Casein

Casein paint, derived from milk casein (milk protein), is a fast-drying, water-soluble medium used by artists.

It generally has a glue-like consistency, but can be thinned with water to the degree that fits a particular artist's style and desired result. It can be used on canvas panels, illustration boards, paper, wood and masonite. Because the dried paint film is inflexible and brittle, it is not appropriate to be applied in heavy impastos on flexible supports such as canvas. Canvas laminated to board is more suitable. Casein paint is reworkable and can be used for underpainting. It generally dries to a matte finish.