Book written on Vellum.
The Schoyen Collection MS2455.

Vellum is calf-skin material for the pages of a book that is thin, smooth and durable. This refers notably to the Book of Armagh which is a manuscript written on such material.

Vellum was a translucent or opaque material produced from calfskin that had been soaked, limed and swiftly moved to be dried at normal temperature under tension, usually on a wooden device called a stretching frame.