Paint made by mixing ground pigments with a solution of casein, a milk protein. Casein paint is water soluble and dries to a smooth, durable matte finish. When dry, the casein paint film is brittle, so using it on flexible surfaces such as paper and canvas is not recommended. Web resource here.

Gustave Klimt. The Beethoven Frieze. Detail: Knight, Ambition and Compassion. Secession Gallery, Vienna. Casein, gold leaf, mixed media. 1902.