Make Your Own Invisible Ink: Tips From the CIA

The History of Invisible Ink

Earlier this spring, the CIA officially released the six oldest documents that had still been classified by the U.S. government. Contained within these documents, which date from 1917 and 1918, are recipes used for invisible ink during World War I. This includes recipes used by Americans, as well as German methods that were uncovered by Allied forces.

The use of invisible ink in the United States dates back at least as far as the Revolutionary War, when George Washington used it to spy on British forces. During World War I, invisible ink was used extensively to communicate when lines of communication couldn't be trusted. Invisible ink allows people to write notes with a pen, stamp or even a finger. When the ink dries, the surface on which it's written appears blank. Because sending a blank piece of paper might arouse suspicion, agents in World War I typically wrote a message in regular ink over the hidden one to mask their intent.

When the message reaches its recipient, the invisible ink can be made visible in a variety of ways. In some cases, the application of heat to the paper reveals the hidden writing. Sometimes, the application of an appropriate chemical is required. Other invisible inks are visible when viewed under an ultraviolet light.

Make Your Own Invisible Ink

Creating invisible ink isn't limited to secret agents with special tools. You and your child can use many common household items to write your own hidden messages. One of the most common homemade invisible inks requires only plain lemon juice. You can dip a paint brush, finger or other pointed object in lemon juice, write a message on a piece of paper and, when it dries, the message will be invisible. Carefully heating the paper with an iron or other heat source will reveal the message.

To make a more complex ink, use a combination of acids and bases to write and unveil your message. There are myriad ways to do this, but one of the most common involves baking soda. Mix equal amounts of baking soda, a base and water. After writing your message and letting it dry, brush the page with grape juice concentrate, which is an acid. The grape juice reacts with the baking soda, revealing the message.

Vinegar, milk, apple juice and white wine are among the many other common items you can use to create invisible ink. Different types of acids and bases will yield different results, including colors that may be unexpected. This easy science experiment helps your child understand basic chemistry, have fun with secret messages and learn a little about military history.