If you make a mistake while using a pencil
and you don’t have an eraser, a piece of bread
will work. Indeed, using bread was the preferred method for erasing marks made with
graphite for many years; some artists still use
bread today to lighten pencil lines to achieve a
desired effect. It is reported that in 1770 the
English engineer Edward Nairne accidentally
picked up a wad of rubber instead of a piece
of bread to erase a mark and found that it
worked quite well.
Once Charles Goodyear discovered the
vulcanization process for rubber in 1844, the
use of rubber in erasers became widespread.
Vulcanization involves adding sulfur to
natural rubber (made of polymer chains)
and heating it. The heat causes many cross-linkages between the polymer chains to
form and creates a durable form of rubber.
The eraser attached to most pencils is made
from rubber, but erasers can also be made of
plastic or vinyl. Gum erasers, favored by artists, are made of a softer type of rubber. Often
there is an abrasive substance added to aid
in the erasing process. The iconic Pink Pearl
erasers contain volcanic pumice, which has
Erasers work by physically removing
graphite particles from the paper. Both rubber and graphite are nonpolar substances, so
there is a mutual attraction involving London
dispersion forces. Paper is made of cellulose,
which is also nonpolar, so only weak forces
act to bind the graphite particles to the paper.
However, the forces between the rubber eraser
and graphite particles are stronger than those
between the graphite and the paper.
If your eraser has been around awhile, you
may have noticed that it doesn’t work well, if
at all. Over time, rubber erasers get hard and
brittle, and they tend to do more smearing
than erasing. This degradation is due to oxidation of the rubber. Ultraviolet light, ozone, and
oxygen can all act as oxidizing agents. Oxidation tends to break up the long polymer chains
and create more cross-linkages, which makes
the rubber harder and less flexible.
The pencil is a marvel of chemistry and
engineering—simple yet exquisite. Even
though the pencil has undergone many incarnations in its history, it is still one of the most
economical ways to write. So the next time
you use a pencil, take a moment to marvel
at the amazing cellulose-encased cylindrical piece of graphite that can so wondrously
transcribe your thoughts. And they don’t even
Ward, J. The Perfection of the Paper Clip: Curious
Tales of Invention, Accidental Genius, and
Stationery Obsession. Simon & Schuster:
New York, 2014; pp 85-107.
Bennett, H. Ever Wondered About the Lead in
Pencils? The Washington Post, Nov 30, 2014:
8cc0a295c773_story.html [accessed Sept 2017].
Helmenstine, A. M. How Do Pencil Erasers Work?
ThoughtCo., Updated on Feb 21, 2017:
eraser.htm [accessed Sept 2017].
Pencil Tech. Middle School Chemistry, American
Chemical Society, June 29, 2012: http://www.
world/2012/06/pencil-tech/ [accessed Sept 2017].
O’Shaughnessy, L. Why Are Pencils Yellow? CBS
News, Last Updated on July 22, 2010: http://
[accessed Sept 2017].
Brian Rohrig is a science writer who lives in
Columbus, Ohio. His most recent ChemMatters
article, “Chemistry Rocks!,” appeared in the
October 2017 issue.
The average pencil contains enough graphite
to make a line approximately 35 miles long.
The first electronic grading machines detected the electrical
conductivity (graphite conducts) of the choices that were
penciled in by running wire feelers along the paper as it was
graded. Today’s computerized image sensors can detect
any type of graphite pencil, but perhaps that high-stakes
exam is not the time to test its limits!
www.acs.org/chemmatters 18 ChemMatters | DECEMBER 2017/JANUAR Y 2018