11 Historical Uses for Invisible Ink


The history of invisible ink veers wildly back and forth between high-tech methods and the humblest of approaches. In her book, Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda, Kristie Macrakis traces invisible ink from daring escapes to love affairs to acts of espionage.

1. Ovid's Advice

The Roman poet Ovid, a noted ladies’ man, wrote elaborate instructions to lovers in his Ars Amatoria (Art of Love). In the section directed at women, Ovid taught wives bent upon deceiving their husbands to communicate in secret. Ask a servant “who is in your secrets” to carry missives in her stocking or her bosom, “under a wide shawl.” Or, failing that, try secret writing, which was apparently common knowledge by 18 BCE, when the book was written. “Characters written in fresh milk are a well-known means of secret communication,” the poet wrote. “Touch them with a little powdered charcoal and you will read them.”

2. Mary's Downfall

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The Catholic Mary, Queen of Scots, kept under luxurious house arrest for eighteen years by her Protestant cousin Elizabeth I, used invisible ink and cipher to communicate with Catholic supporters on the outside. Mary advised correspondents to write to her employing two commonly used substances: alum (hydrated potassium aluminum sulfate) or nutgall (the tannic acid secreted in swellings generated by parasitic wasps colonizing oak trees). Letters written in alum required the recipient to soak the paper in water, while nutgall needed a solution of ferrous sulphate as a developer. Eventually, Lord Walsingham, Elizabeth I’s spymaster, who had been breaking Mary’s codes all along, set a trap for Mary, using a double agent to entice her into partially committing to a plot against Elizabeth’s life. Mary was executed on February 8, 1587.

3. Orange You Glad...

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Another Catholic prisoner in England used invisible ink with far happier results in 1597. The Jesuit priest John Gerard came to England in 1588 to carry out a secret mission for the Catholic underground. Caught and detained in the Tower of London, Gerard was tortured for information. The priest befriended his prison guard and began to ask for oranges, whose juice he saved to write to confederates to on the outside. With the help of this guard, Gerard even communicated with a fellow Catholic prisoner whose cell he could see from his own, miming directions for developing the orange-juice letters over flame. The two eventually conspired to escape the Tower, with the help of outside accomplices who brought a rope—a feat made more impressive by the fact that Gerard’s fingers had been wrecked during his torture sessions.

4. Explosive Ink

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During the eighteenth century, the new vogue for popular science in France and England made invisible ink into entertainment, enacted in public in front of wondering eyes. Poor Jean-Jacques Rousseau experimented with a dangerous type of sympathetic ink in 1736. Macrakis writes that Rousseau probably heard of the recipe from a professor friend, or read about it in a book of recreational experiments. The ink was made with quicklime and orpiment (a rare mineral that’s pigmented orange or yellow, and contains arsenic sulfide). When the philosopher mixed the two, the bottle began to fizz uncontrollably, and eventually exploded in his face. “He swallowed so much chalk and orpiment that it nearly killed him,” Macrakis writes. “He couldn’t see for more than six weeks.”

5. Washington's "Medicine"

The Culper Spy Ring were agents for George Washington who circulated in occupied New York City from 1778 to 1783. The group, recruited by Major Benjamin Tallmadge, used pseudonyms and numerical codes to pass information, fearful as they were of being discovered. They also used invisible ink, manufactured for Washington by James Jay, a doctor who was John Jay’s older brother. This combination of precautions meant that they all got through the war without being discovered, and managed to feed Washington some valuable bits of strategic information. Jay didn’t record the chemical makeup of the fluid, which he and Washington called “the medicine” in letters to each other. In the 1930s, curious doctor and photographer Dr. Lodewyk Bendikson performed ultraviolet and infrared tests on letters written using Jay’s invisible ink. Bendikson found that Jay’s formula was an old one: tannic acid from gallnuts, developed with ferrous sulfate.

6. When Life Hands You Lemons


In World War I, several so-called “lemon juice spies”—German agents operating in England—used citrus as their means of communication. The British government had stepped up its censorship of letters in wartime. One agent, Mabel Beatrice Elliot, flagged letters written by three of these men, heated them up, and unmasked them as spies. The lemon-juice operation was a clumsy one: several spies, once caught, had lemons on their persons, or pens with pulp still stuck on the nibs. In the end, the British executed 11 German spies in the Tower of London in 1915; four of them had used lemon juice. “After the painful and visible loss of … the lemon juice spies,” Macrakis writes, “the Germans began to develop more sophisticated invisible ink methods.”

