As It Happens

Scientists have virtually unfolded a 17th-century locked letter for the 1st time

Hundreds of years ago, letter writers would secure their secrets through the elaborate art of "letterlocking." Now, for the first time, researchers read these secrets from 17th-century Europe without physically opening the letter, using virtual reality technology.

Letterlocking was an 'artistic form of security,' says researcher Nadine Akkerman

An Unlocking History Research Group team member holds a rare unopened example of a letterpacket with a paper lock. (Unlocking History Research Group)

Hundreds of years ago, before the invention of email security or even envelopes, letter writers would secure their secrets through the elaborate art of "letterlocking."

It was meant as a form of security, and was used by everyone from kings to spies to lovers. 

Now for, the first time, a team of 11 researchers from around the world has found a way to read these secrets from 17th-century Europe without physically opening the letters, using virtual reality technology. The findings were published Tuesday in the journal Nature Communications

"They're completely intact," Nadine Akkerman, a lecturer of early modern English literature at Leiden University and one of the co-authors of the study, told As It Happens guest host Peter Armstrong. 

"And that's the beauty of this. We don't destroy the first instances of what is document security. We can still study the folds as well as read the contents." 

The art of letterlocking

Before the invention of the sealed envelope in the 1830s, letterlocking was used for centuries to stop prying eyes, a sort of "artistic form of security," Akkerman said. Writers would fashion the very paper they were writing on into an envelope of sorts.

It was also used as a signature for writers that preferred to keep their identity secret — like a spy. 

"You wouldn't want to sign your letter with 'James Bond.' You would want to use a code name or no name at all. But how would your recipient know that the letter is from you?" she said. 

"You could sort of agree with each other, 'I'll use a very specific triangle shape and the inside will be folded like a little penguin.'" 

A computer-generated unfolding sequence of a sealed letter. (Unlocking History Research Group)

Akkerman said letter writers would also cut off pieces of the paper and weave them together, to act as a lock. This way, they could tell if the letter had been tampered with. 

But those techniques meant that hundreds of years later, when Akkerman and her colleagues came across a trunk full of locked letters, they had no way of opening them without causing damage. 

300-year-old undelivered letters 

In 2014, the team at Unlocking History Research Group was introduced to the Brienne Collection at the Sound and Vision museum in The Hague. 

It's a European postmasters' trunk full of 300-year-old undelivered letters, including 577 unopened locked letters.

"It's absolutely brilliant to be confronted, suddenly, with a trunk full of closed letters. I have never seen one, and then suddenly hundreds," Akkerman said.

The 17th-century trunk of letters belonged to postmaster and postmistress Simon and Marie de Brienne and contained 2600 'locked' letters, 577 of which were unopened. (Sound and Vision The Hague, The Netherlands)

Suddenly they were confronted with a new problem. How do they study these letters without ruining them? 

"You don't know really what you would destroy inside if you would open it normally," Akkerman said. 

"We needed some kind of scanning technology to actually read the letters because we're still interested in the content — we want to know the secrets, especially if they are hidden from us." 

First, the researchers scanned one of the letters using a high-resolution X-Ray machine to create a three-dimensional image. This allowed them to see the writing, but because of all the intricate folds, it was unreadable. 

Then the researchers developed an algorithm that can scan the image and virtually unfold it until the handwriting is revealed. 

"Just imagine that we are the first in 300 years to actually read these letters," Akkerman said.

Hoping for a 'proper secret'

Akkerman was hoping the first letter would entail a "real kind of proper secret," but was disappointed to discover a quite mundane note to a French merchant in 1697. 

It was written by Jacques Sennacques in Lille, France, and sent on July 31, 1697, to his cousin, the French merchant Pierre le Pers, who researchers assumed was in The Hague. Sennacques was inquiring about a death certificate, and Akkerman said it's clear his cousin hadn't answered his previous letters. 

A locked letter sent to a French merchant in 1697. The Unlocking History Research Group was able to virtually unlock the letter for the first time. (Unlocking History Research Group)

Despite her initial disappointment, Akkerman said the discovery is actually "absolutely fascinating" because it lets her peer into a world that has never really been seen before. 

"You sort of read the letters of kings, queens, ambassadors to diplomats, but you don't really see the normal people in the archives, whereas history should be more than just about kings and queens," she said. 

It's absolutely brilliant to be confronted, suddenly, with a trunk full of closed letters. I have never seen one, and then suddenly hundreds.- Nadine Akkerman, co-author of study on letterlocking

The letters likely remained unopened because at that time, the person receiving the letter had to pay for it. The chief postmasters of The Hague, a married couple named Simon de Brienne and Marie Germain, kept it as a "piggy bank" in the hopes that someone would eventually pay for the letters, Akkerman said.  

Akkerman said the letters were full of other kinds of stories, such as women writing to their lovers that they were pregnant.

They've opened four so far, and Akkerman said she is excited for what else they will find. 

"Now that we have this unique collection, plus the technology that allows us to read them, what kind of corners of history do you think will be open to us?" she said. 

"There are probably amazing historical artifacts that we're going to find." 


Written by Sarah Jackson. Produced by Katie Geleff. 

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