(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
Casen is a milk protein which was commonly used by painters in the past as a binder to make a paint with characteristics similar to egg tempera. At one time, casein was a common glue used in cabinetmaking and was know for its great strength. A powdered form of casein is made by drying the curd of soured skim milk (the milk fats from whole milk are nondrying oils and must be removed). Today, casein is prepared commercially by heating and acidifying skim milk and then drying it to produce a powder, which is only soluble in water if made alkaline. One way to prepare a casein solution is to soak the powder in water and then add calcium hydroxide, which can be purchased from a chemical supply company. Fortunately, most casein powder, when it can be found, is in a form that will mix readily with water.
The use of casein is said to date back to the Egyptians. It was also used as an alternative to rabbit-skin glue for sizing canvas because it could be applied cold. The first commercially produced casein was marketed in the nineteenth century. The Pelikan Company of Germany produces casein paint called Plaka, which can be found in most artists’ material stores. Its consistency allows for application by either brush or palette knife. It adheres well to paper, paperboards, wood, plaster, plastic, glass, and metal. It dries matte with a velvetlike finish and becomes waterproof in twenty-four hours. Outdoor use requires a final, protective varnish.
Casein paint has some drawbacks; although it is highly water-resistant, it is not truly waterproof, and it is brittle and becomes more so as it ages. Therefore, the thicker the paint film, the more rigid the support should be. In general, thick applications of casein paint should be avoided. Adding a little drying oil, such as linseed oil, to casein paint will reduce potential cracking, but will slow the drying time and add some yellowing properties. Casein paints dry very quickly and look like oil paint except that they have a matte finish.
The popularity of casein waned with the introduction of polymer emulsion paints, which have almost all the advantages of casein paint with none of the drawbacks.