Everything you wanted to know about the artistic paper restoration process:

Until the 1750s-1700s, the paper was of much higher quality. It was produced from linen, and was significantly thicker than the paper we have today. This is why it is preserved better than later prints. Most of the problems in these books were created later by bookworms, rot, or fungal decay in the case of dampness.

But in later years, for reasons of economy, they used thinner paper made from wood. There is acid in this paper, and over the years the pages become brittle, and they crumble. In particular, if the book is kept at high temperatures, and especially in the USA where heaters are used in rooms and near the bookcases, it can have a bad effect on the book.

The acidic paper hardens and becomes inflexible, until sometimes it is impossible to turn the pages!

Sometimes, the ink itself is acidic, which is encountered mainly in the printing presses in Livorno, Salonika, Istanbul, which causes damage to the paper.