7. Bacon's Blunder

National Archives

George Vaux Bacon, an American journalist recruited by Germans to spy in Britain, was the beneficiary of these new ideas. He smuggled a new kind of invisible ink into the country in a novel way. Bacon’s handler asked him to buy black socks, and then impregnated the socks with the paste-like ink, telling him to soak the socks in water once he reached his destination. Bacon was quickly nabbed, after censors suspected his letters based on their address in the Netherlands. The substance on his socks was Argyrol, a drug used as an antiseptic and anti-bacterial that contains a mild silver protein. Bacon didn’t know how to develop the ink, which was totally new to British censors; eventually, British and French chemists figured out a method using electrolysis. A British court condemned Bacon to death, but his sentence was commuted in exchange for his testimony against his German handlers. Later, he claimed that the whole episode had been a stunt, meant to result in a great magazine story about spying.

8. Flu Ink

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During World War II, chemist Linus Pauling worked on an unusual wartime project, formulating new kinds of invisible ink that would resist all known reagents. Pauling and his colleagues experimented with invisible inks made from pneumococcus bacteria—inert, in this preparation, and so unable to spread pneumonia. The ink-ified microbe would react to an antibody, and then become visible once dipped in a dye solution. The ink never passed the experimental stage; neither did the various inks made with radioactive isotopes that were tested by MIT physicist Robley Evans.

9. What is This? Why is it Here?

James Stockdale, a Navy pilot, was shot down over North Vietnam in 1965 and sent to the “Hanoi Hilton,” where he was to stay for seven and a half years. With the help of U.S. Naval Intelligence, his wife Sybil initiated secret communications with Stockdale by enclosing a photograph of her mother in a letter to him. He was confused, but (as he said later) he thought: “It’s dumb to throw away something from the States without doing more with it. James Bond would soak it in piss and see if a message came out of it.” So he did. After it dried, print appeared on the back, establishing the code that he later used to communicate with the Navy, informing them of conditions at the prison.

10. Ultraviolet Exposure

The East German secret police—the Stasi—prescreened 90,000 pieces of mail every day during the 1980s, using an elaborate conveyor belt system. Agents steamed open letters in bulk, identified suspicious pieces with indentations or scratch marks, then glued them shut, in assembly-line fashion. Renate Murk, a Stasi captain who examined the letters that were under suspicion, used new technology also favored by the CIA to reveal secret writing without using a reagent. (If you chemically develop a letter, that’s an irreversible process, and you can’t send the letter on to its intended recipient; any element of surprise is lost.) Murk used ultraviolet scanners and the Nyom invisible impressions detector to find invisible writing without developing the letter.

11. Prison Speak

In the 1990s, the Aryan Brotherhood used citrus juice and urine to send messages between prisons, orchestrating violent action. In 1997, Brotherhood leader T.D. Bingham, imprisoned at the Supermax prison in Fremont County, Colorado, sent a letter to an outside courier, who passed it to Brotherhood members imprisoned in Lewisburg, Penn. The letter was written in urine, and revealed its secrets upon being “toasted” over a flame. The message: “War with DC Blacks, T.D.” When Bingham and other Brotherhood leaders were tried for ordering this and other attacks in 2006, embarrassed government censors had to admit that they’d missed the message completely.

Sergeant Marshall/Department of Defense, NARA // Public Domain
Would You Be Able to Pass a World War I Military Literacy Test?
Sergeant Marshall/Department of Defense, NARA // Public Domain
Sergeant Marshall/Department of Defense, NARA // Public Domain

Though reading and writing might not come to mind as the first requirement for trench warfare, during the early 20th century, the U.S. Army became increasingly concerned with whether or not its soldiers were literate. Thousands of World War I soldiers couldn't read printed directions on basic military tasks. The Army didn't implement its first major literacy program until the 1940s, but literacy tests were included in a battery of psychological evaluations World War I recruits went through to determine their mental fitness and intelligence, as the blog Futility Closet recently highlighted.

