Articles

    Pencil Chemistry

    Kids, did you ever wonder why everyone calls that stuff in pencils "lead" when it isn't really lead at all? Instead, it is a nontoxic mixture of graphite and clay (more on that later). Way back in the days of the Roman Empire, actual lead rods were used to write on papyrus. But more recently, in the 1500's, a graphite mine was discovered and graphite was found to leave darker marks on paper. At the time, everyone thought that graphite was a type of lead. They called it black lead or plumbago. The chemical symbol for lead is Pb, which stands for the Latin word plumbum (check www.vanderkrogt.net/elements/elem/pb.html for a complete history of lead).

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    By nearly 1800, chemists finally proved that black lead was really a form of carbon. Carbon exists in the elemental form as either graphite or diamond (or, as we have recently discovered, nanotubes and buckyballs). Because graphite is so soft, it needs a holder to support the skinny sticks used for writing. Low quality graphites need to be further strengthened by mixing with clay and water (see the Nov. 1996 ChemShorts to learn more about clays). A slurry of these three ingredients is crushed, mixed for three days, extruded into the thin rods, and then heated to dry out the water. The ratio of clay to graphite affects the hardness of the "lead": the more clay, the harder the pencil lead. This means that less graphite is present to transfer to the paper, resulting in lighter lines. The higher the number, from 1 to 4, the harder the lead. Get a sampling of pencils of various hardnesses and check out their writing ability on different types of paper.

    Various woods have been used for the pencil casings, from red cedar to the now most commonly used incense cedar. This beautiful wood is then coated with five to eight coats of paint. The traditional yellow paint also has a history. When a very pure graphite mine was discovered in China in the 1800's, pencils made with this high quality Asian graphite (no clay necessary) were painted yellow to distinguish them from the rest. Erasers are added and various markings are then stamped onto the pencil shafts. Did you ever notice the word Ticonderoga stamped on many of them? Fort Ticonderoga, a Revolutionary War fort in upstate New York, is near one of the purest graphite deposits ever known at 99.9% pure carbon.

    We don't advocate that you try this, but a pencil will on average write about 45,000 words, or a line 35 miles long! It is claimed that such a line will in fact even conduct electricity because graphite is a known conductor. Colored pencils are made from chalk, clay, or wax mixed with binders and pigments; compare writing with some of these alongside your regular pencils. If it is possible for you to get a sample of a chunk of graphite (maybe at a store that sells gems, minerals, and fossils), take a close look at it and compare it to the stuff in your pencil.

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    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    kcarrado@anl.gov
    April 2003

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    Reference: Steve Ritter, Chemical & Engineering News, ACS, 10/15/01, pg. 35 (http://pubs.acs.org/cen/whatstuff/stuff/7942sci4.html).