A Chemical Counterfeit Test

    Kids, what's so special about the paper that money is printed on? First of all, it isn't really paper at all. Rather, at a blend of cotton and linen, it is more like fabric material. The blend is about 3/4 cotton and 1/4 linen but the precise amounts are kept very secret.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    As you can imagine, the U. S. Bureau of Engraving and Printing uses many different methods to try to stop counterfeiters. We've written about some of them in this column before (see 3/93 for magnetic inks and 10/00 on the "Science of Money"). Here we'll describe one chemical test that you can do to spot the fake in a stack of bills.

    Real paper is either coated or "sized" with starch. Starch sizing means that starch has been added to ordinary paper to fill the gaps between cellulose wood fibers. It acts to stiffen the paper very much like the way laundry starch stiffens a shirt collar. It also makes paper less absorbent to ink. Without sizing, ink would smear out all over the paper fibers and make words blurry. Paper money, however, has to completely absorb and bind ink. Did you ever wash a bill accidentally in the laundry? It comes out good as new without any loss of ink whatsoever. No starch sizing is used in the production of currency paper.

    So, a test for starch is a great way to tell the different between real and fake money. Here's how to do it yourself. To see how a dilute iodine solution (you can find this at a drugstore) reacts with starch, dab a little bit using a cotton swab onto a slice of raw potato. The deep blue-black color that results is a positive test for starch (potatoes are full of starch). It happens when the yellow-red color of iodine combines with starch molecules. Now dab some iodine onto regular paper and see if the same thing happens (it should). Now repeat the test with a dollar bill. Did you get a positive starch test? You shouldn't!


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    March 2003


    Reference: E. Venere, "The Money Makers" in ChemMatters, 2/03, p. 14 and in Chemistry (both American Chemical Society publications, the latter a quarterly newspaper).