These unconventional literacy tests largely took the form of a yes or no questions with obvious answers, according to the 1921 report from the U.S. Surgeon General, Psychological Examining in the United States Army. Edited by pioneering intelligence-testing psychologist Robert Yerkes, who developed the military's first psychology exams for new recruits (and was also famous for his support for eugenics), the volume is a lengthy compilation of all of the methods the U.S. Army used to test the intelligence of its future soldiers. Many of these tests are now considered racist and culturally biased—some of the "intelligence" testing questions required recruits to know things like what products Velvet Joe (a figure used in tobacco campaigns) advertised—but some of the literacy questions, in particular, simply come off as weird in the modern era. Some are downright existential, in fact, while others—"Is a guitar a disease?"—come off as almost poetic.

A long questionnaire to test literacy, including questions like 'Is coal white?'
Psychological Examining in the United States Army, Google Books // Public Domain

One test, the Devens Literarcy Test, asked recruits questions like "Is genuine happiness a priceless treasure?" and "Does success tend to bring pleasure?" Another section of the test asked "Do boys like to play?" and "Do clerks enjoy a vacation?"

Other questions seem like they're up for debate, like "Are painters ever artless individuals?" and "Is extremely athletic exercise surely necessary?" Surely the answers to questions like "Should criminals forfeit liberty?" and "Is misuse of money an evil?" depend on the opinions of the reader. The answer to "Do imbeciles usually hold responsible offices?" might be different depending on how the person feels about their Congressional representative, and could surely be the spark for an hour-long argument at most dinner parties.

Still others are tests of cultural knowledge, not reading skill—a major modern criticism of Yerkes's work. Despite being arguably a pretty literate person, I certainly don't know the answer to the question "Do voluntary enlistments increase the army?" A question like "Are 'diminutive' and 'Lilliputian' nearly identical?" isn't exactly a test of literacy, but a test of whether or not you've read Gulliver's Travels, which doesn't exactly seem like a necessity for military success.

Luckily, some of the questions are pretty obvious, like "Is coal white?" That one I can answer. The full list of questions used in the various versions of the Devens test is below for you to test your own Army-level literacy.

