Secrets of sealed 17th century letters revealed by dental X-ray scanners

For the first time in the world, an international team of researchers read a closed letter from the European Renaissance – without breaking its seal or damaging it in any way.

The research, published in Nature Communications, describes how an X-ray scanner used in dental research and ‘virtual unfolding’ allowed the interdisciplinary team to read the contents of a folded letter in a safe and intricate way that remained closed for 300 years, preserving its valuable physical evidence.

A highly sensitive X-ray microtomography scanner, developed in the dental research labs at Queen Mary University in London, was used to scan a batch of unopened letters from a 17th-century trunk full of undelivered mail.

The senders of these letters closed them using ‘letterlocking’ – the historical process of intricately folding and attaching a flat sheet of paper to become their own envelope. Letter blocking was a common practice for secure communication before modern envelopes came into use and is considered the missing link between ancient physical communication security techniques and modern digital encryption.

Until now, these packets of letters could only be studied and read by cutting them open, often damaging historical documents. Now, the team has been able to examine the contents of the letters without irrevocably damaging the systems that protected them.

Professor Graham Davis, of Queen Mary University of London, said: “We designed our X-ray scanner to have unprecedented sensitivity for mapping the mineral content of teeth, which is invaluable in dental research. But this high sensitivity has also made it possible to solve certain types of ink on paper and parchment. It’s amazing to think that a scanner designed to look at your teeth has taken us this far.

Dr. David Mills, of Queen Mary University of London, said: “We were able to use our scanners for X-ray history. The scanning technology is similar to medical tomographs, but using much more intense X-rays that allow us to see the small traces of metal in the ink used to write these letters. The rest of the team was then able to take our scanned images and turn them into letters that they could virtually open and read for the first time in over 300 years. “

This process revealed the contents of a letter dated July 31, 1697. It contains a request by Jacques Sennacques to his cousin Pierre Le Pers, a French merchant in The Hague, for a certified copy of a death notice for a certain Daniel Le Pers (complete transcript and images available). The letter gives a fascinating insight into the lives and concerns of ordinary people in a tumultuous period in European history, when correspondence networks kept families, communities and commerce together over great distances.

Following the X-ray microtomography scan of the letter packs, the international team applied computational algorithms to the scan images to identify and separate the different layers of the folded letter and ‘virtually unfold it’.

The authors suggest that the virtual unfolding method and the categorization of folding techniques may help researchers to understand this historical version of physical cryptography, while preserving its cultural heritage.

“This algorithm takes us straight to the heart of a closed letter,” explains the research team. “Sometimes the past resists scrutiny. We could have just opened these letters, but instead, we took the time to study them for their hidden, secret and inaccessible qualities. We learned that letters can be much more revealing when they are left closed. Using virtual scrolling to read an intimate story that never saw the light of day – and never reached its intended recipient – is truly extraordinary. ”