Math & Money

How to spot a counterfeit bill

It’s estimated that for every 10,000 bills in the U.S., one of those bills is fake. That may not sound like much, but it adds up to millions of dollars in cold hard cash. Counterfeit money has the potential to cause all sorts of problems, from leaving you short $20 to destabilizing national economies. But don’t worry. You can help catch the counterfeits. All you need are some simple tools and a bit of chemistry.

First up, the anti-counterfeit detection pen. The pen looks like a highlighter and contains a solution of potassium iodide and elemental iodine. It reveals of the presence of starch, which is commonly used to strengthen regular printer paper, but won’t be found in real money. That’s because authentic bills are made of cotton and linen and are threaded with tiny red and blue fibers. That material is made by a single, highly-guarded company called Crane and Company, which has been printing currency since Paul Revere asked them to help finance the Revolutionary War.

The starch in many counterfeit bills, on the other hand, is made of two molecules: amylopectin and amylose. It’s amylose that gives the fake away. Its long chain of sugar molecules connected by oxygen atoms forms a helical structure, like DNA. Iodide likes to squeeze inside this coil, forming a new compound that leaves a dark mark on the paper. However, in the absence of starch, there is no chemical reaction and the mark will look light yellow. So if the fake isn’t printed on starchy paper, iodine solutions can’t help you.

That’s one of the reasons U.S. bills printed since 1996 have been chemically enhanced to include another counterfeit countermeasure: a strip that fluoresces under UV light. That’s the same kind of light used at black light parties and airport security lines. The polyester strip printed with invisble ink is just one millimeter wide and is found in different positions depending on a bill’s value. If you hold your dollar up to natural light, you can see the amount and the word USA printed on the band. But under UV light, these strips really shine.

They contain molecules that can be excited by absorbing certain amounts of energy, specifically, that given off by common UV light sources. As these excited molecules return to their original states, they lose a bit of energy as heat and then radiate the rest as light. Energy is inversely related to wavelength, which means that the longer wavelengths have lower energy.

So the lower energy light given off by the strip means longer wavelengths that fall in the visible range, and suddenly we can see that which had been invisible. And if a glowing strip doesn’t show up on a recent bill, you have a fake on your hands. For times when you’re not dealing with counterfeit masterminds, looking for simple visual cues will do.

Make sure the portrait looks lifelike and not flat, the seal has perfectly even sawtooth points, the inked border is unbroken, and the serial number has precisely equal spacing between each number. So the next time you come across some dubious dough, have a closer look, pull out your iodine solution, or take it to a rave and you just might catch a counterfeit.

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