Emily Becker in Mental Floss:
Sometimes, the act of teaching is a work of art. In the days before clip art and Google image search, artistically-challenged teachers had few alternatives to the chalkboard for their visual-based lessons. Enter Frederik Whitney, author of Blackboard Sketching, who wrote his guide in 1909 with the promise that, with a few basic strokes and some practice, anybody could turn a chalkboard into a canvas. Check out the virtual art gallery below of chalk art that’s too good for the sidewalk.
More here.