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Computational tools reveal secrets of 17th-century sealed letter

In a world first, an international team of researchers has read an unopened letter from Renaissance Europe – without breaking its seal or damaging it in any way. Nadine Akkerman, Reader in early modern English literature at Leiden University, is co-author of the article that appeared on 2 March in Nature Communications.

Solving puzzles with algorithms

The team solved the problem of how to read a sealed letter without damaging it through the use of X-ray microtomography, an advanced scanning technology. A series of thousands of scans isolated the exact position of iron particles in the ink, thus making the writing visible. A computer-controlled algorithm, which itself took four years to develop, was then employed to piece these scans together like a massively complicated 3D jigsaw, allowing the letter to be virtually unfolded.

Letters in this period were folded in such a way as to make the paper on which they were written form its own ‘envelope’, a process known as ‘letterlocking’. Some letterlocking techniques were highly complex and designed to protect the contents from being read without damage – security mechanisms that would be destroyed simply by opening the letter. The technique devised by the team allows them to read the contents while not only leaving the intricately folded historical document intact, but also displaying the manner in which it was folded. Thus the letter as a textual act and a material object can now be read without damaging it in any way.

A computer generated animation showing the unfolding of a letter

Day-to-day worries

The first letter to be unfolded by the team was sent by Jacques Sennacques to his nephew Pierre Le Pers, a French merchant in The Hague, on July 31, 1697. Pierre never read the letter, in which Jacques asked him for a certified copy of an obituary from a certain Daniel Le Pers. This may not appear to be, in itself, a spectacular discovery, but the team are keen to point out that the day-to-day worries of everyday people present a rich seam of historical data that is yet to be mined, as their correspondence is often less well-preserved than that of their more elite counterparts.

The seal on the letter was kept intact, thanks to X-ray microtomography

Groundbreaking collaboration

Of particular importance is that these results have been achieved by a team made of academics from very different disciplines. Enabled partly by a grant from the Netherlands Organization for Scientific Research (NWO), the team allowed humanities scholars from the Netherlands to collaborate with computer scientists from the Massachusetts Institute of Technology (MIT), one of the most prestigious technical universities in the world, amongst others.

Nadine Akkerman, a world renowned expert on early modern manuscripts at Leiden University Centre for the Arts in Society (LUCAS) and member of The Young Academy of Arts and Sciences (De Jonge Akademie), said that ‘What we have achieved at Signed, Sealed, & Undelivered is more than simply opening the unopenable, and reading the unreadable. We have shown how truly interdisciplinary work breaks down boundaries to investigate what neither humanities nor the sciences can hope to understand alone’.

Vulnerable objects preserved

When asked on how this breakthrough would help conservation, she replied ‘it finally removes the need to make a choice between reading a document and preserving it. This means that a document can remain available for future scholars to investigate with new techniques that we have yet to imagine, and also for the public to see as it was, rather than as we have made it’.

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The research team

The research team consists of Nadine Akkerman (Universiteit Leiden; De Jonge Akademie), Rebekah Ahrendt (Universiteit Utrecht), David van der Linden (Radboud Universiteit), Jana Dambrogio (MIT Libraries), Amanda Ghassaei (Adobe Research), Daniel Starza Smith (King’s College London), Holly Jackson (MIT), Erik Demaine (MIT), Martin Demaine (MIT), Graham Davis (Queen Mary University of London) and David Mills (Queen Mary University of London).

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