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.