For seven hundred years British legal documents were written almost exclusively on sheepskin. Parchment made from the skin of sheep has a built-in mechanism for detecting tempering after it has been written on, scientists have found, a feature presumably more popular with lawyers than sheep.
Parchment made from animal skins, while far more expensive to produce, is more durable than paper, and therefore preferred for many uses. Dr Sean Doherty of the University of Exeter, UK, performed mass spectrometry on 645 individual page samples from British legal documents from archives and private collections dating from the 16th to 20th centuries. Doherty identified proteins that showed even those cataloged as calfskin were actually sheepskin for 622 of them, the remaining few being unidentifiable. This surprised him because non-legal documents of similar age were written on a mix of sheep, goats, or calfskin.
Finding sheepskin had been preferred for legal purposes from the 13th century, Doherty was curious whether this was simply a custom that stuck, or if there was a reason for medieval lawyers' sheepishness.
In Heritage Science, Doherty reports sheepskin, like that of humans, consists of several layers. Unlike us, sheep have plenty of fat in between. During parchment manufacturing the skins were placed in lime, which draws out the fat, leaving spaces between layers.
Any effort to scratch off ink from sheepskin parchment causes the upper layers to detach, known as delamination (although it really should be delambination). What has been scratched out may be unreadable, but it will be obvious to anyone that a change has been made, invalidating the document.
Doherty reports that, before being put in lime, sheepskin is 30-50 percent fat, while goatskin is just 3-10 percent. As for cattle, it's just 2-3 percent, so dekidination and decalfination are much less obvious.
"Lawyers were very concerned with authenticity and security, as we see through the use of seals. But it now appears as though this concern extended to the choice of animal skin they used too" Doherty said in a statement.
There are texts that hint at the use of sheepskin as an anti-fraud device utilized by savvy lawyers. In the 12th century, Richard FitzNeal, Lord Treasurer during the reigns of Henry II and Richard I instructed the use of sheepskin for royal accounts as "they do not easily yield to erasure without the blemish being apparent".
As time went on and paper replaced parchment for non-legal uses, awareness of this attribute of sheepskin presumably declined. However, tradition can be a powerful thing, and with sheepskin widely available and usually cheaper than other animal alternatives there was no reason to change. Not all forgot, however. In the 17th century, Chief Justice Sir Edward Coke wrote of the need to write legal documents on parchment, rather than paper because it was “Lease liable to alterations or corruption.” So much for the law being an ass.
"The text written on these documents is often considered to be of limited historic value as the majority is taken up by formulaic rubric,” Doherty said. “However modern research techniques mean we can now not only read the text, but the biological and chemical information recorded in the skin. As physical objects, they are an extraordinarily molecular archive through which centuries of craft, trade, and animal husbandry can be explored."
The finding follows explorations of what can be revealed by the way letters from the same era were folded.