A Letter Sealed for Centuries Has Been Read—Without Even Opening It

Using computer algorithms and an X-ray scanner designed for dental research, an international team of researchers has unlocked the secrets of a tightly folded letter sealed since 1697—without opening it.

The “virtual unfolding” of the letter—the culmination of a four-year project described in a paper published Tuesday in the journal Nature Communications—points to a new line of historical research into the centuries-old practice of letterlocking. That’s the term used to describe the use of origami-like folds to hide the content of letters before envelopes came into wide use in the mid-1800s.

“This is a dream come true in the field of conservation,” said Jana Dambrogio, the conservator at the research library at the Massachusetts Institute of Technology and one of 11 authors of the paper.

An unopened letter from the Brienne Collection.

Photo: Sound and Vision The Hague, Netherlands.

Experts say the technique used to reveal the text of the letter, which includes a type of imaging called X-ray microtomography, could also have applications in healthcare and engineering.

Letterlocks varied in complexity depending upon the expertise of the sender and receiver and how tamper-evident they wished their messages to be during transit. Some involved dozens of folds, resulting in packets about the size of playing cards.

Historians are eager to read these letters because of the insights they can offer into the lives of everyday people in 17th century Europe. “It’s the closest we get to a random sample” of letters from this period, said Virginia Tech historian Rachel Midura, who wasn’t involved in the research. Letters that survive from that period typically were those written by educated elites, she said.

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Until now, the standard way to read a letterlocked letter necessitated physically unfolding it, disturbing the intricate folds and potentially damaging the paper as a result of removing sealing wax and cutting the paper.

The letter researchers finally read was secured by eight folds and was one of hundreds of unopened letters from the Brienne Collection, a rare cache of 300-year-old undelivered messages inside a European postmaster’s trunk acquired by a museum in the Netherlands in 1926.

Written in French and dated July 31, 1697, the letter was revealed to be a request from a man named Jacques Sennacques for a death certificate. “It is important to me to have this extract,” he said in the 200-word message, which was addressed to Pierre le Pers, a merchant in the Netherlands.

The researchers used a multistep process to read the unopened letter. First, they made a 3-D scan of it using an X-ray scanner originally developed by Queen Mary University of London’s dental institute. Then they employed a computer algorithm to reveal the individual layers of paper, and software that showed on a screen how the letter would look if it had been physically unfolded.

The resulting image resembled a photograph of the message, with most of the text legible, said Amanda Ghassaei, a software engineer in San Francisco and an author of the paper.

The Brienne Collection’s trunk, containing about 2,600 undelivered letters sent from various places in Europe to The Hague, in the Netherlands, between 1689 and 1706. About 600 of the letters have never been opened.

Photo: Unlocking History Research Group

The research builds on work that began in 2015 at the University of Kentucky in Lexington, in which a similar technique was used to digitally reconstruct an ancient Hebrew scroll that had been damaged in a fire. The letterlocking research was funded by organizations including MIT Libraries, the research division of the computer software company Adobe Inc. and the Seaver Institute, a private foundation in Los Angeles.

Experts say X-ray microtomography and the algorithmic unfolding process could be used to monitor bone health in people with osteoporosis, allowing doctors to carefully map layers of bone to pinpoint areas of concern. “Right now, you don’t get that precision,” said William Brent Seales, a University of Kentucky computer scientist who worked on the Hebrew scroll research and served as a reviewer of the new paper.

The technique could also be used to check undeployed parachutes and other folded items for defects, said Karen Panetta, a computer engineer at Tufts University School of Engineering and a fellow at the Institute of Electrical and Electronic Engineers, who wasn’t involved in the research.

“I think they barely scratched the surface of the potential of the method,” she said.

Write to Sara Castellanos at sara.castellanos@wsj.com

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