What the marker is actually testing
A counterfeit-detection pen is a paper test, not a bill-authentication test. Its ink contains iodine, which changes color when it meets starch, a common ingredient in ordinary wood-pulp paper. A dark mark can therefore indicate that a note is printed on paper containing starch.
U.S. currency uses a cotton-and-linen paper blend rather than typical wood-pulp paper, so the marker usually leaves a light mark that fades. That difference can help flag some counterfeits—but it says only something about the paper beneath the ink.
Why a fake can pass
A counterfeiter who prints on paper without starch can avoid the reaction. Linen-blend paper may do that, even though it is not genuine currency paper. The pen has no way to tell whether the paper has the precise composition, construction, or security features of an authentic note; it simply responds to starch.
That makes the test useful as a quick screen, not a verdict. A passed mark does not prove a bill is genuine, and a dark mark is a reason to examine the note more closely rather than proof by itself. The practical lesson is that counterfeit detection depends on checking the bill’s other features too—not just watching what a pen does to its paper.
