History of Invisible Ink

While the invention of writing has been attributed to many people, it is widely believed that invisible ink was first created by ancient Chinese calligraphers. The idea for such an ink came from a need for secret messages between two parties who were not in direct contact with each other but still needed to communicate.

The “when was invisible ink invented” is a question that has been asked many times. The answer to the question is: it was invented in 1843 by Sir John Herschel, who used it to write messages in books without being detected.

Vintage men reading invisible ink under light.

The CIA disclosed its oldest secret records, as well as the last from the First World War period, in April of this year. The documents, which date from 1917 and 1918, mostly include formulas for “secret writing”–instructions on how to manufacture invisible ink for operatives of the Office of Naval Intelligence (the CIA did not exist at the time). Invisible ink was once a highly serious business and an essential instrument in a spy’s bag of tricks. It may sound quaint now, but it 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 that was available on the internet and available to every Boy Scout), claiming even in the 1990s that the material served as a foundation upon which more modern tactics could be built and that invisible ink remained a viable tool for its agents.

While the usage of invisible ink has almost completely been replaced by contemporary technology, its history is intriguing, and we’ll look at it today as part of our Man Knowledge series.

The Fundamentals of Invisible Ink

Invisible inks are divided into two categories: organic fluids and sympathetic inks. The former includes the “natural” remedies that many of us experimented with as children, such as lemon juice, vinegar, milk, perspiration, saliva, onion juice, and even urine and diluted blood. Heat, such as that produced by fire, irons, or light bulbs, may be used to create these organic invisible inks, and some can be seen when exposed to UV light. When exposed to heat, the organic fluids modify the fibers of the paper, causing the hidden writing to have a lower burn temperature and brown quicker than the surrounding paper.

Sympathetic inks are more difficult chemical mixtures. Sympathetic inks include one or more chemicals and must be created using a specified “reagent,” such as another chemical or a combination of chemicals.

Invisible Ink’s Evolution

The history of invisible ink is mostly a history of war, since intrigue, espionage, and spying are at their most crucial and required during such times.

The Renaissance and the Ancient World

Invisible ink has a long history, dating back over 2,000 years, and was employed by the ancient Greeks and Romans. Pliny the Elder originally described it in his Natural History in the first century AD, when he recommended utilizing the milk of the tithymalus shrub as an invisible ink. During the Renaissance, invisible ink was still utilized; politicians used it in their letters, and Ovid mentions it in his Art of Love. An ounce of alum and a pint of vinegar were used to create invisible ink by Giovanni Battista della Porta, an Italian polymath. Once painted on the shell of a hard-boiled egg, the message would seep through and be transferred to the albumen of the egg. The inscription was only visible after the egg had been peeled.

The Revolutionary War

Both the British and the Americans utilized invisible ink during the Revolutionary War. Organic fluids and ordinary sympathetic inks were both utilized by the British. Major John Andre, the British intelligence chief, had agents place a letter in the corner of their correspondence to inform the recipient of how the hidden secret message could be revealed; for example, a “F” was placed in the corner of letters that could be revealed by fire, and a “A” in the corner of letters that required the application of acid.


George Washington, on the other hand, desired an ink that could only be exposed by a specifically prepared reagent. Sir James Jay responded to the general’s summons. Jay, a physician who dabbled in chemistry and brother of American hero John Jay, devised a “sympathetic stain” that he provided to Washington. Major Benjamin Tallmadge, the Continental Army’s spymaster, would subsequently pass it on to Abraham Woodhull and Robert Townsend, members of the famed Culper Spy Ring. To avoid suspicion, Washington instructed his spies to inscribe their secret messages “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,” or “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 today’s billion-dollar intelligence services, the CIA did not exist when America joined World War I, and the FBI was just 15 years old. The Office of Naval Intelligence was in charge of collecting intelligence for the government.

Confidential memo via making invisible ink formula.Chemical inks were used at this period, according to a booklet contained in the newly declassified papers described in the introduction, but basic ingredients like lemon juice and milk were still used. While the Americans reverted to old habits, the Germans were pioneering the invention of invisible ink. The Germans utilized inks produced from headache and fever cures, as well as laxatives, during the start of the war; these were convenient since they could be passed off as normal medications. When the Allies realized what was going on, they were obliged to design inks that weren’t based on everyday objects. They used iron sulfate, copper sulfate, and cobalt salt inks, as well as sodium carbonate, ammonia fumes, and potassium ferroscyanide as reagents.

Both parties tried to come up with a universal reagent that could create any invisible ink, regardless of its chemical makeup. When the Allies learned that iodine vapor turned all invisible inks brown, they realized they had a solution. It functioned by exposing where the paper’s fibers had been changed by moisture, rather than by a chemical reaction.

However, the Germans devised a simple counter-measure: after inscribing a secret message, they would steam the whole piece of paper, changing all of the fibers. After allowing the paper to dry, it was sent to its final destination.

