As a filling for cracks and holes in a wooden blackboard, if the crack is tin. or more in width, a slip of wood should be fitted and glued in the opening and afterwards planed down level to the surface of the board. But if the crack is less than 1/8 in. wide, it can be filled in with a mixture of plaster-of-Paris, glue, and a little lampblack. This should be allowed to dry, and then scraped and glasspapered flush with the surface of the board.