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Friday, May 17, 2024

Sleuths Read Old Booby-Trapped Letters Without Opening Them

On July 31, 1697, Jacques Sennacques sent a letter to his cousin—one Pierre Le Pers, a French merchant living in the Hague—begging him, for the love of Pete (that’s paraphrased), to send him a death certificate for his relative, Daniel Le Pers. In a 17th century version of the dreaded “as per my previous email,” Sennacques wrote: “I am writing to you a second time in order to remind you of the pains that I took on your behalf.” Basically, you owe me a favor, and I’ve come to collect.

Sennacques put down his pen and intricately folded the letter, turning it into its own envelope. Today, historians call this technique “letterlocking.” In Sennacques’ time, people had come up with a galaxy of different ways to fold their letters—some so characteristic, in fact, that they acted as a kind of signature for the sender. They weren’t doing this because they wanted to save money on envelopes, mind you, but because they wanted privacy. By folding the paper and tucking corners, they could arrange it in such a way that to open the correspondence, the reader had to rip it in certain places. If the intended recipient opened the letter and found it already torn, they’d know a snoop had gotten inside. Whole bits of paper might rip off, so if they opened the letter and didn’t feel or hear any tearing, yet a chunk still fell out, they’d know they weren’t the first person to read its contents.

It was the early modern period’s version of one of those seals that voids a device’s warranty if you break it. Unlike the self-destructing messages from Mission Impossible, you could still read a torn letter, and if you were familiar with the technique of the person who sent it to you, you might even know tricks to avoid tearing it in the first place. Yet the letterlocking set booby traps that exposed spies.

Unfortunately for all parties involved, Sennacques’ second letter never made it to his merchant cousin. Instead, it ended up in a trunk, known as the Brienne Collection, which contains 2,600 letters sent between 1689 and 1706 from across Europe to the Hague. Sennacques’ letter is one of hundreds that remain unopened, folded tightly in on itself.

How, then, do we know that the man was losing patience with his cousin? Writing today in the journal Nature Communications, researchers describe how they used an advanced 3D imaging technique—originally designed to map the mineral content of teeth—to scan four old letters from the Brienne Collection to unfold them virtually, no tearing required. “The letters in his trunk are so poignant, they tell such important stories about family and loss and love and religion,” says King’s College London literary historian Daniel Starza Smith, a coauthor of the paper. “But also, what letterlocking is doing is giving us a language to talk about sorts of technologies of human communication security and secrecy and discretion and privacy.”

When Sennacques sent letters in the late 17th century, it wasn’t through a postal service like we have today. The recipient, not the sender, had to pay for the letter, and the markup was hefty. A postmaster then had a strong incentive to actually deliver the correspondence. “If you couldn't deliver the letter, you couldn’t get your dough,” says Rebekah Ahrendt, a music historian at Utrecht University and a coauthor on the paper. (Music historians love old correspondence, because—as musicians do now—17th century creative types traveled around to gigs. “Because they led such crazy itinerant lifestyles,” says Ahrendt, “they often didn't stay in one place long enough to ever get any mail.” When Ahrendt does find correspondence, like the letters that ended up in the Brienne Collection, she jumps at it.)

The Brienne Collection comes from the postal professionals Simon de Brienne and Marie Germain, who had hired a “totally neurotic” accountant to run their office for them, Ahrendt says. Maybe the accountant came up with this idea, or maybe it came from his bosses, but this post office started stockpiling the mail of any recipients they couldn’t find, an early version of a Dead Letter Office. “The idea was that if they kept the letters that weren't delivered, then somebody might eventually turn up for them,” at which point they’d get paid, Ahrendt says. And indeed this did, on occasion, work—they'd nicknamed their trunk “the piggy bank.”

Brienne died in 1707. He had no children, and his heirs would have been his family members in France. But he didn’t care much for them: He’d converted to Protestantism, and they remained Catholic. Because of the way postal delivery worked at the time, his trunk of undelivered mail was actually an asset—any number of the intended recipients could have come along and paid for their letters. Brienne couldn’t let this treasure chest fall into the hands of his Catholic relatives, so he bequeathed it to an orphanage. “He donated all his stuff out of spite,” says Ahrendt. That’s right: The orphaned letters ended up at an orphanage.

Eventually the Brienne Collection landed in the vaults of the Ministry of Finance in The Hague. “And then somehow some nerdy postage stamp people, like collectors, got wind of the fact that there was this chest of letters sitting in the Ministry of Finance,” says Ahrendt. “And they're like, ‘Hey, can we have this? Because we actually want to start a postal museum.’ And the Ministry of Finance was like, ‘OK, cool idea. You can have it.’”

