An interdisciplinary team of researchers has documented the widespread use of seal skins in Romanesque bookbindings of manuscripts produced between the 12th and 13th centuries in Cistercian monasteries in France, the United Kingdom, and Belgium. The study, led by Élodie Lévêque and published in the journal Royal Society Open Science, combines archaeology, protein analysis, and ancient DNA to uncover the biological and geographical origin of these unusual bibliographic materials, previously invisible to historians and codicologists.

The starting point for this research lies in a material singularity: a series of manuscripts preserved in their original bindings, particularly in the collection of the former abbey of Clairvaux, founded in 1115 in the Champagne region. These bindings feature a second cover or chemise, made of an unusual type of leather that, unlike typical parchment, retains traces of hair. For centuries, it was assumed that they came from wild boars or deer. However, microscopic examination revealed patterns of hair follicles that did not match any of these species.

Faced with this mystery, the researchers turned to biomolecular analysis techniques. Using an innovative methodology known as Zooarchaeology by Mass Spectrometry (eZooMS)—which allows the identification of proteins without damaging the object—and ancient DNA (aDNA) analysis, they were able to classify the materials as belonging to pinnipeds, that is, seals and other related marine species. In some cases, they even identified the species: harbor seals (Phoca vitulina), harp seals (Pagophilus groenlandicus), and a bearded seal (Erignathus barbatus).

sealskin manuscripts medieval
Left, Romanesque binding from Clairvaux covered with a chemise with hairs on (Médiathèque du Grand Troyes, ms. 35, ca. 1141−1200), sample EL53. Right, Ms 40 (6), Médiathèque du Grand Troyes, detail of skin patching. Credit: Élodie Lévêque et al.

Beyond mere zoological identification, the finding takes on a historical dimension by establishing the geographical origin of these skins. The mitochondrial DNA sequences analyzed in laboratories in York and Copenhagen indicate that the harbor seals came from current populations in Scotland, Norway, Denmark, and the Baltic Sea, while the harp seal and bearded seal point to more northern regions such as Greenland, Iceland, or the Barents Sea.

This information suggests that Cistercian monasteries—frequently perceived as centers of self-sufficiency and retreat—were part of long-distance trade networks linking continental Europe with the North Atlantic Arctic. These networks, in which Nordic peoples played a crucial role, transported not only walrus ivory, leather ropes, and other goods but also skins used as coverings for liturgical books.

There are no written records documenting the acquisition or use of seal skins in Clairvaux. Nevertheless, the genetic, morphological, and archaeological evidence converges to show that the phenomenon was widespread. The collection preserved in the Médiathèque du Grand Troyes and the Bibliothèque de Saint-Omer, along with copies from Laon, Oxford, and Brussels, accounts for at least 105 Romanesque bindings originally covered with seal skin, of which 83% have lost their chemise over time, leaving only traces beneath metal clasps or stitching.

Today, the skins display a dark brown color, the result of hair degradation and the exposure of the subcutaneous fatty layer typical of seals. However, it is unlikely that this was their original appearance. Unlike the Benedictines, the Cistercians preferred white or pale tones for their liturgical objects and vestments. In the Middle Ages, “white” did not mean the pure white of today, but rather any light or desaturated hue, so it is plausible that seal skins, especially those of pups, with their grayish or silvery tones, were perceived as suitable for monastic use.

sealskin manuscripts medieval
Macroscopic examination of a contemporary sample shows the light colour of the natural hair (this sample has been treated with artisanal, greasy techniques and is undyed). A few millimetres have been shaved from the lower end, exposing the dark appearance of the skin surface under the fur. Credit: Élodie Lévêque et al.

The extensive use of these skins does not appear to follow a hierarchical criterion: manuscripts of varying content and value feature similar covers, suggesting a standardized binding norm in Clairvaux and its affiliated abbeys. This pattern reinforces the hypothesis of an established practice within a particular monastic tradition, perhaps facilitated by the relationships between Clairvaux and insular monasteries, such as those in Ireland and England, where the use of seal skins was common.

The progressive disappearance of seal skin use in Cistercian bindings coincides with climatic and economic transformations. By the late 13th century, the cooling of the North Atlantic—the onset of the so-called Little Ice Age—made Nordic hunting and trade routes more difficult. Added to this were political and ecclesiastical factors, such as the imposition of tithes by Pope Gregory X, which had to be paid even from Greenland in the form of goods like skins, tusks, or whale baleen, due to the scarcity of coinage.

Climate change, together with the overexploitation of non-migratory coastal seals, may have contributed to the collapse of these practices, as well as to the disappearance of Norse settlements in Greenland. This circumstance could explain the abrupt cessation of marine skin use in Clairvaux’s books around 1300.

Despite the wealth of recovered data, fundamental questions remain. Did the monks really know they were using seal skins? Did they see them as symbols, offerings, or simply exotic materials? How aware were they of the networks that brought them from distant lands? Medieval iconography rarely features depictions of seals, and some specialists, such as Michel Pastoureau, suggest that the monks may not have associated them with the animals described in bestiaries as “sea calves.”

In the absence of direct written testimony, material remains combined with molecular analysis offer a glimpse into a previously invisible history: that of books covered with skins from the Arctic, traveling along medieval trade routes and protected for centuries in silent libraries.

SOURCES

Élodie Lévêque, Matthew D. Teasdale, et al., Hiding in plain sight: the biomolecular identification of pinniped use in medieval manuscripts. Royal Society Open Science, vol.12, issue 4, doi.org/10.1098/rsos.241090

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