The `lead’ in a pencil has not been made of lead for centuries. The ancient Egyptians, Greeks and Romans all used small lead discs to rule lines on sheets of papyrus before writing on them with a brush and ink. By the 14th century, European artists were using rods of lead, zinc or silver to make pale grey drawings called silverpoint. In the 16th century, Conrad Gesner of Zurich, in Switzerland, in his Treatise on fossils, described a writing rod held in a wooden case.

But lead ceased to be a writing implement in 1564, when pure graphite was discovered at Borrowdale in the North of England  and the modern pencil was born. Graphite is a form of carbon, which is one of the softest minerals. When it is pressed against paper, thin layers flake off to leave a black mark. Some of the best graphite for pencil making comes from Sonora, in Mexico. It is powdery and extremely black.

The wooden outer case of the pencil has to be fairly soft, so that it can easily be sharpened as the graphite wears down. The most widely used pencil wood comes from the incense cedar of the Sierra Nevada mountains in northern California. The best wood is from trees between 150 and 200 years old.

The `lead’ is made by mixing together fine graphite and clay and then firing it in sticks in a kiln. Graphite cannot be ground in an ordinary mill because its flaky structure makes it a natural lubricant. So an `attrition mill’ is used. Jets of compressed air containing graphite particles are blasted at one another, and as the particles collide they break up.

The fine particles are mixed with pure china clay and water, producing a putty-like paste. The mix is fed into a cylinder and forced through a hole at one end, emerging as a continuous length the diameter of pencil lead.

The lengths are cut into pencil sized sticks, and dried in an oven before being fired at a temperature of about 2200° F (1200° C). Finally, they are treated with wax to ensure smooth writing, and sealed to prevent them from slipping out of the wooden casing.

To make the casing, the wood is sawn into slats, each the length of a pencil, seven pencils wide, and half a pencil thick. Grooves are cut into the slats, the leads are inserted, and another grooved slat is glued on top. This `sandwich’ is then fed through machines that cut it into individual pencils of hexagonal or circular cross section.

After shaping, the pencils are painted with non-poisonous lacquer.