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.
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. This is after flattening and before the subsequent mending.
1725 English Indenture written on vellum (parchment) that had been folded and torn across the lower fold. It had become wet which resulted in the vellum skin to dry distorted.
After humidification and flattening the vellum was mended and losses were infilled with toned Japanese paper