Sheepskin was the most commonly used parchment for legal deeds over the past five centuries in Great Britain, even though it is quite fragile. This is most likely because fraud can be more easily detected on it than on vellum.

Sean Doherty at the University of Exeter in the UK and his colleagues analysed 645 pages from 477 legal deeds concerning property in England, Scotland and Wales dating from 1499 to 1969.

They first cut a 2-square-millimetre sample of parchment from the edge of the documents. “We made sure to be well away from the text, any stamps and wax seals,” says Doherty.

The team then chemically treated the animal skin to isolate the protein collagen, which is made up of a mix of sub-units called peptides. “Each animal has a different set of peptides that make up collagen – it is species variable,” says Doherty. This let the researchers work out what type of parchment each deed was written on.

They found that 622 of the 645 pages were made from sheepskin, which was a surprise, as previous research suggested these types of documents were made with a variety of animal skin – most commonly vellum, which is made from calfskin.

“We expected to see a wide range of animals, but they pretty much all turned out to be from sheep,” he says.

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Doherty and his team suspect that sheepskin was used for important deeds because it is difficult to alter without being noticed due to its high fat content.

When animal skin is first processed, it is submerged into an alkaline solution of chalk. This draws out the fat and removes any hair, leaving behind the dermis layer of the skin which is then stretched into parchment.

Sheepskin is between 30 to 50 per cent fat, compared to just 2 to 3 per cent in cattle and 3 to 10 per cent in goats. The removal of the fat causes sheepskin parchment to be very fragile. “The layers will detach because it has all these holes in it where the fat once was,” says Doherty.

As a result, you can see a visible mark where text has been altered on sheepskin more easily than other animal skins, which is useful for important documents.

“If someone intentionally tried to alter a word on a deed made of sheepskin, they would leave behind a telltale smudgy residue,” says Heather Wolfe at Folger Shakespeare Library in Washington DC, who wasn’t involved in the research.

Journal reference: Heritage Science, DOI: 10.1186/s40494-021-00503-6