Natural graphite forms when carbon is squeezed and heated inside the Earth, at pressures near 75,000 pounds per square inch and temperatures near 750 degrees Celsius. Most graphite near the surface formed at plate boundaries, where organic-rich shales and limestones were heated and squeezed in a process called regional metamorphism. A pencil's core is a small piece of that deep history.
Not all graphite forms the same way. Some comes from coal. When coal seams are heated by metamorphism, the heat destroys the organic molecules and drives off oxygen, hydrogen, nitrogen and sulfur. What remains is almost pure carbon that crystallizes into graphite. Miners call this material amorphous graphite, even though it does have a crystal structure.
Flake graphite comes from metamorphic rocks such as marble, schist and gneiss, which contain tiny graphite flakes. When the graphite is concentrated enough, the rock is crushed to free the flakes, and the flakes are separated by their low density using froth flotation or specific-gravity separation. Graphite is a mineral of extremes: very soft, with a Mohs hardness of 1 to 2, and yet it stands up to heat and almost never reacts with other materials.
The first known picture of a wooden pencil dates to 1565. A Swiss naturalist, Conrad Gessner, published a drawing of a strip of graphite inside a tube of wood. Surveyors and artists had been wrapping sticks of plumbago in string or sheepskin to make it easier to use. The word plumbago, like black lead, comes from the old belief that graphite was a form of lead.
In 1779 a German chemist, K. W. Scheele, showed that the black material was pure carbon, and it was named graphite in 1789. Conté's later method mixed graphite with clay. The clay binds the graphite together, and the ratio decides the pencil: more clay makes it harder, and more graphite makes it softer and blacker.
Pencil history also turns on a war. The first American pencils were made in 1812, after the War of 1812 stopped English imports. William Monroe, a cabinetmaker in Concord, Massachusetts, invented a machine that cut and grooved wood slats exactly enough for pencils. Around then, Joseph Dixon cut cedar cylinders in half, set the graphite inside and glued them back together.
The slat method is a sandwich. Graphite rods go into the grooves of one glued slat, and a second slat with empty grooves is turned over on top. The sandwich is clamped in a hydraulic press until the glue dries. Cutters then cut around the top and bottom halves, and one sandwich yields six to nine hexagonal pencils.
Colored pencils are made much like graphite pencils, but the core holds pigment instead. Clay and gum are added to the pigment as bonding agents, and the mixture is soaked in wax so it writes smoothly. The outside of the pencil is then painted to match the color of the core.
A modern pencil gets its smooth finish through many steps. After cutting, sanders smooth the surface, and machines immerse the pencils in a vat of varnish, then pass them through a felt disk to wipe off the extra. They are dipped again and again until the color is right, then get a finishing coat.
Words to know
- metamorphism: rock change
- graphite: soft carbon form
- plumbago: old name
- sandwich: two slices
