|[L to R] Vatman, Coucher, Layer at work.|
Gelatin (produced by boiling animal bones or skins) was first used as an external surface sizing for paper in 1337 and continued as a dominant sizing agent of all European papers through the mid-19th century. Sizing reduces the tendency of dry paper to absorb liquid, allowing inks and paints to dry remain on the surface of the paper, rather than be absorbed into the paper. Thus, sizing provides a more consistent painting and writing surface.
Papermaking, Parchment, Manuscripts: includes demonstration of coordinated actions of vatman, coucher, and layer.
Comprehensive online article on European papermaking techniques from 1300 to 1800.