A crop of Leo Lionni's art from Fish is Fish. Click through to enlarge.
Chalk is calcium minerals compressed into sticks.
Chalk is a dry material composed of calcium sulfate or calcium carbonate used to make a medium white, with colored pigments often added.
Leo Lionni: Fish is Fish
Lionni used chalk as his medium in illustrating Fish Is Fish, with a fish swimming in a pond, a frog ready to emerge from the pond, and multicolored and textured seaweed, water and pond rocks. Chalk properties are visible in the blending of the lines, the subtle colors, and soft edges. The cross hatching designs created by use of chalk is clearly visible in this illustration. While this chalk image is recognizable as a Lionni work, he is perhaps more well known for his works with cut paper and collage, as in his picture books, Frederick and Inch by Inch. Lionni, like many artists, was comfortable with different mediums and techniques in his illustrations.