Casein is a binder derived from milk. A white powder, casein is mixed with water and an emulsifying agent (such as ammonium carbonate) to form a paste, then pigment added. Of course, these days, you can simply buy casein paint ready made, in tubes.
Casein can be used on watercolor paper or on a support that has a layer of gesso on it. As casein is relatively inflexible, it's best suited to rigid supports such as a wood panel, rather than canvas.
Casein is thinned with water and dries quickly, to a matt finish. To get a shinier finish, it can be varnished with a gloss varnish.
Like hand-made egg tempera, casein paint will go off after a few days. Store it in a cool place to extend its 'shelf-life'.
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For information on what it's like to use casein paint, read the article What is Casein? by artist John Molnar, published International Artist magazine.
