A milk protein used as a binder for casein colours. It is prepared by drying the curd from sour milk, then grinding it into a yellowish powder. Casein is only water-soluble in the presence of an alkali such as ammonia, thus casein paints once dry are waterproof. A type of milk curd glue was used by the Egyptians, Greeks and Romans. It has also served as an adhesive for joining the planks of a panel.