molecule, the molecule will act as a sort of antenna, absorbing and channeling light energy to the atom’s electrons. Given such a boost, lanthanides can flare up in vivid colors.

These colors help fight counterfeiting because treasury chemists can print hidden designs with rare-earth dyes on the fronts and backs of bills. These dyes remain invisible under normal, everyday light like sunlight; so a forger might be lulled into thinking he’d made a perfect replica. Put that bill under special ultraviolet bulbs, however, and he’ll start to sweat. The spots treated with rare-earth dyes will suddenly blaze away, revealing incognito features. Security strips embedded in U.S. currency, for instance, will glow green ($20) or yellow ($50) or orange-pink ($100).

These strips inside U.S. bills are effective deterrents but rather utilitarian. European bills display more artistry in the ultraviolet medium. Small fibers embedded into some euros pop out like parti-colored constellations. Charcoal-colored maps of Europe suddenly glow green, as the continent might look to alien eyes from outer space. Pastel wreaths of stars gain a corona of yellow or red, and monuments and signatures shine blue. (Fittingly, the names of some fluorescing lanthanides used in currency have close ties to Europe: there’s rich red europium; regal blue thulium, named after Thule, a Latin name for Scandinavia; and vivid green terbium, named after Ytterby, a hamlet in Sweden that yielded the first minerals containing that element.) There are really two euros on each banknote then: the one we see day to day and a second, hidden euro mapped clandestinely onto the first.

The chemistry-currency symbiosis doesn’t stop with fluorescing dyes either. Banknote paper itself is designed (or tweaked chemically) so it will not glow under ultraviolet light. Certain U.S. denominations have magnetic ink embedded in the portraits of founding fathers, and other countries use ink swatches that shift colors when viewed from different angles. Some treasuries coat their bills with thin layers of nonstick chemicals to prevent dirt from being ground into the paper’s cotton fibers and defacing the currency. A few countries have even ditched cotton-based bills altogether and gone to entirely synthetic bills.

In fact, one of those countries, New Zealand, recently honored its most famous scientific son on its synthetic currency—a genuine but ultimately ironic tribute. Although he came from grubby Kiwi farmers, Ernest Rutherford was a bit of a scientific snoot. Even though his work on atomic structure changed chemistry forever, he once famously declared, “All science is either physics or stamp collecting.” The joke was on him: when the Swedish Academy awarded him the Nobel Prize in 1908, it did so in chemistry. And now Rutherford’s mustached portrait peers out from the New Zealand $100 bill—a portrait imbued with, and enhanced by, some of the most beautiful chemistry of the revolution he helped bring about.

Sam Kean is the author of The Disappearing Spoon: And Other True Tales of Madness, Love, and the History of the World from the Periodic Table of the Elements.