CAMBRIDGE, Mass. — Scientists were able to read a sealed letter from the 17th century for the first time — without even opening it! The team digitally “unfolded” the letter using a dental scanner. They used the highly sensitive device to “virtually unfold” the document dating back more than 300 years, protecting its prized seal.
Ingenious origami-style “letterlocking” was common for secure communication during the 17th century, long before modern envelopes came into use. The so-secured packets could only be studied by cutting them open — until the development of an automatic computational algorithm. The algorithm enabled the international team of scientists — including researchers from the Massachusetts Institute of Technology — to decipher the contents of the Brienne Collection, a postmaster’s trunk containing 2,600 undelivered letters, almost 600 of which are still fastened.
The chest belonged to Simon and Marie de Brienne, a couple at the heart of the European communication network. It was bequeathed to the Dutch postal museum.
The letters were sent from all over Europe to The Hague in the Netherlands between 1680 and 1706. The intended recipients either could not be found or refused to pay the outstanding postage costs, so Brienne held onto the documents, hoping someone would eventually pay for them.
Written in French, Spanish, Latin, Italian, Dutch, and English, some letters were penned by aristocrats. They offer a fascinating glimpse into the lives of Europeans at the time.
“Our work seeks to make an intervention in the conservation of cultural heritage. 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 explain in their paper. “The material evidence a letter preserves about its internal security — including highly ephemeral evidence about tucks and layer order, which usually leave no material trace — can now be retained for investigation. Our methods therefore create an opportunity for the heritage sector to protect the integrity of documents even where there is a need to access their contents.”
Power of X-ray microtomography ‘virtually unfolds’ the 17th century letters
The extraordinary archive includes a message from Jacques Sennacques dated 31 July, 1697, to his cousin Pierre Le Pers, a French merchant, for a certified copy of the death notice of a Daniel Le Pers. Also visible is a watermark in the center of the paper containing an image of a bird.
The letter gives a fascinating insight into the lives and concerns of ordinary people in a tumultuous period of European history, when correspondence networks held families, communities, and commerce together over vast distances. “We demonstrate our method on four packets from Renaissance Europe — reading the contents of one unopened letter for the first time,” the authors write. “Before, we only knew the name of the intended recipient, written on the outside of the letter packet.”
The scientists first scanned the quartet of folded letters using X-ray microtomography. The machine created cross-sections to produce a simulated 3D model.
“We designed our X-ray scanner to have unprecedented sensitivity for mapping the mineral content of teeth, which is invaluable in dental research,” says study co-author Professor Graham Davis from Queen Mary University of London, in a statement. “But this high sensitivity has also made it possible to resolve certain types of ink in paper and parchment. It is incredible to think a scanner designed to look at teeth has taken us this far.”
The algorithm, based on an analysis of 250,000 historical letters, identifies and separates the different layers through “reverse engineering.” Words become visible since ink produces a different contrast than paper. It allowed the authors not only to read them, but to see the crease patterns and discover the step-by-step locking process.
“We have been able to use our scanners to X-ray history,” says study co-author Dr. David Mills, also from Queen Mary University. “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 were 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.”
Similar methods have been used to virtually unroll and reconstruct famous manuscripts, such as the Dead Sea Scrolls.
‘Letterlocking’ an early form of security and privacy
The study sheds fresh light on historical communication security. The letters have unique wax seals, which capture the fingerprints of some senders. They also turn over and overlap to be their own envelopes, preventing busybodies and spies from reading their secrets.
“The challenge is to reconstruct the intricate folds, tucks, and slits of unopened letters secured shut with ‘letterlocking’ — a practice which underpinned global communications security for centuries before modern envelopes,” the study explains. Before the proliferation of mass-produced envelopes in the 1830s a letter, one of the most important communication technologies in human history, was sent using “letterlocking.”
“It 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 authors write.
‘Little time capsules of information’
Virtual unfolding could now be used on hundreds of unopened items in the Prize Papers, an archive of documents confiscated by the British from enemy ships between the 17th and 19th centuries. The scientists hope the project called “Signed, Sealed, & Undelivered” will contribute to broader studies of politics, religion, migration, music, drama, and postal networks in early modern Europe.
“This algorithm takes us right into the heart of a locked letter. 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,” says co-author Dr. Jana Dambrogio, from the Wunsch Conservation Laboratory at MIT Libraries, in a statement per South West News Service.
“We have 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,” she adds, describing the letters as “little time capsules of information.”
The findings are published in Nature Communications.
South West News Service writer Mark Waghorn contributed to this report.