We protect our digital files with passwords, encryption, or blockchain technology to prevent fraud and scams. In the Middle Ages and the early modern period, the authors of official documents had their own ways. The methods used were very ingenious…

From the thirteenth to the nineteenth centuries, British lawyers preferred to write on sheepskin parchment because it is difficult to make changes unnoticed. Anyone who tries to commit fraud would, unlike with goat or calf parchment, be caught. A layer of the sheet would come off as ink was scraped off, blemishing the document.

This is the conclusion of British researchers who examined 477 legal documents from that time. Of the 645 pages examined, 622 were found to be made of sheepskin.

It was already known that medieval lawyers considered the authenticity and security of documents important through the use of seal stamps. “But that concern also extended to the choice of animal hide,” said Sean Paul Doherty of the University of Exeter, one of the authors of the study, published in Heritage Science.
The secret is in the fat percentage.

The researchers took samples from each parchment page to examine the biological material. Sheepskin consists of about 30 to 50 percent fat, while the skin of a goat or beef has a fat percentage of 3 to 10 percent.

During the making of parchment, an animal’s skin is placed in lime water, which loosens hair and fat. The cohesion between the layers of sheepskin diminishes due to this strong decrease in fat, making layers come off more quickly. And that is a handy feature that prevents someone unseen from changing a number or passage by scratching away text.

Cautious hints that lawyers liked to use sheepskin, for this reason, could already be found in written texts. In the seventeenth century, Sir Edward Coke, who wrote several standard works on English common law, noted that this parchment is “the least susceptible to alteration or corruption.”

Sheepskin parchment was also used for the administration of Tudor monarchs (1485-1603). That was even chosen over the more luxurious calfskin vellum. That choice can now be better explained.

Sheepskin parchment was also cheaper because, in England, it was bursting with sheep. In addition, parchment was seen as a material that lasts longer than paper. Until well into the nineteenth century, lawyers, therefore, preferred such important documents of animal skin. In fact, parchment is so durable that old archives are still bulging with it.