Scientists Digitally Unfold 300-Year-Old Sealed Letters

by johnsmith

Using a highly sensitive X-ray microtomography scanner, a team of researchers has scanned four unopened letters from the Brienne Collection, a 17th-century postal trunk full of undelivered mail.

Computer-generated virtual unfolding of the sealed letter DB-1538 from the Brienne Collection. Image credit: Dambrogio et al., doi: 10.1038/s41467-021-21326-w.

Computer-generated virtual unfolding of the sealed letter DB-1538 from the Brienne Collection. Image credit: Dambrogio et al., doi: 10.1038/s41467-021-21326-w.

The letter is one of the most important communication technologies in human history.

Before the proliferation of mass-produced envelopes in the 1830s, most letters were sent via letterlocking, the process of folding and securing writing substrates to become their own envelopes.

Letterlocking was an everyday activity for centuries, across cultures, borders, and social classes, and plays an integral role in the history of secrecy systems as the missing link between physical communications security techniques from the ancient world and modern digital cryptography.

The Brienne Collection, a European postmaster’s trunk preserving 300-year-old undelivered post, provides a rare opportunity to study sealed letters.

It contains 3,148 cataloged items, including 2,571 opened letters, fragments, and other documents, and 577 letterpackets that have never been opened.

Until now, an analysis of such sealed letters has been limited by the standard archival practice of cutting open sealed letters on request, compromising the physical integrity of the unopened letterpacket.

Professor Graham Davis from Queen Mary University of London and colleagues propose an alternate approach grounded in computational analysis, where letters remain intact in their locked state, yet the researchers can still read their contents.

“We designed our X-ray scanner to have unprecedented sensitivity for mapping the mineral content of teeth, which is invaluable in dental research,” Professor Davis said.

“But this high sensitivity has also made it possible to resolve certain types of ink in paper and parchment. It’s incredible to think that a scanner designed to look at teeth has taken us this far.”

“We’ve been able to use our scanners to X-ray history,” said Dr. David Mills, also from 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 scientists revealed the contents of a letter dated July 31, 1697. It contains a request from Jacques Sennacques to his cousin Pierre Le Pers, a French merchant in The Hague, for a certified copy of a death notice of one Daniel Le Pers.

Following the X-ray microtomography scanning of the letter packets, the authors then applied computational algorithms to the scan images to identify and separate the different layers of the folded letter and ‘virtually unfold’ it.

“This algorithm takes us right into the heart of a locked letter,” they said.

“Sometimes the past resists scrutiny. We could simply have cut these letters open, but instead we took the time to study them for their hidden, secret, and inaccessible qualities.”

“We’ve learned that letters can be a lot more revealing when they are left unopened.”

“Using virtual unfolding to read an intimate story that has never seen the light of day — and never even reached its recipient — is truly extraordinary.”

The research is described in a paper in the journal Nature Communications.


J. Dambrogio et al. 2021. Unlocking history through automated virtual unfolding of sealed documents imaged by X-ray microtomography. Nat Commun 12, 1184; doi: 10.1038/s41467-021-21326-w

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