Parchment is a very difficult matter to handle: due to its quality to strongly react to fluctuations in environmental humidity (RH) it is always subject to changes in size. Longterm storage under wrong conditions even changes the characteristics of the material dramatically: the parchment gets hard and breakable.
When restoring damaged parchment bindings most of the times the binding has shrinken, sometimes more than 10%. This results in enormous forces on other parts of the binding: the backbone gets torn or the leather bindings break. Often the strain is that big that the back of the parchment cover breaks on the folds. New material has to be applied to fix these problems and that is a problem as well; it is very difficult to glue new parchment to old. Although we useally use organic (animal- or vegetablebased) glue we apply a synthetic glue here. An additional problem is that parchment (when moist) cannot be pressed to heavy; it becomes pellucid. In the last years we have developped a method however to get it done.
Gluing, colouring and other treatments of old parchment needs a lot of experience-based knowledge, besides of the theoretical background.