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Virtually unfolded letter from the Brienne Collection
Photo: Unlocking History Research Group / dpa
Envelopes have only been mass-produced since around 1830; before that, letter writers had used sophisticated means for centuries to protect confidential information.
Seals and sophisticated folding techniques should protect the content from unauthorized reading.
To date, hundreds of thousands of such letters are unopened in archives.
An international team of researchers led by Jana Dambrogio from the Massachusetts Institute of Technology (MIT) is now presenting a process in the journal Nature Communications with which such letters can be read using a computer without opening them and thus destroying them.
The so-called Brienne Collection is a particularly rich collection of centuries-old letters.
It contains more than 3,100 documents that were sent across Europe in the 17th century and could not be delivered.
The postmaster couple Simon de Brienne and Marie Germain in The Hague kept them in a chest - and 577 are still unopened.
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Chest with undelivered letters in the Hague Postal Museum
Photo: Unlocking History Research Group / dpa
Until now, you could only read such folded letters if you cut them open and thus damaged them.
The complex way in which the letters were folded has also hardly been able to be examined non-destructively so far.
The team from the USA, Great Britain and the Netherlands is now presenting a computer-aided process: First, the letters are x-rayed using X-ray microtomography, then a 3D model is created and the folding technique is determined on a geometric basis before the letters are finally unfolded virtually.
The scan not only checks the material density, but also depicts elements in the ink such as iron, copper and mercury.
In addition, the process provides information about the folding technology used and the associated level of security of a letter.
"Our approach has the potential to open up new historical sources"
The researchers demonstrate the feasibility of the process with four letters from the Brienne Collection.
All of them measure around 5 by 8 centimeters when unopened, but despite their identical rectangular shape, they were folded in completely different ways.
In terms of content, it is often about everyday issues, for example in the now for the first time revealed letter DB-1627 of July 31, 1697. In it, a Jacques Sennacques asks his cousin Pierre Le Pers, a French merchant who lives in The Hague, for a certified death certificate for a Daniel Le Pers.
But the topics go far beyond everyday life: "Our approach has the potential to open up new historical sources," write the researchers.
As an example, they cite the last letter from the Scottish Queen Maria Stuart, which she wrote on February 8, 1587 - only six hours before her execution - to her brother-in-law from her first marriage, the French King Henry III.
A research project took a decade to decipher the complex folding technique.
"Virtual unfolding could bring a result within days," notes the team.
As a further example of the historical value of the process, the researchers refer to the so-called Prize Papers: These approximately 160,000 undelivered letters were collected by the British Admiralty and come from enemy ships that were hijacked between the 17th and 19th centuries.
Worldwide there are still countless unopened letters and documents from different cultures and historical epochs that can now be evaluated without damaging them.
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chs / dpa