Sealed letters folded in intricate patterns have gone unread for more than 300 years, but now researchers have been able to tease out their contents with a combination of X-ray imaging and unfolding algorithms.
Such folded letters were common before the invention of the envelope. Letters were folded multiples times with tucks or slits and often sealed with wax. The paper they are written on is now so fragile that opening them up can damage them.
“Studying folding and tucking patterns in historic letters allows us to understand technologies used to communicate,” says Jana Dambrogio at the Massachusetts Institute of Technology. Now, her team are able to do so without damaging the letters by reconstructing what is inside.
Dambrogio and her colleagues looked at a postmaster’s trunk containing undelivered letters sent from all over Europe to The Hague in the Netherlands, between 1680 and 1706. The trunk, known as the Brienne Collection, contains 2600 letters, of which 600 are unopened.
The researchers first scanned the folded letters with high-resolution X-ray imaging to create a 3D reconstruction. They then used an algorithm to detect individual layers of paper in the scan and analyse the thickness of the crease lines on the paper. This allowed them to to re-create the sequence of folds – thicker creases are more gentle curves involving multiple layers of paper, so come later in the sequence – and unfold the letter.
The team was able to read one letter in full, and get partial content from several other letters. The fully opened letter is dated 31 July 1697 and is from a legal professional in Lille, France called Jacques Sennacques, requesting an official death certificate for a relative. Until now, historians only knew the name of the recipient, not the content of the letter.
Similar techniques have previously allowed researchers to read historical documents with one or two folds, but not the complex folded letters like those in the Brienne Collection. “This is a different sort of virtual unfolding which unfolds letters with special interlocks” says Paul Roisin at Cardiff University, UK.
Journal reference: Nature Communications, DOI: 10.1038/s41467-021-21326-w
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