How does a Counterfeit Money Pen Work?

H. Lo

A counterfeit money pen contains a solution that detects starch in counterfeit paper money. In the United States, legitimate paper currency is not bound with starch in the manner that standard paper is and, therefore, contains either little or no starch at all. When the solution in the pen is applied to American paper currency, it will do one of two things: react to starch in the counterfeit paper money and turn the tested area a deep blue-black color, or simply mark the tested area of legitimate paper money a light golden-brown color.

Iodine is the solvent that detects counterfeit money.
Iodine is the solvent that detects counterfeit money.

The solution in a counterfeit money pen is made up of various solvents, one of which is iodine, which is responsible for detecting counterfeit paper money. This is because when iodine and starch interact, they result in a deep blue-black color that indicates that the money being tested was made on paper containing starch, signifying that it is counterfeit. On the other hand, if a light golden-brown color shows up instead, the money is likely legitimate, since that is the actual color of the solution and it indicates that the iodine did not come into contact with any starch.

Solution in a counterfeit marking pen detects starch in a fake note.
Solution in a counterfeit marking pen detects starch in a fake note.

Many places, including stores and banks, use these pens to test paper money before accepting it. This is probably because the pen is small, easy to use, and will provide quick results. It should also be of note that in some cases, the solution will not leave permanent markings on the money. If the solution in the pen contains hydrogen peroxide, the mark made on the money will go away eventually. Since the iodine has interacted with the starch in counterfeit paper money, though, the deep blue-black coloring will not do the same.

As long as there has been currency, there has been counterfeit money, which is dangerous for an economy. A counterfeit money pen checks the paper that the money is made of, but there are visual ways to distinguish between counterfeit and authentic American paper money. For instance, blue and red fibers are embedded within the paper used for making American money while counterfeit money will try to imitate the look by printing red and blue lines onto the paper. In addition, the quality of authentic American paper money is higher than that of counterfeit paper money, and the details are much clearer, even, and sharp.

Authentic money is typically printed at a much better quality than counterfeit bills.
Authentic money is typically printed at a much better quality than counterfeit bills.

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Discussion Comments


I work at the registers and a lot of the bills have blue marks on it. However, it's a real bill. It's hard to tell nowadays.


The twenty dollar bill is the most counterfeited bill in circulation in the US.


I refuse to use those pens. They give a false sense of security. I trust my ability to know what's wrong and what's right in a bill more than those pens. I know money, and I know when something's funny.

The pens are unreliable. All a counterfeiter has to do is print on starch free paper. It's not hard to get. You can find this sort of paper in some stationary supplies. You can bleach out a legit $1 bill. Honestly? The only time the iodine pens give a true positive is on a very poor fake. In which case, any decent cashier should be able to tell anyway. What's more, these pens can give a false positive. All it takes is a bill going through the wash to pick up enough starch to cause a reaction. How many bills have you ever washed?


@Sunshine31: I have to use the pens at my place of work. I would have agreed with you prior to working with money, but you would understand if you were responsible for $20 every now and then that come up counterfeit. Not fun. It might seem a little over the top but it's not. Nobody wants to pull that money out of their own pocket because they didn't check if it was real or fake. It's bizarre how much fake money is actually out there!


So what about the old US dollar bills? can they be detected if they are real or fake?


@Sunshine31 - I know what you mean, but I think that my bank uses the counterfeit detector pen on all of the bills that are presented. It might be the bank’s regulation.


I just wanted to say that I can understand that there are many counterfeit bills floating around, but I think that when a clerk uses a counterfeit detector pen on a twenty dollar bill, it just seems a little absurd. I can understand the fifty or the hundred dollar bill, but a twenty dollar bill is too much.

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