How to spot a counterfeit bill - Tien Nguyen

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Authentic dollar bills are equipped with many security features to make them difficult to forge. But that doesn’t stop counterfeiters from trying to fool people with fakes. Luckily, anyone can help catch a counterfeit; all you need are a few simple tools and a bit of chemistry. Tien Nguyen details the chemistry behind counterfeit cash.

Lesson by Tien Nguyen, animation by Lippy.

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.