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

First, an international team of researchers read an unopened letter from Renaissance Europe – without breaking the seal or damaging it in any way.

The research, published in Nature communication, describes how an X-ray scanner used in dental research and ‘virtual unfolding’ allowed the interdisciplinary team to read the contents of a letter that had been folded securely and intricately, open for 300 years, while the valuable physical evidence is preserved.

An extremely sensitive X-ray microtomography scanner, developed at the Queen Mary University of London’s dental research laboratories, was used to scan a batch of unopened letters from a 17th-century mailbox full of undeveloped mail.

The senders of these letters closed it using letterlocking – the historical process of folding a flat sheet of paper in a complicated way and securing it to become an envelope. Letter locking was a common practice for secure communication before modern envelopes were introduced, and is considered to be the missing link between ancient physical communication security techniques and modern digital cryptography.

So far, these letter packs could only be studied and read by cutting them open, which often damages the historical documents. Now the team was able to investigate the contents of the letters without irrevocably damaging the systems that secured it.

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

Dr. David Mills of Queen Mary University of London said: “We were able to use our scanners to treat the history of the X-rays. The scanning technology is similar to medical CT scanners, but uses much more intensive 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 photos and turn them into letters that they could open and read for the first time in more than 300 years. “

This process reveals the contents of a letter dated 31 July 1697. It contains a request from Jacques Sennacques to his cousin Pierre Le Pers, a French merchant in The Hague, for a certified copy of a death notice of one Daniel Le Press (complete transcript and images available). The letter provides a fascinating insight into the lives and concerns of ordinary people in a turbulent period of European history, when correspondence networks held families, communities and trade across great distances together.

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

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

“This algorithm takes us to the heart of a closed letter,” the research team explains. “Sometimes resist the investigation. We could have simply cut open these letters, but rather we took the time to study them for their hidden, secret and inaccessible properties. We have learned that letters can be much more revealing than they are. “The use of virtual unfolding to read an intimate story that has never seen the light of day – and never even reached its recipient – is truly extraordinary.”