Both sides had to devise ingenious techniques to conceal their inks. Agents in the United States were told to soak their shirt collars and handkerchiefs in a sodium nitrate solution before soaking them in water to make ink. German spies used a similar method, dissolving their ties in chemicals and then reassembling them. Shaving sticks, hollowed-out soap cakes, and hair brushes were often filled with ready-made inks. Agents also dipped matchsticks in the inks and let them dry, allowing them to be transported without alerting suspicion and utilized as writing instruments for inscribing a secret message.


Secret messages were also written on the bodies of American agents, which could only be read when sprayed with an atomizer. Messages were also etched on toenails, which were visible by sprinkling them with powdered charcoal.

World War II (WWII)

Both the Allies and the Axis forces worked hard throughout WWII to produce their own invisible inks and learn about the enemy’s inks. This battle between laboratories turned into an arms race, with each side attempting to outdo the other in order to develop the holy grail of invisible inks: one that was odorless, could be developed with the fewest reagents possible, and could not be exposed to heat, detected with iodine, or revealed by ultraviolet light.

The Abwehr, Germany’s military intelligence service, had five levels of inks, with the most difficult inks being given to the best operatives (less trusted spies could have possibly been double-agents who would have turned the secrets over to the Allies). The receiver had to dampen the paper, sprinkle it with a red powder containing naphthalene, heat it to 140 degrees Fahrenheit, then expose it to UV light to produce one of the inks. Another clever ink needed blood to activate it, so the agent pierced his finger and dropped a drop into the mix before writing.

When it came to divulging the secret inks employed by the other side, the Allies and Axis forces strove to outdo one other. The US government carefully checked mail entering and leaving the nation, much as it did during WWI. A million pieces of mail were opened by 14,462 censors every day, and communication that raised the censors’ suspicions was transferred to the FBI for additional investigation. 400 of the 4,600 pieces of mail transferred to the government’s laboratories turned discovered to contain secret writing and codes.

Vintage man reading a memo written with invisible ink.Suspicious documents would be exposed to heat, ultraviolet light, and iodine vapors by censors. They’d also stripe them using a gadget made up of numerous brushes that were hooked together. Each brush was dipped in a separate reagent and brushed over the sheet to look for responses.

The Germans countered this detection approach by developing an ink that needed three three-hour intervals between reagent applications.

An encoded message written on a handkerchief.

The Allies and Germans also sought to outsmart each other by writing their messages in different places. They scrawled on the underside of an envelope’s flap, dusted particular words and phrases in a newspaper with ink, and put notes on handkerchiefs, knowing that their letters would be studied. When German spy George Dasch surrendered to the FBI after landing with his co-conspirators in a submarine on Long Island, the FBI discovered a handkerchief in his pocket with the names and addresses of his contacts scrawled in invisible ink.

The Cold War was a period of time when the world was

Countries dedicated significant time and money to creating espionage techniques and technologies that would keep them one step ahead of the opponent during this Golden Age of Espionage. This involves looking at more advanced and effective invisible inks.

A key breakthrough came in the shape of a new writing approach. Wet-writing was a time-honored practice in which a person wrote directly on the paper with ink. However, there were several notable downsides to this method. To prepare the paper, the agent had to steam it, let it dry, write his message, re-steam it to eliminate the indentations caused by the writing tool, let it dry again, and then write a visible message to cover up the unseen one. Even after all of this, qualified experts on the opposite side were able to find remnants of the writing.


During the 1950s, the Soviet KGB and the East German Stasi devised a new method: dry transfer. Rather of placing the ink directly on the paper, a chemically impregnated sheet of paper was sandwiched between two sheets of regular writing paper. The top page was used to write the secret message, which was then transferred to the bottom sheet through chemicals on the middle sheet. The top sheet was usually constructed of a water soluble substance that could be flushed away or dissolved in a cup of water, while the bottom page was left with an invisible message. Chemical sheets might be reused several times before being discarded. During the Vietnam War, American POWs used this dry technique to smuggle secret information inside their letters home.

During the 1960s, the invention of several plastic items allowed agents a new method to convey their messages. Chemicals would be hidden in everyday items like credit cards, pen caps, eyeglass frames, key fobs, and even a Swiss Army knife’s plastic toothpick. To transfer the invisible “ink,” the agent merely rubbed the plastic item on paper.

These and other breakthroughs came from the well-staffed labs of the 1950s and 1960s intelligence organizations. The Technical Operations Sector of the Stasi employed 50 people only for covert writing. The CIA has 36 covert writing experts working for them both at home and overseas.

As the Cold War came to an end, the number of chemists and physicists working on invisible ink shrank, and the decline of snail mail and technological improvements gradually made the employment of invisible ink, if not obsolete, then a considerably less important instrument in a spy’s arsenal. Who knows, however. Perhaps you were actually blowing your nose into the names of suspected terrorists when the “insurance salesperson” you met at the airport handed you his handkerchief.





The “invisible ink recipe” is a method of writing without being seen. The first such ink was used by the ancient Greeks and Romans, who mixed silver nitrate with gum arabic to create a blackish liquid that could be applied to paper.

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