Make of leather
The tanning process is rather different; Dan argues that the best leathers are from the early 1800s, as anything after the Industrial Revolution begins to face deteriorating problems. Before the 19th century most leather was tanned with oak or other vegetable tannin. From the 1850s to the 1920s cheaper catechol (no longer vegetable oil, but chemicals) tanned leathers began to be produced which resulted in the problem of red rot.
Traditional tanning follows these steps: limed and dehaired, delimed, drenched in bran and citric acid, placed in a vat with tannin in it while progressively upping the strength of the tannin, dried, split and then finished. Modern methods of tanning are less time consuming. They involve stronger liquor (chemical in the vat), mechanical action to speed tanning, pH control, and a precise control of acids and salts.
Tanning is subdivided into vegetable tanned (used in historical and traditional work) and chemical tanned (often called chrome tanned and used in clothing), and then combination tanned (provides the flexibility of vegetable tanning and the longevity of chemical). The objective of tanning is to render the hides and skins resistant to decomposition or bacterial decay and to provide it with tensile strength, flexibility and abrasion resistance.
As for vellum and parchment, it is a semantic nightmare. The meaning of the word can vary with country origin, language, period in which the term was used, the animal it comes from, the usage to which it is being put, or a combination of any of those! In the European continent, the term parchment, according to Dan, is used as a generic term for any skin processed in a manner for binding. Whereas the term vellum, which comes from old French, may indicate thin material finished on both sides and used for writing.
In England specifically though, vellum refers to skin that is finished on one side only and used for binding, and parchment is finished on both sides and used for writing.
Both vellum and parchment are made by stretching skin and scraping it over and over again. Vellum is very stable: it’s almost a neutral pH level. Often times the term “limp vellum bindings” is heard and this refers to an old form of binding used to make the cheap “paperbacks” of medieval times – it’s very durable though!
It does caulk easily with water, so relative humidity effects it a lot.