Traditionally, casein (i.e., milk protein) was a popular alternative to rabbit skin glue. This makes it an ideal binder with strong adhesive qualities. Casein paint is made by mixing pigments with casein. The casein paint is water-soluble and fast-drying. A thin layer of paint dries within 24 hours. Once dry, the paint film becomes water-resistant and cannot be reworked. The paint dries to a matte finish like egg tempera and can be varnished to give a gloss appearance.
Casein paint can be applied with a brush or palette knife to porous and non-porous surfaces such as canvas panels, boards, paperboards, plaster, plastic, glass, wood, and metal.
Casein, although it will adhere to paper and canvas, is not suited to flexible surfaces because when dried it has little or no plasticity. The dried paint tends to become brittle with age; therefore, thick layers of paint require rigid support to avoid cracking, flaking, and peeling. Adding a non-yellowing drying oil like safflower or walnut during painting would reduce brittleness but at the same time affect the drying time.