Sidewalk Chalk Is Not Actually Chalk

Most sidewalk chalk is made from gypsum, a mineral entirely different from the calcium carbonate used in traditional classroom chalk. It is specifically designed to be bulky and durable for outdoor use.

While we call it chalk, the material is technically calcium sulfate. This mineral is perfect for pavement because it creates wider, bolder lines that withstand rough surfaces. Traditional blackboard chalk is much more fragile and would crumble instantly on concrete. It is fascinating how a simple everyday art supply relies on such specific chemistry to function on our streets.