Artist’s graphite is a combination of graphite and clay. The higher the proportion of graphite to clay, the softer the medium.
Graphite comes in degrees of hardness from 8H to 6B (the H refers to hard and the B designates black). Hard graphite (H) is good for fine details. Soft graphite (B) can produce a wider range of tones than hard graphite and is good for large tonal areas. Soft graphite requires sharpening more often than hard.