York researchers have helped reveal the anti-fraud device of choice for medieval lawyers - sheepskin.

They say for hundreds of years lawyers used sheepskin parchment (rather than calfskin or vellum) when drawing up legal documents because it made any attempt to later change what was written obvious.

The practice lasted from the 13th to the 20th Century.

A team of researchers from York and elswhere used a technique pioneered in York known as ZooMS, or Zooarchaeology by Mass Spectrometry, to identify which animals the parchment used for legal documents came from.

Co-author of the new study Prof Jonathan Finch of the Department of Archaeology said: “What our research reveals is that there was a sophisticated understanding of the properties of different products and that these could be exploited.

"In the case of sheepskin parchment, its properties were used to prevent fraud by the surreptitious alteration of important legal documents. The structure of the skin clearly showed up any attempt to erase or alter the original text."

York Press:

Prof Jonathan Finch

Sheep deposit fat in between the various layers of their skin. During parchment manufacture, the skin is submerged in lime, which draws out the fat leaving voids between the layers.

Attempts to scrape off the ink would result in these layers detaching - known as delamination - leaving a visible blemish highlighting any attempts to change any details.

Surviving historical texts also record the use of sheepskin as an anti-fraud device.

The 12th century text Dialogus de Scaccario - written by Richard FitzNeal, Lord Treasurer during the reigns of Henry II and Richard I - instructs the use of sheepskin for royal accounts as “they do not easily yield to erasure without the blemish being apparent”.

In the 17th century when paper was common, Chief Justice Sir Edward Coke wrote of the necessity that legal documents were written on parchment “for the writing upon these is least liable to alterations or corruption.”

The new findings are reported in a paper, 'Scratching the Surface: the use of sheepskin parchment to deter textual erasure in early modern legal deeds', published in the journal Heritage Science.

Dr Sean Doherty, an archaeologist from the University of Exeter who led the study, said: “Lawyers were very concerned with authenticity and security, as we see through the use of skins.”

Because they are so durable, millions of old legal documents survive in British archives and private collections, but they are often neglected because of their supposed lack of historic value.

Many were discarded, burnt, or even repurposed into lamp shades during the 20th century after the Land Registry Act of 1925 meant they didn’t need to be kept by owners or lawyers.

Until now so little was known about these documents, many were incorrectly catalogued as calfskin vellum, when they were actually made of sheepskin parchment.