It’s trivia time!  Are you ready?

True or False?
The United States Secret Service was established to protect the president.

If you guessed True, you’re …. wrong!

The Secret Service was established as an agency in 1865, just after the Civil War, to investigate and prevent counterfeiting—the process of making fake money not issued by the U.S. government or other recognized authority.

According the Secret Service website, at the time of Civil War, it has been estimated that 1/3 of all currency in circulation was counterfeit.  The problem was made worse by the fact that, at that time, banks in each state designed and printed their own bills.  With so many different variations, it was nearly impossibly to know which bills were genuine and which were fake.

Although we’ve come a long way since the days of the Civil War, money counterfeiting is still a problem that the Secret Service works to address.  Made easier by the advent of increasingly sophisticated personal computers, graphics programs, and printers, making counterfeit money today may be easier than ever.

A statistic from a Secret Service annual report for 2010 illustrates this disturbing trend.  According to the report, approximately 63 percent of the counterfeit currency passed domestically in FY (Fiscal Year) 2010 was produced using digital printing means, compared with less than one percent in FY 1995.

In 2010 alone, the Secret Service removed more than $261 million of counterfeit currency from circulation, up from $182 million seized in FY 2009.

With so many determined criminals misusing technology to produce fake money, and the power of the technology at their disposal improving all the time, how can the government put a stop to the practice of counterfeiting?

In part, with a little help from science.

Although digital printing and personal computers seem to get better every day, so too do the techniques used by the Bureau of Engraving and Printing, where all US currency is produced.  With features like special paper, UV inserts, and color shifting ink, here are three ways that science helps to keep your money safe:

Special Paper



Despite being one of the oldest anti-counterfeiting security measures, it’s also one of the most ingenious.  Unlike common notebook paper, or the paper we use in our personal printers, U.S. currency is printed on a special kind of paper that is a mixture of 75% cotton and 25% linen.  What makes this paper so special is that unlike the common paper we use every day, this special currency paper does not contain any starch.  Because currency paper doesn’t contain any starch, counterfeit money can be spotted easily by taking advantage of a chemical reaction that takes place between paper containing starch and iodine.

Businesses often supply their cashiers with special iodine pens that employees can use to test for counterfeit bills.  When used on a counterfeit bill that is printed on paper containing starch, the iodine in the pen reacts with the starch and turns a deep purple color.  When the pen is used on genuine currency, it leaves only a faint yellow mark, which is the color of the iodine solution.

The starch causes the reaction because its chemical composition is slightly different from the material that genuine currency paper is made from. The linen and cotton of currency paper are made from a molecule called cellulose. Cellulose is the main molecule in wood and other plant material. Cellulose is a polymer which is a long molecule made up of repeating units called monomers which are linked together. Starch is also a polymer but its monomers are slightly different than those in linen and cotton. This tiny difference is enough for iodine to react with the starch in counterfeit paper but not with the linen and cotton in currency paper.

UV Security Threads

In addition to the paper itself being special, there is another feature incorporated into authentic US currency that helps foil counterfeiters.  During the paper making process, special security threads are inserted into the paper that glow in the presence of ultraviolet (UV) light.

When you hold the bill up to the light, you can see a thin embedded thread that runs vertically to the left or right of the portrait on the bill.  On the newly reissued $100 bill, this security thread glows pink when it is illuminated by ultraviolet (UV) light.  On a $50 bill, the thread glows yellow, on a $20, it’s green, on a $10, it’s orange, and on a $5, blue.

Ultraviolet light is a part of the light spectrum that is invisible to the human eye.  You may recall learning that the light we can see, including all of the colors we observe on a daily basis,  are waves with different wavelengths.  The small part of the light spectrum that we can see is called visible light, but there are parts of the light spectrum that we can’t see with our eyes, including UV light.

So, if we can’t see it, then how can it help keep our money secure?  Although we cannot see it, the security thread embedded into currency paper reacts by glowing in the presence of UV light.  By using special devices that emit UV light, cashiers can quickly pass the light over a bill to confirm that the security thread is present and that it glows the correct color.

Color Shifting Ink

A third scientific spectacle that protects US currency from counterfeiters is color shifting ink.  Also sometimes called “optically variable ink” this special pigment appears to change color as you shift the position of the bill in your hand, relative to the nearest light source.

This security feature can be found on the $100, $50, $20, and $10 bills.  Color shifting inks work because these special inks reflect white light differently depending on the angle at which the light hits the bill.  The angle at which light strikes the surface of the bill is called the angle of incidence.  When you tilt the bill back and forth in your hands, you change the angle of incidence, and therefore, you notice that the color of the ink appears to change from green to black, or copper to green, depending on the age of the bill you are examining.

Angle of incidence.

This popular security feature is also used widely in foreign currency, and has the advantage of being very difficult for counterfeiters to duplicate.

If you were to scan a bill or make a photocopy, for example, no matter how high quality that picture was, it would not be able to capture the color of the ink in both color states, and the copy would be easily spotted as a fake.

You can learn more about the security features of US currency, and take an interactive tour of the new $100 bill at this website jointly developed by the Department of the Treasury, Federal Reserve Board, and US Secret Service.