Pencil leads are made of a mixture of graphite and a clay binder. There is no actual "lead" in pencil leads.
Since it was first developed, in the 16th century, the "lead" pencil - called thus in English because the shiny graphite it contained was mistaken for lead ore - has inspired countless artists time and again, turning them into fans of this seemingly mundane utensil. Vincent van Gogh, for example, the man who paved the way for modern painting, insisted on using solely Faber pencils (now Faber-Castell).
Writing hardness is determined by the mix ratio of graphite and clay: The greater the graphite content the softer the lead and the higher the proportion of clay the harder the lead. The choice of hardness required by the user of the pencil is determined by the type of writing or drawing work, by the paper surface and by the “heaviness” of hand of the user. (A heavy hand might prefer softer leads, and a light hand may work well with relatively hard lead qualities.)
The number and lettering printed on the pencil represents the degree of lead hardness. This indicates how soft or hard the lead is and how dark or light the mark is on the paper. Again this all has to do with the mixing ratio of clay and graphite.
Faber-Castell applies the following designations:
H = hard
B = black = soft
F = firm
HB = hard black = medium hard
Degrees of lead hardness alternatively expressed in numbers:
1 2 2 ½ 3 4
2B B HB H 2H