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11 Historical Uses for Invisible Ink

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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

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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.

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iStock // Ekaterina Minaeva
Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
May 21, 2017
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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Opening Ceremony
These $425 Jeans Can Turn Into Jorts
May 19, 2017
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Opening Ceremony

Modular clothing used to consist of something simple, like a reversible jacket. Today, it’s a $425 pair of detachable jeans.

Apparel retailer Opening Ceremony recently debuted a pair of “2 in 1 Y/Project” trousers that look fairly peculiar. The legs are held to the crotch by a pair of loops, creating a disjointed C-3PO effect. Undo the loops and you can now remove the legs entirely, leaving a pair of jean shorts in their wake. The result goes from this:


Opening Ceremony

To this:


Opening Ceremony

The company also offers a slightly different cut with button tabs in black for $460. If these aren’t audacious enough for you, the Y/Project line includes jumpsuits with removable legs and garter-equipped jeans.

[h/t Mashable]