Very early books and documents from the medieval times, and well into the 20th century were written or printed on vellum (parchment). True vellum is made from the skin of an animal, usually calf, sheep or goat. This material inherently is very robust and will last for centuries. But it is very susceptible to climatic changes. It will expand and contract with variations in humidity or moisture of any kind, which will result in excessive distortions of the surface.
I have pioneered techniques that will vastly improve and simplify the treatment, flattening and mending of vellum. I have taught courses in the US as well as in Switzerland on the Conservation of Parchment Documents and Bindings.
One of the most difficult projects I have been confronted with was to try to improve upon the damage caused by a Frame Shop that miss handled a vellum document.
A previous attempt to preserve this document by dry mounting resulted in tears, and distortion of the vellum. A secondary attempt to remove the lining by a hot water bath caused greater damage.
The vellum was humidified and flattened repeatedly to try to align the tears as much as possible; the image below is before the subsequent mending and after.