Reading Unopened Letters via X-ray Microtomography

Posted in journal articles by Editor on March 3, 2021

An unopened letter, dated 31 July 1697, from Jacques Sennacques to his cousin Pierre Le Pers, virtually unfolded and read for the first time
(Photograph: Unlocking History Research Group)

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The latest from the Unlocking History Research Group, as published in Nature Communications:

Jana Dambrogio, Amanda Ghassaei, Daniel Starza Smith, Holly Jackson, Martin L. Demaine, Graham Davis, David Mills, Rebekah Ahrendt, Nadine Akkerman, David van der Linden, and Erik D. Demaine, “Unlocking History through Automated Virtual Unfolding of Sealed Documents Imaged by X-ray Microtomography,” Nature Communications 12 (2 March 2021), article number 1184.

Abstract: Computational flattening algorithms have been successfully applied to X-ray microtomography scans of damaged historical documents, but have so far been limited to scrolls, books, and documents with one or two folds. The challenge tackled here is to reconstruct the intricate folds, tucks, and slits of unopened letters secured shut with ‘letterlocking’, a practice—systematized in this paper—which underpinned global communications security for centuries before modern envelopes. We present a fully automatic computational approach for reconstructing and virtually unfolding volumetric scans of a locked letter with complex internal folding, producing legible images of the letter’s contents and crease pattern while preserving letterlocking evidence. We demonstrate our method on four letterpackets from Renaissance Europe, reading the contents of one unopened letter for the first time. Using the results of virtual unfolding, we situate our findings within a novel letterlocking categorization chart based on our study of 250,000 historical letters.

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New outlets—including The Art Newspaper, CNN, and The New York Timeshave taken up the story. Here’s coverage from The Guardian:

In a world first for the study of historic documents, an unopened letter written in 1697 has been read by researchers without breaking the seal. The letter, dated 31 July 1697 and sent from French merchant Jacques Sennacques in Lille to his cousin Pierre Le Pers in The Hague, had been closed using ‘letterlocking‘, a process in which the letter is folded to become its own envelope, in effect locking it to keep it private. It is part of a collection of some 2,600 undelivered letters sent from all over Europe to The Hague between 1689 and 1706, 600 of which have never been opened.

The international team of researchers from universities including MIT, King’s College London, Queen Mary University London, Utrecht and Leiden, worked with X-ray microtomography scans of the letter, which use X-rays to see inside the document, slice by slice, and create a 3D image. They applied computational flattening algorithms to the scans to enable them to virtually unfold the letter without ever opening it, and discovered that Sennacques had been asking his cousin for a certified copy of a death notice of one Daniel Le Pers.

“It has been a few weeks since I wrote to you in order to ask you to have drawn up for me a legalised excerpt of the death of sieur Daniel Le Pers, which took place in The Hague in the month of December 1695, without hearing from you,” runs the letter. “I am writing to you a second time in order to remind you of the pains that I took on your behalf. It is important to me to have this extract & you will do me a great pleasure to procure it for me & to send me at the same time news of your health & of all the family.” . . . [as translated by the research team.]

The full article, by Alison Flood (2 March 2021), is available here»

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