How they did it: Reading a locked letter from 1697

A technique used in dentistry, paired with newly developed software, is helping researchers read unopened letters from hundreds of years ago. Even more intriguing is the reason this kind of virtual unfolding is necessary.

Until the 19th century, with postage informal, envelopes still not common and emissaries not always to be trusted, a technique called letterlocking was used to protect confidential missives. It involved folding the letter using such intricate techniques, somewhat like origami, such that the letter essentially became its own envelope.

The letterlocker would cut a corner and pass that sliced-off piece through a slit in a folded letterpacket to secure it shut with a blob of wax. The recipient had to rip the letter open to gain access to the content. Anyone else trying to open it would be sure to rip it too, thereby tipping off the recipient that it had been compromised.

Now, a team of 11 researchers from the UK, US and Europe that call themselves the Unlocking History Research Team has used the scans from a technique called x-ray microtomography (XMT) to write a series of algorithms. Together, the scans and algorithm have allowed them to virtually unfold a 300-year-old letter without breaking its seal. Their account of how they did it was published in the journal Nature Communications earlier this month.

The letter they accessed was among over 2,000 found in a trunk of undelivered letters from all over Europe, dating to the 17th century and preserved at the Sound and Vision museum at The Hague in the Netherlands. It was sent in 1697 by a legal practitioner named Jacques Sennacques from Lille, France, to his cousin in The Hague. He wrote in French asking for a document verifying the death of a relative. Sennacques sealed the letter with an intricate, eight-step fold. Sadly, it was never delivered.

In the virtual unfolding, for the first stage of the analysis, 10 letters from the trunk were imaged via XMT at the dental research labs of the Queen Mary University of London.

“Many of the letters have internal tucking systems to help keep the letterpacket closed,” co- lead author of the report Jana Dambrogio said via email. Dambrogio is with the Wunsch Conservation Laboratory at the Massachusetts Institute of Technology (MIT) Libraries and has been studying the practice of letterlocking for 20 years.

Next came a very high-resolution XMT scan of the selected letter — basically a 3D x-ray image. Co-lead Amanada Ghassaei and co-algorithm coder Holly Jackson then wrote a set of algorithms that work to detect the individual layers and reconstruct the folded geometry of the letterpacket.

“We then virtually unfold the letterpacket to produce an image of what it would look like if it were opened and flattened,” says Ghassaei. “This computational pipeline allows us to observe writing, watermarks, seals, internal folds and any other information hidden inside without doing any damage to the original artefact.”

Ghassaei and Jackson have automated their software so that it can virtually unfold any XMT scan of a letterpacket. They have also made the code freely available on Github.

“We typically have to reduce the x-ray energy to a third of what we’d use for teeth and bones, to be able to image paper and inks,” says XMT scientist David Mills. “We then have to make sure the letters can’t move during a scan. A scan takes three days and the letters can’t move by more than about a half of a thousandth of an inch (15 microns) during that time, or we’d get blurring in the final data.”

The yield has already been rich. The virtually opened letter “is a great example of the everyday business of a lawyer more than 300 years ago,” says Dambrogio. Other letters in the trunk can be expected to as effectively capture the workings of daily life centuries ago.

Perhaps a missive of love, a note to a newly married daughter, a nasty critique of an enemy’s wardrobe… one never knows what lies in wait.