On the eve of her execution for treason in February 1587, Mary, Queen of Scots, penned a letter to King Henri III of France and secured it with a paper lock that featured an intricate spiral mechanism. So-called “letterlocking” was a common practice to protect private letters from prying eyes, but this spiral lock is particularly ingenious and delicate because it incorporates a built-in self-destruct feature, according to a new paper published in the Electronic British Library Journal.

The authors are an interdisciplinary team of researchers working under the umbrella of the Unlocking History Research Group. In this paper, they describe a dozen examples of a spiral lock in letters dated between 1568 and 1638, including one from Mary’s former mother-in-law, Catherine de Medici, as well as her arch-rival, Elizabeth I, who signed Mary’s death warrant.

As we reported previously, co-author Jana Dambrogio, a conservator at MIT Libraries, coined the term “letterlocking” after discovering such letters while a fellow at the Vatican Secret Archives in 2000. The Vatican letters dated back to the 15th and 16th centuries, and they featured strange slits and corners that had been sliced off. Dambrogio realized that the letters had originally been folded in an ingenious manner, essentially “locked” by inserting a slice of the paper into a slit, then sealing it with wax. It would not have been possible to open the letter without ripping that slice of paper—evidence that the letter had been tampered with.

Dambrogio has been studying the practice of letterlocking ever since, often creating her own models to showcase different techniques, eventually forming the Unlocking History Research Group. The practice dates back to the 13th century—at least in Western history—and there are many different folding and locking techniques that emerged over the centuries. “It’s not like people could just go to a shop and buy an envelope,” Dambrogio’s co-author from King’s College London, Daniel Starza Smith, told Ars.

Queen Elizabeth I, Machiavelli, Galileo Galilei, and Marie Antoinette are among the famous personages known to have employed letterlocking for their correspondence. There are hundreds of letterlocking techniques: for example, “butterfly locks,” a simple triangular fold-and-tuck, and an ingenious method known as the “dagger-trap,” which incorporates a booby-trap disguised as another, simpler type of letter lock. And of course, there is the intricate spiral lock that Mary, Queen of Scots, used for her final missive.

Earlier this year, Dambrogio’s team was able to use X-ray tomography to virtually “unlock” a letter written in 1697 by a man named Jacques Sennacque. Their analysis revealed its contents for the first time, right down to the watermark in the shape of a bird, as described in a paper published in the journal Nature Communications. That letter was part of the Brienne Collection, a collection of 2,600 “locked” letters—600 of which had never been opened—found in a 17th-century trunk of undelivered letters preserved in the postal museum at The Hague, the Netherlands.

The unopened letters in the Brienne Collection meant that much more material evidence (crease marks and wax seals, for instance) about a given letter’s internal security was preserved, especially evidence of tucks and layer order, which typically leave no material trace. By contrast, the letters examined in this latest paper have all been opened, presenting a different kind of challenge for the researchers in their ongoing quest to reverse-engineer the creation of letterlocks.

A high percentage of the material evidence for the letterlocks is usually destroyed by opening the letter, and the spiral lock is designed to destroy not just the lock, but also sometimes portions of the actual letter as an added security measure, according to the authors. Subsequent handling by scholars and conservationists can also obscure evidence of the use of a letterlock. Such items are sometimes bound into letter books or stored after flattening and humidification, and the remnants of wax seals might be stored separately, discarded, or reattached incorrectly.

https://arstechnica.com/?p=1819350