Vellum is generally smooth and durable, although there are great variations depending on preparation, the quality of the skin and the type of animal used. The manufacture involves the cleaning, bleaching, stretching on a frame (a “herse”), and scraping of the skin with a hemispherical knife (a “lunarium” or “lunellum”). To create tension, scraping is alternated with wetting and drying. A final finish may be achieved by abrading the surface with pumice, and treating with a preparation of lime or chalk to make it accept writing or printing ink.

Modern “paper vellum” (vegetable vellum) is a quite different synthetic material, used for a variety of purposes including plans, technical drawings, and blueprints.

Vellum is a translucent material produced from the skin, often split, of a young animal. The skin is washed with water and lime (Calcium hydroxide), but not together. It is then soaked in lime for several days to soften and remove the hair. Once clear, the two sides of the skin are distinct: the side facing inside the animal and the hair side. The “inside body side” of the skin is the usually lighter and more refined of the two. The hair follicles may be visible on the outer side, together with any scarring, made while the animal was alive. The membrane can also show the pattern of the animal’s vein network called the “veining” of the sheet. 

Any remaining hair is removed (“scudding”) and the skin is dried by attaching it to a frame (a “herse”). The skin is attached at points around the circumference with cords; to prevent tearing, the maker wraps the area of the skin to which the cord is to be attached around a pebble (a “pippin”).   The maker then uses a crescent shaped knife, (a “lunarium” or “lunellum”), to clean off any remaining hairs. Once the skin is completely dry, it is thoroughly cleaned and processed into sheets. The number of sheets extracted from the piece of skin depends on the size of the skin and the given dimensions requested by the order. For example, the average calfskin can provide three and half medium sheets of writing material. This can be doubled when it is folded into two conjoint leaves, also known as a bifolium. Historians have found evidence of manuscripts where the scribe wrote down the medieval instructions now followed by modern membrane makers. The membrane is then rubbed with a round, flat object (“pouncing”) to ensure that the ink would adhere well. 

The important distinction between vellum (or parchment) and leather is that the former is not processed using tanning techniques. The distinction between vellum and parchment has been made in several different ways, and no one definition can be considered correct, but vellum has always denoted the better quality.

In Europe, from Roman times, the term vellum was used for the best quality of prepared skin, regardless of the animal from which the hide was obtained, calf, sheep, and goat all being commonly used (other animals, including pig, deer, donkey, horse, or camel have been used). Although the term derives from the French for “calf”, except for Muslim or Jewish use, animal vellum can include hide from virtually any other mammal. The best quality, “uterine vellum”, was said to be made from the skins of stillborn or unborn animals, although the term was also applied to fine quality skins made from young animals. Many libraries and museums increasingly use only the safe if confusing term “membrane”; depending on factors such as the method of preparation it may be very hard to determine the animal involved without using a laboratory, and the term avoids the need to distinguish between vellum and parchment.

French sources, closer to the original etymology, tend to define velin as from calf only, while the British Standards Institution defines parchment as made from the split skin of several species, and vellum from the unsplit skin.In the usage of modern practitioners of the artistic crafts of writing, illuminating, lettering, and bookbinding, “vellum” is normally reserved for calfskin, while any other skin is called “parchment”.

Preparing manuscripts

Once the vellum is prepared, traditionally a “quire” is formed of group of several sheets. Raymond Clemens and Timothy Graham point out, in “Introduction to Manuscript Studies”, that “the quire was the scribe’s basic writing unit throughout the Middle Ages”. Guidelines are then made on the membrane. They note “’pricking’ is the process of making holes in a sheet of parchment (or membrane) in preparation of it ruling. The lines were then made by ruling between the prick marks…The process of entering ruled lines on the page to serve as a guide for entering text. Most manuscripts were ruled with horizontal lines that served as the baselines on which the text was entered and with vertical bounding lines that marked the boundaries of the columns.”

Usage

Most of the finer sort of medieval manuscripts, whether illuminated or not, were written on vellum. Some Gandharan Buddhist texts were written on vellum, and all Sifrei Torah (Hebrew: ספר תורה ; plural: ספרי תורה, Sifrei Torah) are written on kosher klaf or vellum.

A quarter of the 180 copy edition of Johannes Gutenberg’s first Bible printed in 1455 with movable type was also printed on vellum, presumably because his market expected this for a high-quality book. Paper was used for most book-printing, as it was cheaper and easier to process through a printing pressand bind.

In art, vellum was used for paintings, especially if they needed to be sent long distances, before canvas became widely used in about 1500, and continued to be used for drawings, and watercolours. Old master prints were sometimes printed on vellum, especially for presentation copies, until at least the seventeenth century.

Limp vellum or limp-parchment bindings were used frequently in the 16th and 17th centuries, and were sometimes gilt but were also often not embellished. In later centuries vellum has been more commonly used like leather, that is, as the covering for stiff board bindings. Vellum can be stained virtually any color but seldom is, as a great part of its beauty and appeal rests in its faint grain and hair markings, as well as its warmth and simplicity.

Modern Usage

British Acts of Parliament are still printed on vellum for archival purposes, as are those of the Republic of Ireland. It is still used for Jewish scrolls, of the Torah in particular, for luxury book-binding, memorial books, and for various documents in calligraphy.

Today, because of low demand and complicated manufacturing process, animal vellum is expensive and hard to find. The only UK company still producing traditional parchment and vellum is William Cowley (Est 1870) who are based in Newport Pagnell, Buckinghamshire. A modern imitation is made out of cotton. Known as paper vellum, this material is considerably cheaper than animal vellum and can be found in most art and drafting supply stores. Some brands of writing paper and other sorts of paper use the term “vellum” to suggest quality.

Vellum is still used on instruments such as the banjo and the bodhran, although synthetic skins are also available for these instruments.

Preservation

True vellum is typically stored in a stable environment with constant temperature and 30% (± 5%) relative humidity. If vellum is stored in an environment with less than 11% relative humidity, it becomes fragile, brittle, and susceptible to mechanical stresses; if it is stored in an environment with greater than 40% relative humidity, it becomes vulnerable to gelation and to mold or fungus growth. The optimal temperature for the preservation of vellum is 20 ± 1.5 °C (68 ± 3 °F).

(Wikipedia).