Quote:
Originally Posted by sheepherder
Those only react to the starch in the paper. Real currency isn't really 'paper', it's rag.
What you want for currency is an UV pen that will show the stripe and mini-writing on the stripe. The stripe is in different locations on the bill depending on denomination, and it's a different color too. Green on a twenty.  |
Rich is right, they're basically iodine markers. The swipe that turns black indicates starch. Starch was supposedly used as part of an obsolete counterfeiting process from the 20s, however some starch can be found in today's printer paper.
James Randi used to create a little excitement by sourcing a brand new Ben Franklin from the bank and spraying it with Niagara spray starch before he took it to a merchant with one of the bogus pens. It would show up as bad money in the test at the checkout counter, the police would be called. But the brou-ha-ha would be quelled after the bill was verified at the bank!