October 22, 2021

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X-Rays Help Scientists Read ‘Letterlocked’ Renaissance Mail

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A computer-generated depiction of the letter’s step-by-step virtual unfolding.
Image: Courtesy of the Unlocking History Research Group archive.

In July 1697, Jacques Sennacques of Lille, France, scribbled off a missive to his merchant cousin, Pierre le Pers, in The Hague. The subject of discussion was a death certificate for their relative, a topic which the cousins had discussed previously but le Pers had neglected to follow up on. The letter was the Renaissance equivalent of a “per my previous email,” and it was only just read for the first time since it was sealed 324 years ago.

But though it was read, the letter remains unopened. It’s letterlocked, a term coined by MIT conservator Jana Dambrogio for letters that use specific folds and slits to seal themselves, without the need for an envelope. Letterlocking was the typical way of sealing messages in the days before mass-produced envelopes; Queen Elizabeth I of England had at least five different letterlocking variations for privatizing her correspondence.

In a unique application of the technology, Dambrogio’s team “unfolded” Sennacques’ epistle virtually using X-ray microtomography, which allowed the researchers to circumvent the often damaging process of a manual letter opening. The team’s research was published on Tuesday in the journal Nature Communications.

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A virtual unfolding of the centuries-old letter.
Gif: Courtesy of the Unlocking History Research Group archive.

“I remember a feeling of elation, as in, [oh my god] we finally did it,” said co-author Rebekah Ahrendt, a musicologist at Utrecht University, in an email. “Having worked with this collection for a number of years, the effect of ‘I’m probably the first person reading this since it was written’ has admittedly worn off a bit…That said, this letter is such a wonderful example of the concerns of normal people at this time.”

It’s not known why le Pers never got the letter—given his profession, he may have moved. But the sealed letter remained in the care of the chief postmasters of The Hague, Simone de Brienne, and his wife, Marie Germain. The couple didn’t discard the enclosed familial matter because in those days letters were purchased by recipients, not paid for by their senders. Some postmasters kept unclaimed letters in case someone eventually came along to buy them. The couple in charge of The Hague’s post were either hoarders or resolutely optimistic, because they held onto the letters until they died. Thousands of letters in Brienne and Germain’s charge were preserved in an old trunk, and 600 of them are unopened letterlocked messages; it’s an amazing assemblage of European conversation suspended in time, now called the Brienne Collection. The collection resides at The Hague’s Sound and Vision museum.

Shooting X-rays through the letter penned by Sennacques yielded the spread of iron-rich ink he jotted across each fold in the letter. The X-rays’ intensity were about a third of those used by the same machine for its original purpose—imaging teeth and bones.

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The team’s legible results, with the letter’s faint watermark at center.
Image: Courtesy of the Unlocking History Research Group archive.

“We start with a very high resolution CT scan of the folded letterpacket, basically a 3D x-ray image,” said co-lead author Amanda Ghassaei, the algorithm engineer lead on the project and who previously has worked on simulating the folds in origami, in an email. “From there, our algorithm detects individual layers of paper in the scan and reconstructs the folded geometry. This computational pipeline allows us to observe writing, watermarks, seals, internal folds, and any other information hidden inside the letterpacket without doing any damage to the original artifact.”

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But that wasn’t enough. The team also had to decrypt the folded letter, understanding which characters fell where in the unfolded version. To do this, they employed a computational flattening script, to deconstruct the letter without touching it. Though an imperceptible jumble of characters from the outside, sheathed in the khaki paper, the research team was able to extract the message without issue.

The research team didn’t describe any of the folded layout of Sennacques’ letter in code; the algorithm did the heavy geometrical lift.

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The Renaissance chest contains missives from around the world sent to The Hague.
Photo: Courtesy of the Unlocking History Research Group archive.

“The message and intricate internal mechanics of these letters are only known to us because they have been virtually reconstructed,” said co-author Holly Jackson, an undergraduate at MIT and an algorithm engineer on the project. “Our methods are fully automatic, unbiased to scan orientation, and require no prior knowledge about a letterpacket’s folded geometry.”

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So, to author the new paper, the team used X-rays to detect the layout of ink on a piece of centuries-old paper, they built and deployed an algorithm to unfold that paper virtually, and they described the contents of that letter alongside a complex dictionary for the diverse techniques of letterlocking as a greater practice in days before envelopes. Essentially, the work was threefold.

The sum of these efforts is a clear-cut plan of attack for the roughly 600 letterlocked items that remain in the chest. The qualms of cousins, marital disputes, state secrets—who knows?

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It’s as close as history can get to holding its breath.

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