Man Knowledge: The History of Invisible Ink

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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.

The Basics of Invisible Ink

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.

The History of Invisible Ink

The history of invisible ink is mainly the history of war, for it is during such times that intrigue, espionage, and spying is at its most vital and necessary.

Ancient Times and the Renaissance

The history of invisible ink goes back more than 2,000 years and was used by the ancient Greeks and Romans. The first record of it comes from Pliny the Elder in the first century AD, who mentioned using the milk of the tithymalus plant as an invisible ink in his Natural History. Invisible ink continued to be used during the Renaissance; statesmen used it in their letters, and Ovid references the practice in his Art of Love. Giovanni Battista della Porta, an Italian polymath, developed a formula for invisible ink that consisted of an ounce of alum and a pint of vinegar. Once painted on the shell of a hard-boiled egg, it would seep through and transfer the message onto the egg’s albumen. The writing could only be seen once the egg was peeled.

The Revolution

During the Revolutionary War, both the British and the Americans used invisible ink. The British used both organic fluids and common sympathetic inks. Major John Andre, the chief British intelligence officer, had agents put a letter in the corner of their correspondence to inform the recipient as to how the hidden secret message could be developed; for example, an “F” was placed in the corner of letters that could be revealed by fire, an “A” for those that needed the application of an acid.

But George Washington wanted something more, an ink that could only be revealed by a unique, specially formulated reagent. Sir James Jay answered the general’s call. Jay, brother of American patriot John Jay and a physician that dabbled in chemistry, created a “sympathetic stain,” which he supplied to Washington. Washington would then pass it on to the Continental Army’s spymaster, Major Benjamin Tallmadge who in turn provided it to the members of the famous Culper Spy Ring: Abraham Woodhull and Robert Townsend. To avoid suspicion, Washington instructed his spies to write seemingly banal letters between the lines of their secret messages, or to inscribe them “on the blank leaves of a pamphlet. . . a common pocket book, or on the blank leaves at each end of registers, almanacks, or any publication or book of small value.”

World War I

In contrast to the billion-dollar-budgeted intelligence agencies of today, when America entered the First World War, the CIA did not exist, and the FBI was barely 15 years old. The Office of Naval Intelligence coordinated the gathering of the country’s intelligence.

In a pamphlet included in the recently unclassified documents mentioned in the introduction, we can see that chemical inks were employed during this time, but basics like lemon juice and milk were still in use as well. While the Americans were falling back on old favorites, the Germans were at the leading edge of invisible ink development. At the beginning of the war, the Germans used inks made from headache and fever remedies and laxatives; these were handy as they could be passed off as common medicines. But when the Allies caught on, they were forced to develop inks outside those based on common household items. They utilized inks made from iron sulfate, copper sulfate, and cobalt salts, and employed reagents of sodium carbonate, ammonia fumes, and potassium ferroscyanide.

Both sides worked to find a universal reagent that could develop every invisible ink, no matter its chemical composition. The Allies struck upon a solution when they discovered that iodine vapor would turn all invisible inks brown. It worked not by chemical reaction, but by revealing where the the paper’s fibers had been altered with moisture.

But the Germans then came up with a simple counter-measure; after inscribing a secret message, they would wet the entire paper by steaming it, thus altering all of the paper’s fibers. After the paper was allowed to dry, it was sent on to its destination.

Both sides had to come up with sneaky ways to hide their inks as well. American agents were advised to impregnate their shirt collars and handkerchiefs with a solution of sodium nitrate; these items could later be soaked in water to create an ink. German spies employed a similar tactic by soaking their ties in chemicals that were later reconstituted. Ready-made inks were often placed inside of shaving sticks and hollowed out soap cakes and hair brushes. Agents also dipped matchsticks into the inks and let them dry; the matches could then be carried without arousing suspicion and used as writing utensils when it came time to inscribe a secret message.

American spies also wrote secret messages on their body which could only be developed when sprayed by an atomizer. Messages were even engraved on toenails; a dusting with powdered charcoal revealed the etchings.

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