A black crayon made from charred twigs of wood, usually willow or vine. It is one of the oldest known drawing materials and was used for sketching in free-hand mural compositions by the Greeks and Romans and during the Middle Ages and the Renaissance. Like pastel it smudges easily and can be manipulated on the drawing surface with a finger or with the artist's tool known as a stump.
Text source: 'The Concise Oxford Dictionary of Art Terms' (2nd Edition) by Michael Clarke