A paint much like opaque watercolor. Casein, a kind of glue made from milk the binding agent in this paint.
Casein paint can be used on wood or paper, but not canvas (consistency is too inflexible). Can be used for underpainting or impasto. It dries quickly with a waterproof surface, and can then be varnished.
information gleaned from, but not c&pd from,
http://www.artlex.com/