Letter written by Benedict Arnold
A letter from the Revolutionary War with lines written in invisible ink. Credit: Wikimedia Commons

Last year, the US Central Intelligence Agency released documents detailing recipes for invisible ink from 1917 and 1918. Among the recipes was a favorite ink recipe of German soliders during World War I — crushed aspirin mixed with water. Though the ink is easily available, uncovering it required a complicated chemical developer.

Other recipes in the CIA documents used solutions of iron, silver or copper salts as ink and developed the writing by heating the paper. This bit of history inspired my inner scientist and spy. Without access to chemicals in a lab, I experimented with invisible ink recipes using things I found around my house.

1. Lemon juice and heat
Dip a cotton swab or thin paintbrush in lemon juice. Write your message on white paper and let it dry. Hold the paper over a lamp, radiator or candle (but don’t let it catch fire!). The heat breaks down the acid into light-brown compounds, revealing your message forever.

This is how my experiment turned out. The top part of the picture is the paper before I held it over a candle.

Lemon juice invisible ink
Using lemon juice as invisible ink, before and after developing with heat. Credit: Melissae Fellet

2. Laundry detergent and black light
White shirts glow under black lights at a haunted house because the whitening agent in laundry detergent glows under ultraviolet light. Try writing your message using liquid laundry detergent. Holding the paper to a black light reveals a glowing message without damaging the paper.

But if spies intercept a message written with one of these inks, it’s likely they could decode it. We all can find something to heat paper. And black lights, though less common than candles, are still widely available.

To solve this problem, I borrowed a trick from George Washington: using two different chemicals to write and decode the message. Washington wrote with an watery ink containing iron salts. The receiver decoded the message by painting the paper with sodium carbonate, a chemical cousin of baking soda.

Maintaining ink supplies was crucial during Washington’s day. Now a trip to the grocery store will keep you well supplied.

3. Baking soda and grape juice
Make a paste of baking soda in water. Write your message with this paste on a piece of paper and let it dry. To decode the message, paint the paper with thawed grape juice concentrate. The acidic juice reacts with the basic baking soda and the purple grape juice turns gray. This worked best when I used plenty of baking soda paste and undiluted grape juice concentrate:

Baking soda and grape juice invisible ink
Writing in baking soda and developing with grape juice. Credit: Melissae Fellet

In the pictures above, you can tell that the paper has been altered because the paper buckles as the watery lemon juice or baking soda paste dries. Traditionally, writers would re-steam the paper to remove the bumps after scribbling a message with wet ink.

During the Cold War, invisible ink technology improved enough to remove the time-consuming steaming step. Soviet and East German spy agencies developed chemical-coated paper similar to carbon paper we use today. Agents sandwiched this coated paper between two blank pieces of paper. Writing on the top sheet transferred the chemicals from the middle sheet to the bottom paper.

Perhaps hacking an ink-jet printer to print lemon juice invisible ink would transfer your message without damaging the paper as well. I’d love to hear from someone who has tried this.

Have fun sending secret messages!

Try This at Home: Invisible Ink 20 September,2015Melissae Fellet

  • Anonymous

    You can get glass “art pens” or try plain old calligraphy pens. With a thinner line and less ink, I bet the paper won’t buckle as much.

  • anwar ali Malik
  • Anonymous

    @KiltBear Thanks for the great suggestion. I found that I needed a lot of “ink” for the baking soda paste, so glass pens might not work for that variation. But it would be great to try them with lemon juice ink.

  • Brigid Franey

    Specifically what compounds does the citric acid decompose into that give the brownish color?

  • Melissa Fellet

    @Brigid: That’s a great question. I’ve always heard it explained generally as the oxidation of acid, but I wouldn’t expect citric acid to turn brown. This set of instructions says the ascorbic acid (also known as vitamin C) in the juice oxidizes. I also asked an expert in paper conservation. He thought the sugars in the juice (including ascorbic acid, which is sugar-like) could be charring before the paper burns.

    Here’s the experiment that I would do to figure out if one of these ideas is right: Try writing with concentrated sugar water, like simple syrup or homemade hummingbird nectar. If that doesn’t turn brown with heat, then I would use a paste of vitamin C tablets as ink. A vitamin/electrolyte powder like Emergen-C might have sugars in the ingredient list, so would I avoid using that to make the paste. I would look for a source of vitamin C in the drugstore that doesn’t contain sugars.

    If you try it, let me know what you find out!

  • chris

    I keep finding all different kind of ChapStick’s oils etc..and diffent colored lights like keychains and little nick nacs mostly red and blue and green and pieces of paper blank I’ve tried all kinds of stuff please help something is up w my wife

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Melissae Fellet

Melissae Fellet is a freelance science writer obsessed with electrons, atoms and molecules. Writing about chemistry, physics and technology, she hopes to reveal how the invisible building blocks of matter influence things like plastics, perfumed shampoos and the speedy computer chips we use everyday. She holds a BS in biochemistry and microbiology from the University of Florida and a PhD in chemistry from Washington University in St. Louis. She spends sunny days at her home in Santa Cruz either watching otters in the bay or tromping around the redwood forests.

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