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 writing.

Sheepskin has a very high fat content, accounting for as much as 30 to 50 percent, compared to 3 to 10 percent in goatskin and just 2 to 3 percent in cattle. Consequently, the potential for scraping to detach these layers is considerably greater in sheepskin than those of other animals.

The work was carried out by academics from the University of Exeter and Universities of York and Cambridge.

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 seals. But it now appears as though this concern extended to the choice of animal skin they used too".

Dr. Doherty said: "The text written on these documents is often considered to be of limited historic value as the majority is taken up by formulaic rubric. 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 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".

(Via PhysOrg.)

Science fiction writers are fascinated with writing materials, as you might imagine; in his 1960 novel Dorsai!, Gordon R. Dickson in particular devised the idea of a paper contract that could not be altered in any way once written:

The single sheet he held, and even the words and signatures upon it, were all integral parts of a single giant molecule which in itself was well-nigh indestructible and could not be in any way altered or tampered with short of outright destruction...
(Read more about Single Sheet Molecule)

You might want to read these past stories, before they are altered or the memory they are kept on is erased to make way for new content:

• eScroll Design Takes E-Books Way Back
  What kind of electronic book would a time-traveler used to scrolls pick?
• Reading A Scroll Burned To Charcoal
  The effort to read the ancient text began when researchers at the Israel Antiquities Authority (IAA) in Jerusalem took high resolution x-rays...
• Diamond Light Source Illuminates Manuscripts
  A reading light so powerful that scientists will be able to read books without even opening them.
• Electronic Erasable Paper - Xerox Seeks E-Palimpsest
  Electronic reusable paper could reduce waste; as many as 40% of paper copies in offices are used for only a single day.