In 1697, a man named Jacques Sennacque wrote a letter to his cousin, a French merchant named Pierre Le Pers, requesting a certified death certificate for another man named Daniel Le Pers (presumably also a relation). Sennacque sealed the letter with an intricate folding method known as “letterlocking,” a type of physical cryptography, to safeguard the contents from prying eyes. That letter was never delivered or opened. More than 300 years later, researchers have virtually “unlocked” the letter to reveal its contents for the first time, right down to the watermark in the shape of a bird. They described their results in a new paper published in the journal Nature Communications.
Co-author Jana Dambrogio, a conservator at MIT Libraries, coined the term “letterlocking” after discovering such letters while a fellow at the Vatican Secret Archives in 2000. The Vatican letters dated back to the 15th and 16th centuries, and they featured strange slits and corners that had been sliced off. Dambrogio realized that the letters had originally been folded in an ingenious manner, essentially “locked” by inserting a slice of the paper into a slit, then sealing it with wax. It would not have been possible to open the letter without ripping that slice of paper—evidence that the letter had been tampered with.
Dambrogio has been studying the practice of letterlocking ever since, often creating her own models to showcase different techniques. The practice dates back to the 13th century—at least in Western history—and there are many different folding and locking techniques that emerged over the centuries. Queen Elizabeth I, Machiavelli, Galileo Galilei, and Marie Antoinette are among the famous personages known to have employed letterlocking for their correspondence.
For instance, a February 8, 1587, letter from Mary, Queen of Scots, to her brother-in-law, King Henri III of France, was sealed using a so-called “butterfly lock”—just one of hundreds of locking techniques Dambrogio has compiled into a dictionary of letterlocking. Other techniques include a simple triangular fold-and-tuck and an ingenious method known as the “dagger-trap,” which incorporates a booby-trap disguised as another, simpler type of letter lock.
Individuals would often have their own unique style of letterlocking, most notably English poet John Donne, who used at least five different letterlocking styles, one unique to him, according to Dambrogio. “So we’ve got this guy who’s known as the most inventive and witty poet of his generation, and he’s doing one of the most inventive and witty and brilliant interlocking methods you could imagine,” she told Atlas Obscura in 2018. “That is the kind of evidence you can use to say, ‘Ah, so you can actually see something of people’s personalities in the way they fold letters.'”
In 2012, Dambrogio hit the jackpot: a Yale researcher named Rebekah Ahrendt found a 17th-century trunk of undelivered letters preserved in the postal museum at The Hague, the Netherlands. The trunk belonged to Simon and Marie de Brienne, a highly connected postmaster and postmistress of their day. Now known as the Brienne Collection, the trunk contains 2,600 “locked” letters sent from all over Europe, 600 of which had never been opened.
And therein lay the challenge. “Once a document such as an unopened letter is damaged in the opening process, we lose a sense of the object as untouched and intact,” the authors wrote in their paper. Virtually opening these letters helps preserve “the material evidence” about a given letter’s internal security, “including highly ephemeral evidence about tucks and layer order, which usually leave no material trace.”
So Dambrogio et al. turned to virtual “unwrapping” techniques, which are becoming increasing popular for the study of fragile historical documents. For instance, in 2016, an international team of scientists developed a method for virtually unrolling a badly damaged ancient scroll found on the western shore of the Dead Sea, revealing the first few verses from the book of Leviticus. The so-called En-Gedi scroll was recovered from the ark of an ancient synagogue destroyed by fire around 600 CE.
In 2019, we reported that German scientists used a combination of cutting-edge physics techniques to virtually “unfold” an ancient Egyptian papyrus, part of an extensive collection housed in the Berlin Egyptian Museum. Their analysis revealed that a seemingly blank patch on the papyrus actually contained characters written in what had become “invisible ink” after centuries of exposure to light. And earlier this year, we reported that scientists had used multispectral imaging on four supposedly blank Dead Sea Scrolls and found the scrolls contained hidden text, most likely a passage from the book of Ezekiel. While it took 10 years to determine that Mary, Queen of Scots’ final letter employed a letterlocking technique, Dambrogio et al. assert that their new virtual unfolding method could make the same determination in mere days.
For the first stage of analysis of the Brienne Collection letters, Dambrogio created her own test set of 10 model letters, which were then imaged using X-ray tomography by collaborators at Queen Mary University of London’s dental research labs, along with four original letters from the trunk. The scanner in question was designed to be especially sensitive to mapping the mineral content of teeth, but it works just as well on certain types of ink in old paper and parchment. This was followed by the painstaking process of developing the algorithms to identify and separate different layers of the folded letters, enabling Dambrogio et al. to virtually unfold and “read” the unopened letters. It also allowed them to better explore the various complicated folding systems of each letter because the algorithm can visualize crease patterns.
“We’ve been able to use our scanners to X-ray history,” said co-author David Mills of Queen Mary University of London. “The scanning technology is similar to medical CT scanners, but using much more intense X-rays which allow us to see the minute traces of metal in the ink used to write these letters. The rest of the team was then able to take our scan images and turn them into letters they could open virtually and read for the first time in over 300 years.”
In addition to finally reading Jacques Sennacque’s 1697 letter, the team found evidence for tracing the evolution of the letterlocking technology. The letters in the Brienne Collection showed a marked shift in letterlocking techniques over time, moving away from the “fold, tuck, and adhere” method, for example, to more of a “fold and adhere” approach that seems to foreshadow the modern envelope. Subsequent analysis is likely to reveal even deeper historical insights.
“Conserving intact these records of human interaction with materials, while making their secrets visible, enables a new perspective on history that is both kinetic and tactile, and which encourages new ways of thinking about the lives, emotions, and creativity of historical individuals and communities,” the authors wrote. “Doing so also challenges cultural historians to reconceptualize hidden, secret, and inaccessible materials as sites of critical inquiry. Letterlocking and virtual unfolding point to the ways that history sometimes resists scrutiny, and that resistance itself deserves patient study.”
One likely source for further study is hundreds of unopened, undelivered letters in an archive known as the Prize Papers—all confiscated by the British from enemy shops between the 17th and 19th centuries. Ultimately, “We envision a thorough, data-driven study, encompassing tens of thousands of known unopened letters plus millions more opened letters, drawing together letterlocking data globally to make persuasive, consequential statements about historical epistolary security trends,” the authors concluded. “By synthesizing traditional and computational conservation techniques, we can help further integrate computational tools into conservation and the humanities—and show that letters are all the more revealing when left unopened.”
Listing image by Unlocking History Research Group archive