Parchment is made from thin pieces of dried animal skins. Vellum, which costs the same as well-crafted parchment, is thicker and can be reused by scraping the surface with a razor. Parchment evolved from early techniques of writing on wax tablets with a stylus.[1a]
Source[]
1: Warhammer Fantasy RPG 2nd ED -- Old World Armoury
Warhammer Wiki