  • Do dogs bark?
  • Is coal white?
  • Can you see?
  • Do men eat stones?
  • Do boys like to play?
  • Can a bed run?
  • Do books have hands?
  • Is ice hot?
  • Do winds blow?
  • Have all girls the same name?
  • Is warm clothing good for winter?
  • Is this page of paper white?
  • Are railroad tickets free?
  • Is every young woman a teacher?
  • Is it always perfect weather?
  • Is the heart within the body?
  • Do clerks enjoy a vacation?
  • Is the President a public official?
  • Would you enjoy losing a fortune?
  • Does an auto sometimes need repair?
  • Is it important to remember commands?
  • Are avenues usually paved with oxygen?
  • Do we desire serious trouble?
  • Is practical judgment valuable?
  • Ought a man's career to be ruined by accidents?
  • Do you cordially recommend forgery?
  • Does an emergency require immediate decision?
  • Should honesty bring misfortune to its possessor?
  • Are gradual improvements worth while?
  • Is a punctual person continually tardy?
  • Are instantaneous effects invariably rapid?
  • Should preliminary disappointment discourage you?
  • Is hearsay testimony trustworthy evidence?
  • Is wisdom characteristic of the best authorities?
  • Is extremely athletic exercise surely necessary?
  • Is incessant discussion usually boresome?
  • Are algebraic symbols ever found in manuals?
  • Are tentative regulations ever advantageous?
  • Are "diminutive" and "Lilliputian" nearly identical?
  • Is an infinitesimal titanic bulk possible?
  • Do all connubial unions eventuate felicitously?
  • Is a "gelatinous exaltation" ridiculous?
  • Are "sedate" and "hilarious" similar in meaning?
  • Is avarice sometimes exhibited by cameos?
  • Can a dog run?
  • Is water dry?
  • Can you read?
  • Do stones talk?
  • Do books eat?
  • Do cats go to school?
  • Are six more than two?
  • Is John a girl's name?
  • Are there letters in a word?
  • Is your nose on your face?
  • Can you carry water in a sieve?
  • Do soldiers wear uniforms?
  • Does it rain every morning?
  • Are newspapers made of iron?
  • Are "forward" and "backward" directions?
  • Do many people attend motion-picture theatres?
  • Do handkerchiefs frequently injure human beings?
  • Do magazines contain advertisements?
  • Are political questions often the subject of debates?
  • Are empires inclosed in envelopes?
  • Are members of the family usually regarded as guests?
  • Is genuine happiness a priceless treasure?
  • Do imbeciles usually hold responsible offices?
  • May chimneys be snipped off with scissors?
  • Is moderation a desirable virtue?
  • Are apish manners desired by a hostess?
  • Do conscientious brunettes exist?
  • Do serpents make oblong echoes?
  • Do voluntary enlistments increase the army?
  • Is hypocrisy approved by honest men?
  • Is virile behavior effeminate?
  • Do alleged facts often require verification?
  • Do pestilences ordinarily bestow great benefit?
  • Are painters ever artless individuals?
  • Do the defenders of citadels sometimes capitulate?
  • Do physicians ameliorate pathological conditions?
  • Is embezzlement a serious misdemeanor?
  • Do vagrants commonly possess immaculate cravats?
  • Are "loquacious" and "voluble" opposite in meaning?
  • May heresies arise among the laity?
  • Are piscatorial activities necessarily lucrative?
  • Do tendrils terminate in cerebral hemorrhages?
  • Does a baby cry?
  • Can a hat speak?
  • Do hens lay eggs?
  • Is a stone soft?
  • Is one more than seven?
  • Do the land and sea look just alike?
  • Are some books black?
  • Does water run up hill?
  • Are stamps used on letters?
  • Do 100 cents make a dollar?
  • Are we sure what events will happen next year?
  • Do ships sail on railroads?
  • Do stones float in the air?
  • May meat be cut with a knife?
  • Are ledges common in mountain districts?
  • Does success tend to bring pleasure?
  • Are diamonds mined in mid-ocean?
  • Is misuse of money an evil?
  • Should criminals forfeit liberty?
  • Is special information usually a disadvantage?
  • Are attempted suicides always fatal?
  • Are exalted positions held by distinguished men?
  • Does confusion favor the establishment of order?
  • Is a civil answer contrary to law?
  • Is a dilapidated garment nevertheless clothing?
  • Are textile manufacturers valueless?
  • Do thieves commit depredations?
  • Does close inspection handicap accurate report?
  • Do transparent goggles transmit light?
  • Do illiterate men read romances?
  • Is irony connected with blast furnaces?
  • Do avalanches ever descend mountains?
  • Are scythes always swung by swarthy men?
  • Do pirates accumulate booty?
  • Are intervals of repose appreciated?
  • Are intermittent sounds discontinuous?
  • Is an avocational activity ordinarily pleasurable?
  • Are pernicious pedestrians translucent?
  • Are amicable relationships disrupted by increased congeniality?
  • Are many nocturnal raids surreptitiously planned
  • Are milksops likely to perpetrate violent offenses?
  • Are "precipitancy" and "procrastination" synonymous?
  • Is snow cold?
  • Can a dog read?
  • Do houses have doors?
  • Has a horse five legs?
  • Are three more than ten?
  • Do mice love cats?
  • Does a hat belong to you?
  • Do animals have glass eyes?
  • Should fathers provide clothing for children?
  • Is it true that lead is heavy
  • Do poor men have much money?
  • Is summer colder than winter?
  • Can a horse tell time by a watch?
  • Is a city larger than a country town?
  • Does Christmas ever fall on Tuesday?
  • Do Christians often overlook faults?
  • Are difficult problems easily solved?
  • Do convicts sometimes escape from prison?
  • Should the courts secure justice for everybody?
  • Are scoundrels always intoxicated?
  • Is a guitar a kind of disease?
  • Do jugglers furnish entertainment?
  • Should we build on insecure foundations?
  • Do annual conventions take place biweekly?
  • Does persistent effort favor ultimate success?
  • Is a shrewd man necessarily admired?
  • Is manual skill advantageous?
  • Are elaborate bonnets inexpensive?
  • Are petty annoyances irritating?
  • Are false arguments valid?
  • Do you approve of ruthless massacres?
  • Do blemishes occur in complexions?
  • Is air found in a complete vacuum?
  • Do robins migrate periodically?
  • Are weird tales sometimes gruesome?
  • Do felines possess locomotor appendages?
  • Do demented individuals frequently have hallucinations?
  • Are laconic messages sometimes verbose?
  • Are perfunctory endeavors usually efficacious?
  • Would a deluge extinguish a smouldering trellis?
  • Are devastated suburbs exhilarating vistas?
  • Are "contingent" and "independent" alike in meaning?

[h/t Futility Closet]

10 Not-So-Small Facts About the Volkswagen Beetle

While Volkswagen has announced—for a second time—that it's going to cease production on the Beetle, people are still singing the praises of the quirky little car. Here are 10 not-so-small things you need to know about the German car that was once named one of the top four cars of the century.