Today, the Brienne Collection serves as an invaluable snapshot of life around the turn of the 17th century. Typically, you hear about the surviving letters written by royalty or authors or composers—you know, famous folk. People tend to hold on to that correspondence, given its value. But the Brienne Collection holds the correspondence of all manner of regular people from the period, including our friend Jacques Sennacques. “What's really remarkable about this collection is that it is letters from everybody,” says Ahrendt. “It's letters from people who were illiterate and had to hire a letter writer. It’s letters from mothers who are worried about their children. It’s letters from lovers who are left in the dust. It's all classes of society. That is truly remarkable.”

Hundreds of those letters, though, are letterlocked, which is where a little magic called X-ray microtomography comes in. Think of it like a CT scan, only at an extremely high resolution—we’re talking on the micron scale. Just as bones will show up brightly on a CT, so too does the ink used to write these letters from the early modern period. Back then, ink was infused with metals, particularly iron, which makes the writing glow against the darker background of paper.

With X-ray microtomography, these researchers could map out the writing, layer by layer. They also reconstructed the folded shape of each letter, then used geometry processing techniques to virtually “flatten” out the paper. “We can take the information of the brighter regions and the darker regions of the scan, and we can lay that out on an image,” says Adobe Research’s Amanda Ghassaei, a coauthor on the paper. “We can actually create a picture that looks like if you were to unfold this letter packet and lay it out flat, and take an image of it and see all the text.” (That’s what you see in the animation above.)

For each of the four letters the team studied, the senders folded the paper in unique ways. Sennacques, for instance, seems to have first folded the two corners on one side of the sheet inward, creating a sort of house shape. He then kept folding and tucking the paper in on itself until it formed a neat little rectangle.

The researchers can’t yet say exactly how many letterlocking techniques might actually have existed, but they do know that there’s a whole lot of them, and that a single individual might use more than one strategy. “Some had what seemed like easy-to-break-into techniques, and others required more steps, and seem to have more tamper evidence,” says MIT conservator researcher Jana Dambrogio, a coauthor on the study. “Like if somebody tore this open that it wasn't intended for, you couldn't undo a tear, but you had to tear the lock to get inside.”

That tearing would create both a feel and a sound to assure the intended recipient that they were the first to read the letter. (Today, you get the same kind of tactile and auditory signals from a pickle jar, which pops to indicate you’re the first one to open it.) That turns out to be invaluable data about how each letter might have been folded, so the researchers couldn’t just steam in and tear the things open. “The minute you open that letter, you lose the evidence of internal tucking, which we see a lot of in the flapping and tucking of the Brienne trunk,” says Dambrogio. It’s delicate origami that’s as ephemeral as a sneeze, she adds.

With this new imaging technique, researchers can both read letterlocked correspondence without opening it, and begin to better understand the incredible diversity of security strategies employed by the senders. At the time, a simple wax seal just wouldn’t do. “If you're a spymaster, you can heat up that seal, cut through it, open the letter, read the contents, and put it back,” says MIT computer scientist Erik Demaine, a coauthor of the research. Do it with enough care, and the recipient would never know. “But with fancy letterlocking techniques, you will forcibly rip some part of the paper, and then that will become detectable,” he says.

Some letterlockers folded the paper using complex patterns, making the letters particularly secure, even though their outsides looked plain. Some flipped that technique, making the outside of the letter look complex, without adding internal intricacies, making them less secure. It’s the equivalent of modern-day homeowners putting up alarm signs without actually having an alarm. “You can use it either to have a simple letter with super high security on the inside, or a letter that looks like it's high-security with super low security on the inside," says Holly Jackson, an undergrad at MIT and a coauthor on the new paper. "And there's a whole range of everything in between.”

Scanning letterlocked correspondence not only preserves this diversity of techniques, but also preserves the material itself. Even if you were able to unfold one of these letters and keep it relatively intact, repeated handling over time causes old paper to deteriorate. By contrast, these 3D scans allow researchers to peruse the material virtually however many times they want. “That makes it even more important, because so many more scholars, even interested laymen, can be allowed to actually access the information and give their opinion on what's in there,” says classical archaeologist Rubina Raja of Aarhus University, who wasn’t involved in the research but did peer-review the paper.

And so Jacques Sennacques’ secrets become extremely accessible because they were secrets. Let’s hope he appreciated irony.

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