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An international team of archaeologists, bioinformatic specialists, and historians has discovered that many medieval books were bound using seal skin. In their published in the journal Royal Society Open Science, the group describes their biocodicological analysis of medieval books.

Prior research has shown that most medieval books were made in European abbeys and monks used parchment made of animal skin for the pages. Such books were then typically covered using leather, wood or some type of cord. Additionally, some books were given a second cover, called a chemise, to better protect the book against the elements.

Until now, it was believed that most chemises were made using local animal skins, such as deer or boar. In this new effort, the research team has found that many were in fact made using seal skin.

To learn more about chemise construction, the researchers began with 19 books held at the Library of Clairvaux Abbey, in France. All were made between the years 1140 and 1275. Testing was done initially using mass spectrometry. Follow-up tests involved conducting ancient DNA analysis of the material used to make the chemises.

The research team found that all the books they tested had a chemise made of sealskin. The team then studied an additional 13 books from abbeys in France, England and Belgium dated from 1150 to 1250 and found that they were also bound in sealskin.

The researchers noted that seal species varied, from harp, to harbor to bearded varieties. They were also able to trace the origin of the seals, from off the coast of Scandinavia, to Greenland, Scotland, Iceland and Denmark.

The research team showed that all the books were made by monks working in abbeys located along 13th-century trading routes—and also along Nordic trading routes. The different abbeys appeared to have different tastes in sealskin, based mostly on color.

They suggest the monks began using the sealskin when it was found to be more water resistant than skin from land-based animals. They also noted that it is not clear if the monks even knew what sort of animal skin they were using—there was no word for seal in their language at the time.