Counterfeit banknote detection pen

[1] The chemical properties of US banknotes printed before 1960 make marking pens useless, resulting in false positives.

[2] One example of this complication taking place happened in 2016, in which a 13-year-old student was arrested by police after she attempted to pay for lunch with a red seal Series 1953 $2 bill after the school's counterfeit pen was unable to prove its authenticity.

[8] US counterfeiters bleach small denominations and print more valuable bills on the resulting blank paper to evade this test,[9] although changes to the currency since 2004 have made this method easier to detect.

Simply having a banknote pass through laundry, depending on the soaps and bleaches used, can cause a bill to fail the test when it is otherwise accepted.

Additionally, it was discovered that treating a counterfeit note with a dilute solution of vitamin C will cause a false negative: that is, it will respond to the iodine-based ink as though it were made of the same paper as a valid banknote.