Sealed letters folded in intricate patterns have been unread for more than 300 years. Now researchers can work out their content using a combination of x-ray imaging and deconvolution algorithms.
Such folded letters were common before the envelope was invented. The letters were folded several times with tucks or slits and often sealed with wax. The paper they are written on is now so fragile that opening it can damage it.
“If we study folding and hiding patterns in historical letters, we can understand the technologies used to communicate,” says Jana Dambrogio of the Massachusetts Institute of Technology. Now your team can do this without damaging the letters by reconstructing what is inside.
Dambrogio and her colleagues examined the trunk of a postmaster with 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 2,600 letters, 600 of which 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 identify individual layers of paper in the scan and to analyze the thickness of the crease lines on the paper. This allowed them to recreate the sequence of folds – thicker folds are smoother curves with multiple layers of paper, so come later in the sequence – and unfold the letter.
The team was able to read one letter in its entirety and received some of the content from several other letters. The fully opened letter is dated July 31, 1697 and is from a lawyer in Lille, France, named Jacques Sennacques, requesting an official death certificate for a relative. Until now, historians only knew the name of the recipient, not the contents of the letter.
Similar techniques have so far allowed researchers to read historical documents one or two times, but not the complex folded letters as in the Brienne collection. “This is a different kind of virtual unfolding, where letters are unfolded with special interlocks,” says Paul Roisin of Cardiff University in the UK.