The graphite found in common household pencils is mixed with clay, and the hardness and color of the mark it leaves is dependent upon the ratio of its components. In this work, Picasso first drew a basic composition with faint strokes, applying very little pressure to the pencil. As the image progressed, he used thicker, stronger lines that emphasize certain shapes while allowing others to act as background or subtle detail. To create the textures of the beard and face, Picasso turned his pencil on its side, using long strokes for one effect and crisscross marks for another. The passage of light and shadow on the figure’s knee has the heaviest application of graphite. Close examination reveals a network of fine and partially erased lines, an indication that Picasso altered his earlier composition lines and details as he drew.
For more information, see:
Karmel, Pepe. "Incessant Invention: Picasso's Drawings, 1914–1916." In Cubism: The Leonard A. Lauder Collection (MMA, 2014), pp. 188–192, 317.