Adolf Hitler checks out a VW Beetle
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It’s long been said that Adolf Hitler was the man behind the Beetle, and that’s sort of true. The dictator wanted German families to be able to afford a car, so he enlisted automaker Ferdinand Porsche (yes, that Porsche) to make “the people’s car.” But the basis for the Beetle had been around since long before Hitler’s demand; the Bug was heavily influenced by Porsche's V series. Rumors that Hitler directly designed the car are probably false; though he was the one who reportedly said that the car should look like a beetle, because “You only have to observe nature to learn how best to achieve streamlining,” it’s likely that he was regurgitating something he had read in an automotive magazine. Still, one thing is for certain: Hitler himself placed the cornerstone for the Porsche factory in Wolfsburg, Germany.


Perhaps still wary of anything imported from Germany, Americans shunned the Beetle when it was introduced in the States in 1949: Only two were sold in the first year. But after that, sales grew quickly. By the 1960s, hundreds of thousands of Bugs were sold every year, topping out at 570,000 in 1970.


A pink VW Beetle

We have the public to thank for the car’s distinctive nickname. Originally known as the Volkswagen Type 1, the car’s curves and rounded top led to its later, insect-like moniker. Volkswagen must have realized they had a good thing on their hands, because they started referring to the car as the VW Beetle in the late 1960s.


The UK and the U.S. aren’t the only countries that bestowed a new name on the Volkswagen Type 1. In France, it's called Coccinellewhich means ladybug. It's Maggiolino and Fusca in Italy and Brazil, respectively, both of which mean "beetle." Mexico calls it Vocho; it's Peta (turtle) in Bolivia; and Kodok (frog) in Indonesia. 


In 1999, Advertising Age declared the car's not-so-small ad campaign to be the best campaign of the last 100 years, besting Coca-Cola, Marlboro, Nike, and McDonald’s. The quirky concept and copy—which, according to Advertising Age, “Gave advertising permission to surprise, to defy and to engage the consumer without bludgeoning him about the face and body”—was a game-changer for the entire industry.

The "Think Small" line and accompanying self-deprecating copy was written by Julian Koenig, who was also responsible for naming Earth Day and coming up with Timex’s “It takes a licking and keeps on ticking” tagline. He’s also half-responsible for daughter Sarah Koenig, whom you may know from NPR’s This American Life and Serial.


Herbie the Love Bug

Because of their distinctive aesthetic, VW Bugs have been associated with everything from the Beatles to Transformers. A few highlights:

  • The Beetle with the license plate “LMW 28IF” on the cover of The Beatles' Abbey Road album was sold at an auction for $23,000 in 1986. It is now on display at Volkswagen's AutoMuseum at the company’s headquarters in Wolfsburg, Germany.
  • The Fremont Troll sculpture in Seattle, a huge statue lurking under the Aurora Bridge, clutches an actual VW Beetle. An in-progress picture shows that the car was once red. It also once contained a time capsule of Elvis memorabilia, which was stolen.
  • The Herbie the Love Bug series was a big hit for Disney in the late 1960s and early 1970s. One of the original Herbies sold for $126,500 at an auction in 2015.
  • In the original Transformers cartoon, Bumblebee transformed from a VW Bug. The car was changed to a Camaro for the live-action movies.


The so-called “blumenvasen,” a small vase that could be clipped to the dashboard, speaker grille, or windshield, was porcelain when it was originally offered. The nod to flower power became such a symbol of the car that it was incorporated into the 1998 redesign. Sadly, it didn’t make the cut for the most recent overhaul: The vase was eliminated in 2011 by marketing execs apparently seeking to make the car more male-friendly.


When the millionth VW Beetle rolled off the line in 1955, the company capped the achievement by plating the car in gold and giving it diamante accents. They also created a Bug with a wicker body in collaboration with master basket-maker Thomas Heinrich.


After WWII, the VW factory in Wolfsburg, Germany, was supposed to be handed over to the British. No British car manufacturer wanted to take responsibility for the company, though, saying that "the vehicle does not meet the fundamental technical requirement of a motor-car," "it is quite unattractive to the average buyer," and that "To build the car commercially would be a completely uneconomic enterprise." Whoops.


The last VW Bug
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Beetle #21,529,464—the one celebrated by the mariachi band—is now at Volkswagen's AutoMuseum.


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