How Is a Pencil Made? Graphite, Clay and a Lead That Was Never Lead

The Borrowdale graphite was the only large deposit of pure, solid graphite ever found, and for two hundred years it made the English pencil. The mineral was sawn into thin square rods, wrapped in string or set in grooved wooden holders, and it wrote so well and was so valuable, as a lining for cannonball moulds as much as for writing, that the mine was guarded, flooded when not in use and worked only a few weeks a year, and the export of graphite was made a crime punishable by transportation. Keswick, at the foot of the valley, became the pencil town it still is, and the Crown controlled the seam until it was exhausted in the nineteenth century.

Everyone else had to make do with impure graphite, and in 1795, with England's supply cut off by war, the French government asked Nicolas-Jacques Conte, an officer, painter and balloonist, to find a substitute. His answer was to grind poor graphite to powder, mix it with clay and water, extrude the paste into rods, and fire them in a kiln, which produced a lead as good as Borrowdale's from any graphite at all and, by varying the proportion of clay, a lead of any hardness. That is still the process, and the grading system that came from it, from 9H, hardest and palest, through HB in the middle to 9B, softest and blackest, is Conte's ratio of clay to graphite in a code. An American number 2 pencil is an HB.

A modern pencil factory runs Conte's method at scale:

• •Graphite, now mined in China, Brazil and Sri Lanka or made synthetically, is milled with clay and water into a paste, extruded through a die into thin rods, dried and fired at around 1,000 degrees
• •The fired leads are soaked in wax or oil to make them write smoothly
• •Slats of incense cedar, cut to half a pencil's thickness and the width of several pencils, are grooved by machine
• •Leads are laid in the grooves, a second grooved slat is glued on top, and the sandwich is pressed
• •The sandwich is cut into individual pencils, hexagonal or round, and each is sanded, painted with several coats, stamped and, if it is to have one, fitted with a ferrule and eraser

Graphite is carbon in sheets, each sheet a hexagonal net of atoms strongly bonded within and barely held to the sheet above, so that the sheets slide off each other at a touch. Drawing a pencil across paper shears flakes from the lead, and the flakes lodge in the fibres of the paper's surface; the mark is a smear of graphite crystals, which is why it is grey rather than black and why it shines a little when thick. Harder leads, with more clay, shed fewer flakes and leave a thin pale line; softer ones shed more and leave a black one. An eraser works because the flakes are only sitting on the fibres, and a rubber that is slightly sticky and slightly abrasive lifts them off; ink, which soaks in, cannot be erased the same way. A pencil writes in a vacuum, on wet paper and at minus 40 degrees, which is why it went to the Moon in every astronaut's pocket, though the story that NASA spent millions on a space pen while the Russians used pencils is a myth, since a broken pencil tip floating in a spacecraft is a hazard and both sides bought the pen.

The Faber family in Nuremberg industrialised the pencil in the nineteenth century and gave it the hexagonal shape that stops it rolling off a desk; Hymen Lipman of Philadelphia patented the attached eraser in 1858, and his patent was later struck down for merely combining two known things. The yellow paint on American pencils, from the 1890s, advertised the Chinese origin of the best graphite, yellow being the imperial colour. About 14 billion pencils are made a year, a single cedar tree supplying around 300,000, and the pencil remains the tool of the surveyor, the carpenter, the examination hall and the first draft, an instrument that a Cumbrian shepherd would recognise and that was perfected by a man trying to get round a blockade.

A pencil's lead is graphite, a form of carbon first mined in Cumbria in the sixteenth century, and since 1795 it has been made by Conte's method of firing powdered graphite with clay, the ratio setting the hardness from 9H to 9B; the lead is glued between grooved slats of cedar, cut, sanded and painted. It writes because graphite's sheets of carbon shear off onto paper fibres, which is also why it can be erased, and the design has not changed in two centuries.