What is Casein? Casein (kay’seen) is a quick-drying, water based medium with a milk-based binding agent. Casein has the wash capabilities of watercolor, the smooth opacity of tempera and gouache, and the richer textures of oils and acrylics. Brushes dipped incasein keep their finesse, producing clear, crisp lines. Unlike oils, casein is a clean, water-soluble medium requiring no strong solvents. And because it dries quickly, it’s possible to lay on a glaze and move onto the next stage within a few hours. In comparison to watercolors, the main advantage of casein is that it’s easily correctable. It can be removed with a cloth, brush or eraser. Why cottage cheese is bad for you.