It’s not uncommon for Medieval manuscripts to be bound in animal skins. The hides of deer and goats were used, as were those of boars. But a new analysis of 16 manuscripts held in a French abbey has uncovered the use of a mysterious, surprising skin.

The study homed in on a set of Romanesque volumes in the Clairvaux Abbey in France. Established in 1115, the monastery has in its collection a whopping 1,450 manuscripts, which date back to the 12th and 13th centuries. Among these are 168 Romanesque bindings, of which 28 are still intact, recognizable for their lush illumination and intricate patterns. At Clairvaux, they’re also distinctive for being encased in secondary covers, long been assumed to have been crafted out of boar or deer skin.

Not so, the new research found. Upon scrutiny, the authors of the paper, published in Royal Society Open Science, discovered that the distribution of hair follicles on these covers was unlike that of either animal. These surfaces, in fact, were hairier, with the team characterizing them as “furry skin chemise[s].”

A primary hair, flat at the base. Ms 31 (Médiathèque du Grand Troyes). Photo: E. Lévêque et al., Royal Society Open Science / doi.org/10.1098/rsos.241090.

To pinpoint their source, the researchers adopted biocodicological techniques including electrostatic zooarchaeology by mass spectrometry (eZooMS) and ancient DNA studies. The former examination, a biomolecular analysis method, identified the origin of seven manuscript covers as originating from pinnipeds, also known as seals. DNA sequencing confirmed the finding, further identifying harbor seal and harp seal skins in five out of the seven samples.

Another 43 manuscripts—18 from Clairvaux and other comparable ones from Medieval libraries around France and the U.K.—were subject to deep forensic visual analysis. Seen under extreme magnification, their covers evidenced common elements across the morphology, arrangement, and distributions of hair on the skins. Pores and follicles, as well as the fat layers on the surfaces, led the researchers to identify the presence of phocids, or earless seals, also of the pinnipedia suborder.

Hand-colored engraving of the Harp Seal (1825). Photo: Universal History Archive / Universal Images Group via Getty Images.

So, why seals? For one, the light coloring of their hides. While the manuscript covers appear brown today, the team posits that the shade is simply due to degradation. The Cistercian monks who compiled these books likely did not choose a brown color for these bindings, they said, as the hue was more commonly associated with the Benedictine order.

“Cistercians are known for their affinity with white clothing and objects,” they wrote, “and, therefore, it is likely that pinniped skins were chosen for the chemises due to their light grey or white coloration.”

Close-up of one of the manuscripts at Clairvaux Abbey. Photo: Matthew Collins.

From the findings, too, the Cisterians appear to have acquired these skins from global trading networks, contrary to long-held beliefs that the manuscripts were created with locally sourced materials.

The study has traced six of the sealskin covers to harbor seal populations in Scandanavia and Scotland and harp seal species in Iceland or Greenland—key locations on Norse trading routes. A Medieval trading network, namely the Hanseatic League, could feasibly have seen these hides reach Clairvaux Abbey, which is situated close to a land trading route and engaged with the Champagne fairs, where exchanges were conducted.

Engraving of St. Bernard of Clairvaux and the Cistercian Monks Taking Possession of the Abbey of Clairvaux, from Military and Religious Life in the Middle Ages (c. 1880) by Paul Lacroix. Photo: Universal History Archive / Universal Images Group via Getty Images.

Did the monks know what they were buying? Seals, while cherished commodities on the Scandinavian coast, were rarely depicted in European Medieval art; bestiaries simply referred to the creature as a “sea calf,” which was depicted with the head of a wolf and four legs. No written records exist to explain why the monks chose sealskin to bind their books, or whether the material was of particular value to them.

Quite likely, the team concluded, the Cisterians had “a detachment from the animal’s true identity, making it difficult for contemporaries to recognize sealskin as a specific material.” Still, that didn’t stop them from using the skin to bind some of their most prized texts, including a historical account of St. Bernard.