Flaking Ink and Pigment
Until the invention of the printing press in 1450, most books were handwritten on parchment pages. Parchment is made of stretched animal skin (typically calf, goat or sheep) treated with an alkaline solution. It is not as porous a surface as paper, and is highly reactive to changes in humidity. Inks and pigments sitting atop this constantly flexing surface may crack and flake off. Conservators can help halt this process, if it's not too late, by consolidating the media using an adhesive to reattach the loose fragments.