ChemMatters | DECEMBER 2017/JANUARY 2018 17
rods, but they were hard to hold. Originally,
they were wrapped in string. Sometime
around the late 1500s, perhaps tiring of get-
ting their hands black, someone came up with
the idea of sandwiching the graphite between
two pieces of wood. Thus the modern pencil
was born, which has always been lead-free.
The mark graphite leaves is easily smudged,
since it is a very soft material. It wasn’t until
1790 that clay was added to graphite to make
it harder. When pencils are made, clay and
graphite are ground into powder and then
water is added, which forms a gray sludge.
After intense heating in a kiln, the final graph-
ite product is formed. Since a harder pencil
does not leave as much graphite behind,
the mark is lighter. The most popular grade
is the ubiquitous No. 2 pencil, which is still
fairly dark, but durable enough that the “lead”
doesn’t break easily.
Graphite is an allotrope of carbon. An allotrope is a different form of the same element
due to a different arrangement of its bonds.
Other allotropes of carbon include diamonds
and buckminsterfullerene (commonly known
the graphite layers slide off in flakes and stick
to the paper.
As you write or draw, flakes from the graphite -and-clay mix that make up your pencil’s
“lead” cling to the cellulose fibers that make
up your piece of paper. The fibers have a huge
surface area that catches lots of flakes. Since
graphite and cellulose are both nonpolar, the
flakes attract to the paper via London dispersion forces.
as the buckyball). Each carbon atom in graphite covalently bonds with three other carbon
atoms, forming layers of very thin sheets of
rings (Fig. 1). These thin sheets are attracted
to one another by London dispersion forces.
Between these sheets of atoms are a vast
number of delocalized electrons—electrons
that are not tightly bound to any specific atom.
The sheets can slide easily, which is why
graphite is soft and slippery to touch. When
force is applied to your pencil as you write,
Figure 1. In graphite, carbons (black) form
a two-dimensional sheet in ring formations.
These sheets are attracted to each other, but not
chemically bonded, and justify graphite’s brittle
The substance rubber gets
its name for its ability to
effectively rub out
A typical pencil mark on a piece of paper
has a height of around 20 nanometers,
which is 60 atoms thick.
core in a standard pencil
has a diameter of 2
Most pencils sold in the United
States are yellow. In the 1800s, the
world’s best graphite was found in
China. The Chinese painted
their pencils yellow,
which signified royalty.
Other companies followed
suit and it became the
Even though you cannot get lead
poisoning from pencil “lead,” you
may get it from the paint used on
older pencils. Before 1978, some
types of pencils contained up to
12% lead in their paint.
HB is a European scale. H refers to
hardness and B for blackness. The larger
the H number, the harder the pencil, and the
higher the B value, the blacker the pencil
writes. HB is in the middle of the scale.
The eraser is made of
The eraser is attached
to the pencil with a
little metal ring
known as a fer-
rule; despite the
name it is not made
of iron but rather alu-
The No. 2 pencil
gets its name from
the fact that it is
shade that a pencil
can make, with No.1
being the darkest.
Pencils are typically hexagonal
to minimize rolling.
Cedar, the wood used to
make pencils, resists decay,
sharpens well, and is resistant
to cracking and warping.
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