The Basics of Invisible Ink

In April of this year, the CIA released its oldest classified documents and the last from the World War I era. Dating from 1917 and 1918, the papers mainly contain recipes for “secret writing”–instructions for agents of the Office of Naval Intelligence (the CIA did not yet exist) on how make invisible ink. Such a low-tech espionage method may seem quaint today, but invisible ink was once a very serious business and an important tool in a spy’s bag of tricks. So much so that the CIA bizarrely waited almost a century before revealing its most basic recipes to the public (information which was available on the web and to every Boy Scout), claiming even in the 90s that the material constituted a foundation upon which more modern tactics had been built and that invisible ink remained a viable tool for its agents.

While the use of invisible ink has now been almost entirely eclipsed by modern technology, its history is incredibly fascinating, and today as part of our Man Knowledge series we’ll explore its use through time.

There are two categories into which invisible inks fall: organic fluids and sympathetic inks. The former consists of the “natural” methods many of us tried our hand at as kids: lemon juice, vinegar, milk, sweat, saliva, onion juice, and even urine and diluted blood, to name a few. These organic invisible inks can be developed through heat, such as with fire, irons, or light bulbs, and some can be seen when placed under ultraviolet light. The organic fluids alter the fibers of the paper so that the secret writing has a lower burn temperature and turns brown faster than the surrounding paper when exposed to heat.

Sympathetic inks are more complicated chemical concoctions. Sympathetic inks contain one or more chemicals and require the application of a specific “reagent” to be developed, such as another chemical or a mixture of chemicals.