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.

Subscribe to the MAKE Podcast in iTunes, download the m4v video directly, or watch it on YouTube and Vimeo.

Download a PDF of the project instructions from MAKE Volume 16.

Matt Richardson

Matt Richardson

Matt Richardson is a San Francisco-based creative technologist and Contributing Editor at MAKE. He’s the co-author of Getting Started with Raspberry Pi and the author of Getting Started with BeagleBone.

  • Becky Stern

    Awesome video, Matt!

  • Ismail Selman Kimyacioglu


    It is very amazing and interesting video, indeed. And the idea is great. I am very glad to watch it, indeed.

    Thanks for the video

  • Matt Moore

    That’s really cool, takes the stuff we did as kids and moves it into the modern age.

  • Cj Griesmeyer

    Note to self

  • Samantha Karkare

    This Video is amazing ,i like it..very informative…Keep it up

  • Samantha Karkare

    This Video is amazing ,i like it..very informative…Keep it up