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Scientists read 300-year-old sealed letter without opening it

"Virtual unfolding" allows researchers to look inside intricately folded historical letters without damaging them.

Letter opening
Scientists are using technology to read centuries-old letters sealed using "letterlocking."
Nature Communications

In a letter, dated July 31, 1697, Jacques Sennacques asks his cousin Pierre Le Pers, a French merchant in The Hague, for a certified copy of a death notice for Daniel Le Pers. That's no revelatory request by any means, but for the past 300 years, the letter has remained sealed away, its contents unseen. 

Now, thanks to a technique that let scholars peek inside virtually without damaging the intricately folded historical document, Sennacques' request has been uncovered. The new technique could hold promise for unlocking sealed correspondence containing historical gems across time and place.

All those years ago, Sennacques' letter was closed using a process called "letterlocking," a complex folding technique used globally to secure post before the invention of envelopes. Think of it like ancient encryption: Letters sealed this way couldn't be opened without getting torn, and rips indicated a note had been tampered with before reaching the intended recipient. 

"Letterlocking was an everyday activity for centuries, across cultures, borders and social classes," said Jana Dambrogio, the Thomas F. Peterson Conservator at MIT Libraries and one of the authors of a paper published Tuesday in the journal Nature Communications that details the virtual unlocking technique. 

No paper was damaged in the reading of this letter: It was unfolded virtually. 

Nature Communications

Letterlocking played an integral role in securing physical communications before the age of modern digital cryptography. Some of the earliest letterlocking examples can be found in the Vatican Secret Archives dating back to 1494. Researchers could have just torn the letter open, but they wanted to conserve all of its folds and creases, which themselves amount to evidence about communications practices. 

"This research takes us right into the heart of a locked letter," Dambrogio said in a statement. 

To unlock the letter, an interdisciplinary team of researchers from MIT and King's College London turned to advanced X-ray machines designed for dentistry to produce high-resolution 3D scans that showed exactly how the paper is configured. An automated computational algorithm developed by one former and one current MIT student then produced legible images of the letter's contents and intricate crease patterns. 

"Virtual unfolding is a computational process that analyzes CT scans of folded letterpackets and creates a flattened image of their contents," the team said. "Our virtual unfolding pipeline generates a 3D reconstruction of the folded letter, a corresponding 2D reconstruction representing its flat state and flat images of both the surface ... and each letterpacket's crease pattern." 

Computational algorithms have been successfully applied to scans of scrolls, books and documents with one or two folds. But the complexity of the letterlocked documents posed their own challenges. 

The letter came from the Brienne Collection, a European postmaster's wooden trunk that contained 3,148 items, including 577 letters that were never unlocked. The research team unlocked several letters using their new technique and believes it holds promise for many other unopened letters. 

"One important example is the hundreds of unopened items among the 160,000 undelivered letters in the Prize Papers, an archive of documents confiscated by the British from enemy ships between the 17th and 19th centuries," the study reads. "If these can be read without physically opening them, much rare letterlocking data can be preserved." 

Before the researchers' computational analysis, they only knew the name of the intended recipient written on the outside of the locked letter. 

"When we got back the first scans of the letter packets, we were instantly hooked," said Amanda Ghassaei, who helped write the publicly available code for virtually unfolding the letters. "Sealed letters are very intriguing objects, and these examples are particularly interesting because of the special attention paid to securing them shut." 

Let the epistolary history unfold. 

CNET's Corinne Reichert contributed to this report.