Virtual unfolding: New digital techniques for opening complex documents

An exciting breakthrough has been announced this week, one that permits researchers to read letters without unfolding them, offering new ways of managing sealed and fragile documents. Using a combination of X-rays and 3-D imagining techniques, researchers virtually “opened” four letters from the Brienne Collection, a trunk filled with 2,600 notes sent from Europe to the Hague between 1689 and 1706. The team published its findings in the journal Nature Communications.

A gif animation of the virtual unfolding process
The researchers virtually opened the letters with an advanced X-ray machine. They then used computers to analyze the folds and create a readable, digital model of the unfolded message. 
(Unlocking History Research Group Archive)

The following is drawn from a report in The Smithsonian Magazine:

The documents concerned were especially intriguing and challenging because they used a technique known as “letter locking”. Here the page was folded to create its own envelope with the complex folding, cutting and interlocking techniques both preventing others from viewing the contents and creating a kind of tamper-proof lock.  Study co-author Daniel Starza Smith, a literary historian at King’s College London, told Wired, “what letterlocking is doing is giving us a language to talk about sorts of technologies of human communication security and secrecy and discretion and privacy.”

Lead author Jana Dambrogio, an MIT Libraries conservator, said in a statement, “Letterlocking was an everyday activity for centuries, across cultures, borders, and social classes,” says “It plays an integral role in the history of secrecy systems as the missing link between physical communications security techniques from the ancient world and modern digital cryptography.”

The team’s insights also also point to new possibilities for working with fragile paper documents that cannot be safely handled.

The full article in The Smithsonian Magazine can be found here.

The team’s research is published in the article Dambrogio, J., Ghassaei, A., Smith, D.S. et al. Unlocking history through automated virtual unfolding of sealed documents imaged by X-ray microtomography. Nature Communications 12, 1184 (2021).

Abstract: Computational flattening algorithms have been successfully applied to X-ray microtomography scans of damaged historical documents, but have so far been limited to scrolls, books, and documents with one or two folds.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.

Brienne trunk
During the late 17th and early 18th centuries, postal workers at the Hague held on to unclaimed letters. 
(Sound and Vision The Hague / Brienne Archive)

To read more about paper and materiality, check out the posts Brittle Paper: What can it stand? and New book: On manuscripts, materiality and ‘thinking through paper’.