Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

parchment

In the manufacture of parchment, the skin is first limed and unhaired (or dewooled), a process generally accomplished by scraping the skin with a blunt knife. Following unhairing, the skin is dried under tension. While this is taking place, more lime is applied to remove moisture and grease, particularly the latter in the case of sheepskins. Finally, the parchment is finished while still in a taut condition; the surface is smoothed by shaving it with, a semi-circular (often semi-lunar) knife, and rubbing it with pumice. Despite this treatment, the flesh side can usually be distinguished from the grain side of an unsplit skin by its rougher texture and often darker color. In books the pages are usually arranged grain- to grain-side and flesh- to flesh-side in order to provide a more uniform appearance.

Parchment manufacture, which subjects the fiber network of the skin to the simultaneous action of stretching and drying, causes changes in the skin very different from those which take place in the manufacture of leather. The dermal fiber network is reorganized by the stretching, and the network is then permanently set in this new and highly taut form by drying the fluids—i.e., ground substance of the skin—to a hard gluelike consistency. The fibers of the skin are thus fixed in a stretched condition and, as long as the skin remains more or less dry, they will not revert to their original three-dimensional network. This gives a taut, highly stressed sheet that is relatively inelastic and has a stiff handle. In addition, the alignment of the fibers into layers parallel to the grain and flesh surfaces of the skin (resulting in very low or virtually zero angle of weave) involves a certain extent of breakage of fibers in the dermal network. It is this variation in mechanical processing that results in the fundamental difference between parchment and leather. The extent of the alteration of fiber orientation depends on several factors, including the species of the animal; the age, sex, diet, etc., of the skin being processed; the intensity of the liming it has received; and the tension and rate at which the wet, stretched skin is dried. The fiber orientation of parchment is such that it tends to tear fairly easily into a number of thinner sheets, whereas leather cannot be torn in this manner because it retains its original three-dimensional network.

Originally, parchment was made from the full thickness of a skin, and was made thinner in the Middle Ages by shaving. The modern practice, however, is to use only the flesh layer of a split skin, which means that neither side of the finished parchment has any grain pattern. The remaining grain split is generally used to make a thin leather, usually a skiver.

As in leather manufacture, it is usually the skins of animals slaughtered for their meat that are used in making parchment. As the blood drains from the animal, the minute network of dermal blood vessels becomes colorless and is usually undetected in the flayed skin. Proper drainage of the blood vessels is essential, otherwise the iron compounds of the blood will react with the lime liquors to form dark colored pigments which are extremely difficult, if not impossible, to remove. If, however, some blood does remain in the vascular system during the processing of a skin into parchment, so that a colored pattern of blood vessels is left on the finished material, the parchment is said to be "veined." Assuming the veining pattern is of an attractive character, it may actually enhance the aesthetic appeal of the parchment for use in bookbinding. The colors and depths of shading of finished parchment vary with the animal skin, ranging from the greenish markings of goatskin, through the light brown patterns of calfskins, to the brown-black shades of sheepskins.

Contrary to the process in leather manufacture, the lime used for unhairing skins in parchment is not subsequently removed. The presence of so much alkaline substance may explain why parchment is not affected as much as leather is by the action of acids resulting from atmospheric pollution, mainly from the presence of sulfur dioxide (SO.,). Parchment, however, is readily affected adversely by water, and water can very easily permit the multiplication of bacteria, which can rapidly degrade and even destroy parchment. In addition, if parchment is permitted to absorb large quantities of water, the setting and fixing action of its dried ground substance will eventually break down allowing the fiber network to relax since it is no longer fixed in a taut condition. When the parchment is subsequently dried in this relaxed condition, the very properties that made it parchment initially are lost, and a hard, horny sheet, not unlike rawhide, is all that remains. It follows then, that parchment should not be washed, at least in the manner that leather may be washed, or allowed to remain in an environment saturated with water vapor.

Medieval scribes usually pounced their parchment a second time (also with pumice) before writing. It was also given a coat of glue sizing before it was illuminated. In the 18th century, however, a new method of sizing parchment was developed in which the size was actually formed in the surface of the sheet by dissolving it with hot water. As a result of this new technique, pouncing is no longer usually required.

In the modern method of parchment manufacture, the shaving knife is still used, but the skins are rapidly unhaired by the use of sodium sulfide, split by machine, and dried in an oven under tension. If a transparent skin is required, the tension is relaxed somewhat. Transparent parchment was at one time used in decorative schemes displaying paintings underneath the covering material of books. A patent for making transparent parchment (vellum) was issued to James Edwards in 1785. See: EDWARDS OF HALIFAX .

Ideal storage conditions for parchment are temperatures between 0° and 20° C. (32° and 68° F.), with a relative humidity of 50 to 65%. Although, under no circumstances should parchment be allowed to become saturated with water, neither should it be allowed to dry out.

The manufacture of parchment dates back to at least the Middle Kingdom of Egypt, or approximately 2000 B.C. Its manufacture arrived in Northwestern Europe along with Christianity, where it became the most important writing material of the Middle Ages. From the 12th century onwards, however, its use slowly declined in favor of paper. Its use today is limited, being restricted largely to state and legal documents, certificates, and the like; in the construction of musical instruments; and in certain aspects of archival conservation.

Some authorities use the terms parchment and VELLUM interchangeably, contending that vellum is simply a superior form of parchment—one made from the unsplit skin of a calf. Others, however, maintain that, whatever the historical derivations may be, parchment is a material made from the flesh split of a sheepskin, while vellum is made specifically from an unsplit calfskin. In either case, both materials are produced by the same process. See also: GOLDBEATER'S SKIN ; IMITATION PARCHMENT . (198 , 236 , 263 , 291 )