He rendered the illustrations in casein paint. Casein paint is reworkable and can be used for underpainting. It was illustrated using casein paint, and won the Caldecott Medal for illustration in 1953. Casein paint loses its solubility with time and exposure and becomes water-resistant. Casein paint is a fast-drying, water-soluble medium used by artists. The paints used can e.g. be casein paint, tempera, oil paint, silicate mineral paint.