U.S. postal inspectors, ca. early 20th century

Photo of a censor’s office lab, ca. early 20th century. Postal inspectors during both world wars examined mail for secret messages sent by spies.

Library of Congress

Under Cover of Ink

A recent book reveals the long history of secret writing.

Kristie Macrakis. Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda. Yale University Press, 2014. 392 pp. $27.50.

I held in my hands what looked like a blank piece of paper, but I knew it was inscribed with a message I couldn’t see. Slowly, carefully, I held the paper to the heat. One by one the words appeared, curling across the paper in brown cursive:

These were the common kitchen items I had used to create invisible ink. Not the most exciting thing ever written in invisible ink, to be sure, but part of a long tradition that Kristie Macrakis chronicles in her book Prisoners, Lovers, and Spies.

You’d think the original invisible inks would have been made from fruit juices, like those in my own kitchen experiment, but they sprang from something else: trees. Gallnuts, insect-produced swellings on oak trees, have been used to make ink for thousands of years. By mixing ground-up galls with iron sulfate, you get what Macrakis calls “the most important ink in Western civilization”: the dark black ink of Shakespeare, Bach, and van Gogh. This hardly seems secret, but it wasn’t long before people noticed that if they wrote using only one ingredient of the ink (the ground-up galls, for instance), the message remained invisible. Wash the iron sulfate over the top of the hidden message and the words appeared as if originally written with regular ink. Stories of invisible inks used for military purposes that worked much like iron-gall ink date back to about 300 BCE.

Fruit juices, though simpler, came later to the invisible-ink game. First used in the Arab world around 600 CE to deliver secret messages between desert towns, lemon inks became popular in Europe in the 16th century. Monasteries used them to protect trade secrets. Juices were even used during World War II by prisoners of war to relay hidden messages to those back home, despite everyone knowing the trick by then: just add a little heat.

The most infamous story of fruit inks has to be that of England’s Lemon Juice Spies of World War I. German émigré Carl Muller recruited English baker John Hahn, and together they reported British troop movements to the Germans using lemon juice. Between the easily detectable ink and the fact that Hahn signed a secret letter with his real name (amateurs!), British Postal Censorship soon caught on. Scotland Yard staged a raid on Hahn’s home, finding a lemon poked full of holes by a pen nib. Then the police went for Muller, who had pieces of lemon in a drawer and a whole lemon in his coat pocket. These lemons, along with the pen nibs degraded by citric acid and clogged with lemon bits, were used as evidence in court. (The whole lemon still exists by the way, blackened beyond recognition, in the British National Archives.) Hahn got off with seven years in prison. Muller was convicted and executed alongside three other German spies who had used lemon-juice ink. A fifth lemon-juice spy hanged himself in his jail cell before his trial could be completed.

The Lemon Juice Spies used an outdated technology, and it cost them their lives. Invisible inks had progressed far beyond juices by then, but they needed careful preparation, special paper, or specific decoders. Many of these secrets eventually became not so secret and found uses beyond spycraft. Color-changing inks were popular in the fireplace screens of 18th-century Parisian salons, transforming from barren winter landscapes when cold into blooming, colorful spring scenes when warmed. Some 19th-century dolls came with cobalt chloride–infused dresses that changed from blue to pink depending on humidity levels. Twentieth-century chemistry sets capitalized on color-change experiments to lure young scientists.

Macrakis covers more than just invisible ink, looking at secret writing more generally, from the opaque language used by alchemists in their books, to Cold War microdots, to modern terrorists hiding messages in digital photographs. (If you want to try that last method at home, the easiest way is to open a JPEG file with a text editor and add your message at the end of the hodgepodge of letters and numbers on the screen. But if you try it and the feds catch on to your shady dealings, don’t blame me.) Macrakis even recounts the classic story of Histiaios, a Greek leader who around 500 BCE shaved his slave’s head, tattooed it with a secret message meant to incite a revolt against the Persians occupying his city, and waited for the hair to grow back before sending the slave on his way. (Messages and events moved a lot slower then.)

Despite the exciting subject material, something is lacking in Prisoners, Lovers, and Spies. It never becomes the espionage adventure it wants to be: it can’t quite bridge the gap between an academic text and a popular book, leaving it floundering in tone.

The author is at her strongest in the chapter on the American Revolution and the Culper Spy Ring. In 1776 British forces captured Staten Island, Manhattan, and parts of eastern Long Island before occupying Manhattan. Patriots moved out; loyalists moved in. Thousands of American colonists were held in deplorable British prison ships moored just off the East River.

New York City was in Britain’s iron grip—and George Washington desperately needed to know what was going on there. A number of patriots with jobs that enabled them to move around relatively easily (like merchants) or put them in a position to overhear information (like newspaper reporters) were recruited. James Jay, brother of the first chief justice of the Supreme Court, John Jay, had invented an invisible ink that worked much like iron-gall ink. Jay became Washington’s supplier of the ink and its decoder (code-named “the medicine”), and soon secret messages written in letters, pamphlets, almanacs, and other unassuming publications were making their way to the commander in chief. The Culper Spy Ring is credited with at least four significant successes—including the first hints that Benedict Arnold was about to turn traitor.

While I may argue with Macrakis’s style, I cannot argue with her research; this book is a good reference to keep around if you’re a devotee of secret writing. Macrakis even includes an appendix full of invisible-ink recipes that go far beyond my little kitchen experiment. (Though I must say, all three of my “inks” worked quite well and would do the trick if you found yourself in a tight spot. No wonder they were used for so long.) Get out the aspirin, find some alum, or even buy some phenolphthalein or iron sulfate and experiment for yourself. It’s just as fun now as it was in your elementary school science fair.