Reading a letter that's been sealed for more than 300 years — without opening it

An unopened letter that was mailed back in 1697 but never delivered has been read by researchers who have developed a way to virtually "unfold" sealed letter packets without having to actually break the seal.

The new technique, described in the journal “Nature Communications,” should allow historians to learn more about "letterlocking," the practice of using elaborate slits, folds, creases and tucks to turn a flat sheet of paper with a written message into a tamper-resistant package.

Such security measures were an everyday part of life for centuries. "The envelope as we know it, the gummed envelope, wasn't invented until the 1830's," says Jana Dambrogio, a conservator with the Massachusetts Institute of Technology Libraries in Cambridge, Mass. "And so before then, everyone letterlocked."

Letterlocking technologies varied greatly, with people folding and sealing their messages in hundreds of different ways. "They can make it look like it's less secure and have security hidden on the inside," says Dambrogio. "Or they can make something look very secure, but it actually isn't that secure on the inside, when you open the letter packet."

Letterlocking hasn't gotten much attention until recently, however, and in the past, archivists would open locked letters by simply cutting them. "What do we lose when we open the unopened?" asks Dambrogio.

She was intrigued to learn of a seventeenth-century postmaster's trunk from The Hague in the Netherlands that contained 577 unopened letter packets. Back then, if a letter couldn't be delivered for some reason, workers would keep it around in case the intended recipient ever showed up to claim it and pay the postage.

Dambrogio and a team of researchers now say they've managed to read one of these unopened Renaissance letters, with the help of a medical scanner.

"It was originally designed to study teeth. It's this really high-resolution X-ray scanner," says Amanda Ghassaei of Adobe Research in San Francisco, who explains that the device can create a detailed three-dimensional X-ray image of a folded letter.

Because the inks used back then contain a lot of metal, says Ghassaei, the writing "shows up as a very bright region on the scan, kind of like the way that your bone would show up really bright on an X-ray."

The trouble is, the ink markings look all jumbled up, because many layers of folded paper are pressed close together and words appear to overlap.

"The challenge here was really to try to find a way to manipulate that data and actually virtually unfold it so that we could get it into a flat state," she says, "and actually kind of generate something that looks like an image of the letter if it had been opened and flattened. But in reality, we haven't even touched the letter."

Their first success, using a brute-force algorithm to study a partially-opened letter, came at the end of 2016, recalls Holly Jackson, a student at MIT who has been working on the project.

"Since then we've kind of been refining this pipeline, trying to make it fully automated, fully generalizable to lots of different intricate folding patterns," says Jackson.

The unopened 1697 letter from the postmaster's trunk has an especially lovely folding pattern, says Dambrogio, even though the letter's contents make it clear that it's just an ordinary bit of family business, with one cousin writing to another to request a copy of an official death certificate for one of their relatives.

"In one sequence it makes an arrow shape, which folds in on itself and is sealed with a little piece of adhesive," says Dambrogio. "So it's quite beautiful and it's thrilling that we can read it without tampering with the letter packet, leaving it to study as an unopened object."

The work impressed Brent Seales, a computer scientist at the University of Kentucky in Lexington who has worked on virtually "unwrapping" ancient scrolls that are too fragile to be unrolled.

"They revealed the text, which was awesome. To do that without opening the letter is itself a sort of miracle, which I love," says Seales. "They were also complete about the technology of the artifact itself, because we can't forget that this stuff is embodied in physical form. And that's important."

He notes that the practice of sealing written documents dates back to the clay tablets used by the Sumerians, and it can put historians in a difficult position because to open something is to partially destroy it.

"There's this idea of the physical artifact that needs to be preserved and then the scholarly work that we would like to do, which is to know what it says," notes Seales. "How do you break that deadlock?"

Virtual unwrapping can provide an answer, he says, but its methods always have to be fully described and transparent, as they were in this case, so that others can follow the process step by step. "They told you in their paper what a letter says inside without ever opening it. You have to have some kind of trust in that. Because the artifact itself will never be opened," Seales points out.

Many, many unopened letters await further study, including hundreds in the Prize Papers, a collection of mail and other materials confiscated from enemy ships by Britain from the 17th to the 19th centuries.

Howard Hotson, a historian with the University of Oxford, believes that studying unopened letters and others that were once locked could perhaps "reveal patterns of trans-cultural, potentially global technological exchange, as sophisticated letterlocking techniques are passed from one country, sphere of activity, or continent to another during the long period in which these techniques were in use."

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