Long before digital encryption, elaborate methods were used to keep the contents of letters secret from prying eyes. Centuries after the death of both author and intended recipient, scientists have worked out how to learn what was written without damaging the letter itself.

Before envelope mass-production, letter-writers developed a sort of origami known as letterlocking, where the paper a letter was written on was folded to hide its contents, sometimes adding wax seals for extra security. This creates a dilemma for archivists and historians. Should they open a letter that may provide important insights into people's past lives, destroying the seal and folds in the process, or leave the contents unknown?

The invention of X-Rays brought an alternative option, making it possible to read old letters without breaking the seal or harming the document. However, until now this has only been effective where letters had a few simple folds. Multiple complex folds, used for more secret documents, left the problem back where it started before modern technology. Now an unexpected collaboration has changed that.

"We designed our X-ray scanner to have unprecedented sensitivity for mapping the mineral content of teeth, which is invaluable in dental research,” said dentistry Professor Graham Davis of Queen Mary University of London in a statement. “But this high sensitivity has also made it possible to resolve certain types of ink in paper and parchment.”

Besides technology, it proved essential to understanding the way a letter was folded to explore its message. This was no simple task, since letterlocking evolved into fine art, with regional variations which changed over time.

Davis's collaborators investigated 250,000 old letters to create a taxonomy of letterlocking methods, some of which enabled recipients to determine if the letter had been unfolded in transit, even without seals. This allowed the team to program scanners to know which way the writing should be facing in a particular letter. More powerful versions of the precision X-Ray machines Davis helped develop could then pick up tiny traces of metal in the ink, producing readable images. “We've been able to use our scanners to X-ray history,” said co-author Dr David Mills.

The technique was then applied to the contents of the Brienne Trunk, a collection of 300-year-old undelivered letters, of which 577 have never been opened.

In Nature Communications Davis, Mills and an interdisciplinary team describe the contents of DB-1627. one the Brienne letters. “It contains a request from Jacques Sennacques, dated July 31, 1697, to his cousin Pierre Le Pers, a French merchant in The Hague, for a certified copy of a death notice of one Daniel Le Pers,” the paper reports. Not romantic or exciting perhaps, but a glimpse into the lives of people of the day, who may have needed such death notices to gain access to estates.

Three of the trunk's other unopened letters were read in the same way, after one of the more than 2,500 that had already been unfolded, was used to test the process.

“We've learned that letters can be a lot more revealing when they are left unopened. Using virtual unfolding to read an intimate story that has never seen the light of day - and never even reached its recipient - is truly extraordinary,” the authors said. 

Anyone disappointed by the contents of DB-1627 might like to know of the Prize Papers, a 160,000-strong archive of documents British ships seized from other captured ships in the 17th-19th Centuries, many of which have never been opened.