Rare books covered with seal skin hint at a medieval trade network

Science is helping researchers judge books by their covers — and revealing surprising beneficiaries of medieval trading routes in the process.

Dozens of rare, fur-covered volumes from 12th and 13th century French monasteries are wrapped with seal skins that may have come from as far away as Greenland, researchers report April 9 in Royal Society Open Science. The findings challenge the assumption that the books’ makers used only locally sourced materials and suggest that they were part of an extensive trade network.

The books hail from Clairvaux Abbey, founded in 1115 by Cistercian monks in northern France, and its daughter monasteries. Some tomes are nearly 900 years old. Researchers had thought they were wrapped with boar or deer skin. But when book conservator Élodie Lévêque looked at them through a microscope, she was stumped.

The primary covers were obviously made of sheepskin, but Lévêque struggled to identify the skin used for the furry chemise — the outermost protective cover. So she had scientists compare proteins from chemise samples with known animal proteins. It turns out that the skins belonged to seals.

“I was like, ‘that’s not possible. There must be a mistake,’” says Lévêque, of Panthéon-Sorbonne University in Paris. Seals didn’t frequent France’s northern coast at the time, she says. “I sent it again, and it came back as seal skin again.”

Comparing DNA from five chemises with DNA from seals confirmed that the covers were indeed seal skin. Four of the chemises are genetically similar to harbor seals from Scandinavia, Denmark and Scotland, while the fifth chemise is genetically similar to harp seals, likely from Greenland or Iceland. The researchers visually identified other furry chemises and eventually cataloged 43 seal-skin books.

Norse hunters in those regions may have caught seals and brought their skins to northern France through trading routes, Lévêque and colleagues say. The monks may not have known that they were covering their books with seal skins, the team suggests.

The worn, brownish covers may have been furrier and a different color in their heyday, Lévêque says. “At the time, it would have looked completely like a teddy bear, but light in color.”