An odd resource for codicologists popped up a few years back when Philobiblon started a bookbinding journal called Bonefolder (which refers to one of the most basic bookbinding tools). Every now and then they publish an article that would be of interest to someone involved with the study of early Christian origins, such as an article on The Preservation of Torah Scrolls by Daniel Stuhlman. The big contribution of the article is a plea for more Jewish involvement with the preservation of Jewish book and scroll materials, as most conservation departments don't have someone on hand that is schooled ways to deal with artifacts that are still holy even if they are no longer useful. But leading up to this interesting point are some helpful descriptions of how parchment is made, and a survey of its most important preservation factors. At the very least, having some of this process in mind is helpful when looking at bits of NT parchment fragments. (Those fragments are from de Hamel's collection, recently photographed.)
The process of making parchment hasn't significantly changed since antiquity. There are references to the making of parchment in Pliny and Herodotus, and a slew of different words for the material, but by and large our knowledge of parchment preparation has been handed down to us through some Middle Age references and instructions that had been standardized centuries beforehand. Oddly enough, modern parchment preparation uses chemicals and enzymes that create weaker book materials.
From the article:
"The source for the skin is from kosher animals slaughtered for meat. Animals may not be killed solely to use the hides. The finest leathers and parchment come from fetal calves. The next grade comes from young calves. Older animals have hides with stains from the environment that are a challenge to remove. After skinning the animal, the skin is soaked in water. Lime is used to help removed the hair. The skin can be made into either leather or parchment... In general, the younger the animal at time of slaughter the thinner the hide, the smaller surface area, the smoother and finer the grain structure, and the less likelihood of damage due to disease, injury, or insects."
There was allowance in Jewish law for scrolls to be written on leather, but leather scrolls of any length are far heavier and more cumbersome than the alternative, which is parchment. Parchment is made by soaking the skin in lime for weeks, which softens the epidermis in such a way that hair can then be easily removed. In the Middle Ages, lime was predominantly used for dehairing, but in antiquity fermented organic pastes were used. The Dead Sea Scrolls, for example, were prepared in a characteristically Jewish way that involved smearing the skins with rotten flour paste and stacking them up for a few days. The old Jewish process made very fine parchments, but as the heat and enzymes built up during this process can quickly destroy skin, they eventually began to use the less volatile lime. (Throughout history, urine has also been used as a dehairing catalyst, which is why curators will ask you not to touch parchment materials directly.) After this, remaining fat and tissue are scraped from the other side. The skin is then carefully stretched, which aligns the skin fibers into a consistent grain, and it is scraped again while drying. By now the skin is pure collagen, and these additional scrapings allow the tanner to determine the final thickness and quality of the parchment. There are many ways to polish stretched parchment. Pumice, or rolls of bread with bits of glass baked into them, make the skins smooth and receptive to ink. Lime, egg whites, and other materials made the skins whiter. This process produces a writing material that is very receptive to ink and other sorts of decoration. It is more durable than paper from the binder's perspective, and very pleasurable to work with as its stiffer properties make it easier to sew and arrange in complex ways. But long term, parchment is more susceptible to poor environmental conditions. It is easy to see this difference in the curling, discoloration, and chemical reactions to various inks that characterize early parchment fragments (see this excellent summary of how leather and parchment decay).
In scholarship on fragments of biblical texts, parchment and vellum are sometimes used as interchangeable terms. Vellum is actually a subcategory of parchment made from the skin of calves, and graded by the age of the calf as noted above from the article. Technically, Hebrew scrolls are written on vellum, as parchment can be made from sheep, goats, horses, or cows. The Mishneh and Talmud call the vellum most scrolls were written on "gevil," which refers to the outer layer of a split-calf skin. (I am pretty sure the DSS were written on this layer of skin, which is a handy example from antiquity.)
There is a nice short summary of this process as applied to the medieval codex here. Either way, I think it is helpful to have these processes in mind when looking at fragments and folios. Not only does it help explain some of the physical anomalies that pop up every now and then in the form of bumps, splits, and holes - but it further helps to remind us that texts and textual variants aren't the only history told by manuscripts and codices.