A liquid in which water is combined with an oily or resinous substance in such a way that they will not separate out. Oil proverbially will not mix with water, but if an emulsifying agent—such as albumen—is added, it will surround the drops of oil and prevent them from coming together. The medium of tempera painting is always an emulsion. The natural emulsions used most commonly in painting are egg yolk and casein. Both have the advantage that once they have set they are not soluble in water.