Carbon dust

Carbon dust is an artistic technique in which carbon dust is applied to a prepared surface via dry brushes. Highlights may be painted on or scratched out later in the process. Dust is prepared by rubbing carbon pencils against an abrasive surface, such as a metal file. The technique was popularized by Max Brödel (1870-1941) and became widely used among medical and scientific illustrators in the twentieth century.

Hodges, R.S. Elaine (2003). The Guild handbook of scientific illustration. Hoboken, NJ: John Wiley & Sons, Inc. 

...
Wikipedia