Before envelopes began to be used, "letter locking" was common, a technique in which letters are closed by folding them several times in different ways. 

Today, therefore, it is difficult to open certain historical letters without breaking them, which can destroy their historical value.

Special X-ray technology

In order to still be able to read the contents of these letters, researchers have found out how the letters used to be folded and tried to analyze them with a special X-ray technique.

It shows a new study published in the journal Nature Communications. 

The technique is called microtomography and makes it possible to produce 3D images of an object based on X-ray images. 

"Digital twins"

Fredrik Forsberg is a senior lecturer at Luleå University of Technology and has worked with microtomography in his research for about 20 years.

He has taken part in the new study.

- A positive aspect of microtomography is that you get a digital image and twin of the object itself.

The risk with old objects is that they are gradually destroyed and this is a way to preserve them digitally, says Fredrik Forsberg.

Collection of letters from the 17th century

In the study, the researchers analyzed old sealed mail that had been preserved in a suitcase since the 17th century.

The contents of the suitcase are called "the Brienne Collection" and are a collection of letters sent from all over Europe to The Hague.

600 of a total of 3148 letters have never been opened.

- In this study, they have produced 3D images of the folded letters and then with image processing managed to unfold the letters and make the text readable.

Being able to handle it on such complicated structures is very new, Fredrik Forsberg explains. 

600 unopened letters from the 17th century have been preserved in the suitcase.

Photo: Sound and Vision The Hague, The Netherlands

300 year old letter

One of the major discoveries is the reading of a letter dated July 31, 1697. The letter was written by a man named Jacques Sennacques and the intended recipient was his cousin, a French merchant named Pierre Le Pers.

Jacques requests a copy of a death certificate from a suspected relative, Daniel Le Pers.

Prior to this discovery, only the intended recipient was known because it was written on the outside of the letter. 

Only the imagination sets limits

Through virtual opening of letters, more historical letters can be analyzed without destroying them at the actual opening.

It may reveal historical events that we did not know about before, the researchers believe in the study. 

Fredrik Forsberg also sees many opportunities with technology. 

- Before a famous artist became famous, perhaps that person painted over the motifs and reused the canvas.

Now there is the opportunity to study a work to see if there are any hidden paintings under the surface.

It is only the imagination that sets the limits for what you can do with this, says Fredrik Forsberg.