Unlocking Sealed Letters

Some 600 letters in the Brienne Collection have always remained closed. It is the Museum’s policy to preserve them in their closed state. But what if you are a curious scholar and investigator? Would it be possible to peek inside the letters while preserving their unique material features?

This new room (added in 2021, a year after the initial launch of this exhibit) explores the groundbreaking possibilities of modern technology.

An opened letter cannot be unopened…

Meet letter DB-1627, which is still unopened.

From the address panel we learn that it was directed to Pierre Le Pers, a merchant in The Hague. The recipient had to pay a postage of 10 stuivers, as indicated by the red chalk.

But for the rest we don’t know much – and the reverse side doesn’t help much either.

Why not rip the letter open?

The only way to read the letter, would be to cut it open. That would certainly be a quick and efficient route – yet it would also permanently damage the letterpacket as a material object.

We deliberately want to save the still locked letters. Millions of letters are stored in their opened state. But to preserve letters in their original, locked state is quite exceptional.

If you want to conserve the letterpacket as object, you keep it closed. It proves the practice of letterlocking and it best conserves the wax seals and paper.

When you open and unfold letters and keep them flat, unique information and meanings are lost forever.

X-RAYS

One way to look inside a sealed letter is to use X-rays. Because of their short wavelengths, they can penetrate organic materials – not only human tissue, but also carbon objects like paper.

To explore the possibilities of X-rays, the SSU team enlisted the help of Graham Davis, Professor of 3D X-ray imaging, and David Mills, X-ray microtomography expert at Queen Mary University of London, School of Dentistry.

Museum curator Koos Havelaar securely packaged a batch of letters and safely transported them through the Channel tunnel. Once in London, Davis and Mills scanned the batch of letters in their MuCat X-ray machine to see whether we can see ink on paper. They also scan letter DB-1627. Scanning takes 72 hours…

Paper, like other organic materials including parchment, primarily consists of hydrogen, carbon and oxygen. X-rays easily pass through the paper, but it nevertheless shows up on an image.

The common elements in early modern ink, on the other hand, include metals such as iron, copper, silver, tin, gold, and mercury. These are high density materials, resulting in bright regions within the scan – hence ink shows up very well with X-rays!

As you can see below, letters and words are clearly visible.

X-rays can therefore produce a very high resolution 3D x-ray image, or in technical terms, a CT scan of the locked letter.

But the text is far from legible. What is needed is another technology to change the 3D scan into a 2D image of the letter.

Algorithm

For further study, the X-ray scan crosses the pond to the Massachusetts Institute of Technology in Cambridge, MA (USA).

Algorithm engineers Holly Jackson and Amanda Ghassaei coded a computer algorithm to detect individual layers of paper in the scan. Together with detailed knowledge of letterlocking formats, this allows for a reconstruction of the folded geometry.

The picture above shows virtual unfolding :

a) the 3D x-ray image

b) identification of layers

c) the unfolded reconstruction of letter DB-1538 (UH0218).

d-g) the difficult process of meshing and flattening the paper surfaces.

Letterlocking format of DB-1627.

Based on a close examination of the X-ray image, Jana Dambrogio and Daniel Starza Smith can identify the specific letterlocking format – the special way in which this letter was folded.

Knowing the specific letterlocking format makes it possible to adapt the algorithm and assist the virtual unfolding process.

Thanks to the algorithm and letterlocking format, the computer can virtually unfold the letterpacket to produce an image of what it would look like if it were opened and flattened.