Make: Online readers who are secret agents—or secret agent wannabes—will love this project, which comes to us from MAKE Volume 16. By replacing the ink in a color inkjet cartridge with concentrated lemon juice, you can print with invisible ink. The secret message is revealed by spraying or wiping the paper with iodine tincture. Here’s how it works: the reaction between the starch in the paper and the iodine turns the paper purple, however, ascorbic acid (also known as vitamin C) in the lemon juice acts as a reducing agent that breaks up the iodine complex before it reacts with the starch. If you don’t have iodine handy, you can also reveal the message by heating up the paper with a hot iron.
However you do it, you’ll be fairly sure that your super top secret dispatches are a bit safer from interception.
Download a PDF of the project instructions from MAKE Volume 16.