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

The contents of a handwritten European letter sealed for 300 years are no longer a secret, thanks to a technique that allows researchers to peek inside with virtually no damage to the finely folded historical document.

In the letter of July 31, 1697, Jacques Sennacques asked his cousin Pierre Le Pers, French merchant in The Hague, for a certified true copy of a death notice from Daniel Le Pers. It’s not a historical reveal, but the technique that revealed the claim may hold promise for unlocking a sealed match containing historical gems across time and place.

All those years ago, the Sennacques letter was closed using a process called “letterlocking,” a complex folding technique used around the world to secure mail before the invention of envelopes. Think of it like ancient encryption: Letters sealed this way couldn’t be opened without being torn, and the tears indicated that a note had been tampered with before reaching the intended recipient.

“Locking down letters has been a daily activity for centuries, across cultures, borders and social classes,” said Jana Dambrogio, Thomas F. Peterson curator at MIT Libraries and one of the authors of a article published Tuesday in the journal Nature Communications which details the virtual unlocking technique.

Locking letters played a vital role in securing physical communications before the era of modern digital cryptography. Some of the earliest examples of letter locking can be found in the Vatican secret archives dating back to 1494. Researchers could have torn up the letter, but they wanted to keep all of its folds and folds, which themselves are evidence of communication practices. .

“This research takes us to the heart of a locked letter,” Dambrogio said in a statement.

To unlock the letter, an interdisciplinary team of researchers at MIT and King’s College London turned to advanced x-ray machines designed for dentistry to produce high-resolution 3D scans showing exactly how the paper is set up. An automated calculation algorithm developed by a former and current MIT student then produced readable images of the letter’s contents and intricate fold patterns.

“Virtual unfolding is a computational process that analyzes CT scans of folded packets of letters 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 the surface … and the fold pattern of each packet of letters.”

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

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

“A prominent example is the hundreds of unopened articles 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 said. “If these can be read without physically opening them, a lot of rare letter lock data can be retained.”

Prior to the researchers’ computer analysis, they only knew the name of the intended recipient written on the outside of the locked letter.

“When we got the first scans of the letter packets, we were instantly hooked,” said Amanda Ghassaei, who helped write the publicly available code to virtually unfold the letters. “Sealed letters are very intriguing objects, and these examples are particularly interesting because of the special attention paid to their closure.”

Let the epistolary story unfold.

CNET’s Corinne Reichert contributed to this report.