Black Chalk
Natural black chalk is a soft stone, a composite of carbon and clay known as carbonaceous shale. The carbon provided the color, and the clay acted as a binder. Since the quality of the stone was extremely variable, it was critical to obtain the mineral with just the right combination of friability and hardness to suit the draftsman's hand.

Black chalk first became popular in the late fifteenth century and continued to gain favor in the sixteenth. It declined in the eighteenth, as chalk of acceptable quality became more difficult to obtain, and was eventually supplanted by synthetic chalk, crayons and graphite.
 


Carlo Maratta. Study Sheet. MMLV Fine Arts
 

by Michael Miller
Information supplied by: http://www.nyu.edu/classes