Casein on Claybord
Rigid panel supports like Claybord are ideal for painting with casein. Canvas cannot be used, because a dry casein paint film is the most inflexible of the permanent paint binders and becomes increasingly brittle as it ages. Casein colors are paints made by mixing artists' pigments with a solution of casein, a milk protein. They are usually applied with a brush on a wall-board panel coated with gesso. For the same reason, the palette knife should be used with restraint, as the paint can crack if laid on too thickly; a heavy impasto should be avoided. The colors dry rapidly to a pleasing and durable matte finish, although some artists varnish their pictures to obtain a glossy finish more characteristic of oil paints. Casein may be used to produce effects ranging from smooth areas of flat color to a moderate impasto, for which a full bristle brush is used. Casein is also frequently used as an underpainting for oil paints and glazes.
Gareth Jones, The Road Through Defiance, 8x10, casein on Ampersand Claybord
Artists are drawn to the matte quality of casein colors and their high drying speed and simplicity of the material. They don’t need a complex set of materials or solvents; they can be mixed with water. Although dried casein paint may be damaged if spattered with water, it is not readily water-soluble and will resist dampness. A casein painting may be sprayed with a fixative to further protect the finished artwork.
More information on casein can be found in Ralph Mayer's book, Art Terms and Techniques.