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A rare unopened example of a letterpacket with a paper lock. This locking mechanism was used since the 1500s, but until now, only opened and “unlocked” examples had been seen in archival collections. Virtual unfolding algorithms helped to determine that the packet was locked using a low-security technique without tearing open the letter. The letters are from the Brienne Collection, Sound and Vision at the Hague in the Netherlands. (all images courtesy the Unlocking History Research Group archive)

A dead letter office has sprung vividly to life, as researchers who specialize in the ancient practice of “letterlocking” reveal the contents of letters that were previously impenetrable. The letters in question belong to the Brienne Collection, a legacy left in 1707 from the postal professionals Simon de Brienne and Marie Germain, in the form of a trunk of undeliverable letters. Among these are numerous missives sealed with a specific kind of epistolary origami.

Letterlocking is the term for a historical practice of folding letters so that they not only served as their own envelopes, but they signaled to the recipient whether or not they had been read along the way. By folding in specific sequences, known only to sender and recipient, any tampering with the letter or attempts to read it in passage would become obvious, in the form of tears or even entire chunks detached from the message.

Letterpacket DB-1627 was virtually unfolded and read for the first time since it was written 300 years ago. The letter contains a message from Jacques Sennacques, dated July 31, 1697, to his cousin Pierre Le Pers, a French merchant, for a certified copy of a death notice of Daniel Le Pers.  Also visible is a watermark in the center of the paper containing an image of a bird.

But this, of course, proffers a specific challenge to historians and researchers, who would dearly like to read the contents of these lost letters, but quail at the thought of destroying them in the process. However, recent strides have been made among letterlocking researchers. A recent article in Nature Communications details the use of X-ray microtomography (XMT) — routinely used by dentists to analyze variation in tooth composition) — to virtually unlock these intricately folded letters and read their contents without needing to physically dismantle them.

Computer-generated unfolding sequence of sealed letter DB-1538.

These efforts have yielded a treasure trove of information from previously inaccessible historical communications. The study of 250,000 letters — including those in the Brienne Collection — has produced the first systematization of letterlocking techniques thus cataloging a crucial and longstanding form of cryptography from the annals of human communication.

“One of our major results is to reveal for the first time the contents of DB-1627, a letter typical of day-to-day communications of the time,” the study says. “It contains a request from Jacques Sennacques, dated July 31, 1697, 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. Before computational analysis, we only knew the name of the intended recipient, written on the outside of the letterpacket.”

Computer-generated unfolding sequence of sealed letter DB-1538.

This work is a triumphant achievement for the Unlocking History research group, whose champions include researchers, lecturers, historians, engineers, and conservators. It’s also a boon for those with a desire to peek into previously unexplored crevices in history, and those who believe that no matter what, on a long enough timeline, the mail must come through.

CT scan cross-sections of the letters.
A 17th-century trunk of letters bequeathed to the Dutch postal museum in the Hague. The trunk belonged to one of the most active postmaster and postmistress of the day, Simon and Marie de Brienne, a couple at the heart of European communication networks. The chest contains an extraordinary archive: 2600 “locked” letters sent from all over Europe to this axis of communication, none of which was never delivered. Sealed letterpackets from this trunk were scanned by X-ray microtomography and “virtually unfolded” to reveal their contents for the first time in centuries. The trunk is part of the Brienne Collection. (Courtesy of the Sound and Vision The Hague, The Netherlands.)

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Sarah Rose Sharp is a Detroit-based writer, activist, and multimedia artist. She has shown work in New York, Seattle, Columbus and Toledo, OH, and Detroit — including at the Detroit Institute of Arts....