Among my favorite educational maps and charts are those that are "slated" or have a blackboard surface. They are often referred to as chalk plates. These were designed to be written on in chalk by the instructor and wiped clean with a damp cloth. The graphics were usually very simple and clear- often just an outline. The simplicity of these maps and charts can be very artful and interesting. Chalk plates are relatively rare and difficult to find. Many are worn and cracked from being written on and wiped off countless times in the classroom.
Pictured below are a few examples from my collection: