How should one read a medium that is meant to be erased? What can such a medium tell us about art, science, literature, pedagogy, or modernity? While slates and other erasable media date to ancient times, they took on powerful new uses and meanings around the turn of the nineteenth century. At the dawn of the modern era, large-scale blackboards swept across scientific institutions and into schoolhouses and other sites of civic and cultural production. In many ways, they helped define and inculcate what it meant to be a part of the modern world---for pupils learning letters, for urban publics who crowded into lecture theaters, for elite scientists tackling the mysteries of nature, and for many others. Based on my own ethnographic and historical research and recent work by collaborators and colleagues, I will offer a humanistic account of the blackboard in the modern world and examine some of the central methods and stakes of such blackboard humanities.