Vellum and parchment are animal skins that have been soaked in limewater, scraped clean of flesh and fat, depilated, stretched, and polished or sanded smooth. Their method of preparation was invented during the reign of Eumenes II of Pergamon, in the second century before Christ. Within a few centuries, it had largely replaced papyrus in bookmaking, being more suitable for the newly-invented, and characteristically Christian, form of the book, the rectangular codex. Although paper was introduced into Europe by the 11th century, the vast majority of European books predating the advent of printing were written on vellum or parchment, including almost all of the most beautifully illuminated manuscripts.
The terms vellum and parchment are sometimes used interchangeably, but I tend to use the former word to refer to calfskin and the latter to refer to goatskin or sheepskin. I have drawn many works on calfkin, and a few on goatskin. I have never tried sheepskin; I read somewhere that sheepskin is oilier and that therefore it is more difficult to make corrections on it (which is why it was preferred for legal documents rather than artistic manuscripts). In medieval times, the thinnest sheets of calfskin were reserved for the best books (slunk or uterine vellum, made from the skins of miscarried calves, was especially prized). Personally, I prefer to work on thicker sheets of calfskin; these lay flatter and are easier to handle; since I am not binding the sheets into codices, their flexibility is not especially important. Goatskin is much thicker and more opaque than calfskin.
Medieval scribes and illuminators worked on both sides of a sheet of vellum or parchment. The flesh side of the sheet is somewhat whiter, and the hair side somewhat yellower; the difference is more pronounced in goatskin. Tanners sometimes create a fuzzy nap surface on a sheet, which is preferred by calligraphers. I prefer as smooth a surface as possible, and will draw on whichever side of the sheet is smoother.
I would rather draw on vellum or parchment than on any kind of paper. This is less because of its historic connections to medieval art than because it is a far superior surface for drawing. It is possible to draw a finer line on vellum and parchment than on paper. Any kind of paper is made of pulverized vegetable fibers; when a vegetable fiber touches liquid ink, it pulls the ink further along its length, like a capillary. Skin cells do not do this, so a line drawn in ink over them has less bleed. Because skin cells are naturally arranged in layers, it is possible to scrape them away very cleanly. Entire sheets of vellum and parchment were sometimes scraped clean and reused; these are called palimpsests and are especially interesting to historians becuase the erased text and images can be recovered with special photography. An artist drawing on vellum or parchment can make easy corrections with a knife, or scratch details into inked areas, with a precision that is simply impossible when drawing on paper.
The only real disadvantages to vellum and parchment are that they are more expensive than paper, and that large sheets of them are seldom perfectly flat or homogenous. Only a few of my drawings are so large that this problem compels me to use paper instead.
The first two photographs here show drawings in progress on calfskin; the third shows drawings in progress on goatskin.
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