One of the most interesting bits of chemistry in the typical classroom is so common that it is used every day without any thought to its characteristic properties or its history.  It is the common pencil.

The history of writing is filled with various techniques that changed and evolved as our understanding of materials improved.  Some of the earliest writing was done with a sharp stick on wet mud tablets.  Sharpened sticks or reeds were used as crude pens.  The Egyptians wrote on thin layers of wax coating on tablets.  Early ink was developed by the Chinese, by combining soot with gelatin and musk.

Quill pens made from feathers (and dipped in ink) were the dominate way to write for a thousand years, starting around 700 A.D.  But these pens were fairly fragile and only lasted for a few days of work.  Plus they had to be sharpened frequently.  Perhaps the biggest difficulty with quill pens was their lack of portability.  They were very difficult to use anyplace other than a nice, stable desk.

This all changed dramatically with the discovery of the mineral graphite in 1564.  You may have heard the story of how Isaac Newton ‘discovered’ gravity while sitting under an apple tree and observing one of the apples fall to the ground.

In a topsy-turvy twist of this discovery, graphite was first discovered when a large apple tree in Borrowdale, England fell over.  Attached to the roots of the fallen tree were chunks of a shiny black mineral that was initially called blacklead or plubagoi.  Like lead, it left dark marks, making it ideal for writing.  In 1789 Abraham Werner discovered the material was a form of pure carbon, not lead,  and named it ‘graphite’ from ancient Greek, meaning ‘to draw or write’.

Pure lead was used for writing as early as the first century B.C., but graphite was much better for making clear, dark letters on paper.  Now over two thousand years later, we still see confusion over what pencil ‘leads’ are really made of.

The only problem with graphite was that it was quite brittle.  This was solved by sandwiching a thin rod of it between two piece of wood—the first pencil!   In 1795 a French chemist improved the technology by combining powdered graphite with clay and water, making a slurry.  When this slurry was heated in an oven it produced thin graphite rods that were uniform and efficient for making pencils.

Pencils were an instant success and were very popular.  Pencils were cheap, durable, easy to carry, could be used anywhere and were erasable!

There is a humorous story about the early days of space exploration when NASA spent millions of dollars trying to make a pen that would work in zero gravity.  The Russians just used pencils.  (Interestingly, pencils were later banned because of fear the wood in pencils would be a flammable hazard in the pure oxygen atmosphere of spacecraft and the zero gravity pen took over.)

But what is the chemistry behind graphite? Graphite is made of pure carbon.  Carbon comes in many crystalline forms, called allotropes, the most well known being diamond and graphite. Allotropes are various forms of an element that have different physical or chemical properties. The way atoms are connected to each other in solid materials has a huge impact on their overall properties.

A diamond and a piece of graphite are so different that you would never guess that they are both made of the same element—carbon. Diamond is a very hard and transparent mineral that is ejected through volcanic eruptions while graphite is a black and lightweight material found as a mineral.

In diamond, the carbon atoms are connected to four other carbons. This is a very strong arrangement that makes diamonds one of the hardest know materials. In graphite, on the other hand, the atoms are linked to one another in layers of hexagonal (six-sided) shapes that look like chicken wire. The bonds within the hexagonal sheets are strong, but each layer is only weakly bonded to the next, which allows the layers to slip by one another.

This last feature is what makes graphite so good for writing.  As we move the tip of the pencil across paper the layers of ‘chicken wire’ are scrubbed off and deposit on the paper, leaving a durable mark.

It is a good thing that tree in England decided to fall over when it did, otherwise you might be spending all your spare time sharpening quills!