Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Why is lead used in pencils even though lead is poisonous?

Category: Chemistry      Published: April 4, 2013

Standard pencil cores have always been made of graphite and not lead. This substance was historically misidentified as lead and the name stuck. Public Domain Image, source: Christopher S. Baird.

The core of a pencil does not contain lead and never has. Pencils contain a form of solid carbon known as graphite. According to the book The Pencil by Henry Petroski, the graphite pencil was first developed and popularized in the 1600's. The first users of graphite simply dug this mineral out of the hills and discovered it could be sawed into sticks and used as an excellent writing tool. During the 1600's, no one knew the chemical nature of this material, as chemistry itself was still in its infancy. Since this writing material behaved similar to metallic lead, but had a darker color, people began calling it "black lead". Eventually, the name of the core of the pencil got shortened to "lead". In 1779, German chemist K. W. Scheele finally determined pencil lead to be composed of pure carbon. A decade later, A. G. Werner decided that this carbon material needed a new name and proposed the name "graphite" based on the Greek word "graphein" which means "to write". Black-core pencils currently contain and have always contained graphite, not lead.

Carbon is the sixth element on the periodic table and is famous for forming the backbone of molecules found in fuels and in all living organisms. Aside from biological molecules, carbon can also be found in nature in pure mineral form. Depending on the shapes in which the carbon atoms are bonded, pure carbon can take on many forms. In diamond, each carbon atom is bonded tetrahedrally to its four nearest carbon atoms. This tightly packed arrangement of carbon atoms makes diamond the hardest naturally occurring material on earth. In contrast, graphite contains a stack of carbon sheets. Each carbon sheet is one atom thick and consists of a hexagonal lattice of carbon atoms bonded together. Each carbon atom in the sheet is bonded to the three nearest carbon atoms. The carbon atoms within a sheet are very strongly bound together, but the sheets themselves are very weakly bound. As a result, it is very easy for one carbon sheet in graphite to slip past the other sheets. This slippery-sheet structure is what makes graphite so oily to the touch and makes it such a good material to write with. The carbon sheet fragments readily rub off the pencil core and onto the paper. This property also makes graphite powder an ideal dry lubricant. When a small single graphite sheet is bent into a circle and connected back to itself, it can form a carbon nanotube, which is a promising object that should find application in future technology.

Christopher S. Baird is a physics professor at West Texas A&M University and author of the book, The Top 50 Science Questions with Surprising Answers.

Topics: carbon, graphene, graphite, lead, pencil, pencil lead, pencils