The earliest documented usage of chalk on a blackboard in the United States goes back to September 21, 1801, during a mathematics lecture course delivered by George Baron. Coloured chalk was invented in 1814 by James Pillans, who had a mixture that included crushed chalk, colors, and oatmeal. This early form of crayon was used by teachers to draw pictures on schoolrooms walls. In 1816, Joseph Dixon developed a method for making bone chalks that were more durable than Pillans' ink, which was often washed out by rain. Chalk has been used for drawing on sidewalks, streets, and even ice since the early 19th century. It was also used as a surface for painting. In fact, Charles Willson Peale is considered the father of American art because of his use of chalk to create paintings on wood. He got this idea from Thomas Hancock, who had the idea while watching children draw with chalk on the sidewalk outside of his shop. Chalking up a career, then, is how people communicated in the early days of Philadelphia. As time went on, people started using chalk for other purposes too. For example, William Gedney used it as a pathfinder in 1872 when he marked out roads with chalk before laying down gravel or tar. The same year, Henry Chapman sold the world's first commercial chalkboard advertising space for $1000. It was painted on a wall at No. 17 Market Street in Philadelphia.