It remained sealed and unread for 300 years: With the help of a dental X-ray scanner and algorithms, researchers non-destructively coaxed secrets from a “cryptically” folded letter: it could be unfolded and read virtually. In this way they were able to show that, in addition to providing insight into texts, the process can also provide information about the historical art of folding, which was once supposed to protect documents from unauthorized viewing.
The standardized envelopes that are still in use today have only dominated the postal system since the 1830s – before that, they were custom-made – and sometimes with considerable effort: In addition to wax seals and other processes, special folding techniques were used to transform a flat sheet of paper into a complex letter package that could not be opened non-destructively. This so-called letter locking was supposed to protect the secrecy of the letter: A recipient could see that a letter was reaching him unopened. The procedures were thus forerunners of modern cryptography.
Unopened letters in sight
Investigating the historical techniques of letterlocking is problematic, however, because most of the time the old documents are of course in open form. But there are exceptions: a collection of undelivered letters in the possession of the Postal Museum in The Hague also includes many as yet unopened copies. These are letters from all over Europe that were sent between 1680 and 1706. So far, these letters could only be examined and read by cutting them open, which damaged them. But the international research team are now presenting a method that enables a non-destructive examination of the texts and their security systems.
As the scientists report, they used an X-ray scanner to recognize the writing and the material structures, which was originally developed for dental research. A virtual development was then also possible through computer calculations. “The X-ray scanner has an extremely high sensitivity to map the mineral content of teeth,” explains co-author Graham Davis of the Queen Mary University of London. “This also makes it possible to detect certain types of ink in paper and parchment.” His colleague David Mills adds: “This enables us to shed light on the stories. The scanned images can also be used to transform them into virtual letters that can be opened and read on the computer, ”says the scientist.
Insight into content and folding
This process revealed the contents of an unopened letter from the collection dated July 31, 1697. As it turned out, it was a question from a Jacques Sennacques to his cousin Pierre Le Pers. In the letter he asks the French merchant to send him a certified copy of a Daniel Le Pers obituary. In addition to the text, the process also uncovered a watermark in the center of the paper depicting a bird. As far as the production of the letter is concerned, the researchers found that although it was provided with the so-called letterlocking system by means of special folds, it was not in a very complex way: They arranged the letters as part of the categorization system that the researchers also presented in their study The security level of this letter is comparatively low.
The researchers now hope that their method can help uncover more historical texts, while at the same time providing insights into the historical version of cryptography. “It turns out that letters can be much more informative if you leave them unopened. Using the virtual unfolding to read a personal story that has remained hidden for so long – and has not even reached its recipient – is something very special, ”said the researchers.
Source: Queen Mary University of London, Article: Nature Communications, doi: 10.1038 / s41467-021-21326-w
Video: Credit: Unlocking History Research Group