Pencil

A pencil is three materials doing three jobs. The core is graphite mixed with clay, fired, and impregnated with wax. The casing is wood, traditionally incense cedar, split into two slats and glued around the core. The ferrule is aluminium or brass, holding a rubber eraser.

The grade is the clay-to-graphite ratio and nothing else. More clay gives a harder, lighter line — that is what H means — and more graphite a softer, darker one, which is B. HB is the middle. The system is a proportion, so a 4B and a 4H are the same two ingredients in different amounts, fired in the same kiln.

Graphite marks paper because it is a stack of carbon sheets held together so weakly that they shear off under almost no pressure. Every mark is layers of graphite left behind on the paper's surface roughness. That is the same property that makes it a lubricant, and the same structure that makes it, in a different arrangement of the same element, diamond.

There is no lead in a pencil, there never has been, and the confusion is a sixteenth-century mineralogical error that the language never corrected.

A very large and very pure deposit of graphite was found at Borrowdale in Cumbria in the 1500s — solid enough to be sawn into sticks and used directly, which is unique and is why the English pencil industry started there. It was black, it made a mark, and it was assumed to be a form of lead ore, so it was called plumbago, black lead, or in German Bleistift — lead pencil. The name stuck in most European languages.

Carl Wilhelm Scheele showed in 1779 that it was a form of carbon, and Abraham Werner named it graphite in 1789 from the Greek for 'to write'. By then the wrong name had been in use for two hundred years and it was too late.

The Borrowdale deposit was valuable enough to be guarded by armed men and an Act of Parliament, and worked out by the nineteenth century.

The modern pencil is a wartime substitution, which is not how it is usually remembered.

Britain controlled Borrowdale and, during the Napoleonic wars, controlled the supply. Nicolas-Jacques Conté, working for the French government, solved the problem in 1795 by grinding lower-grade graphite powder, mixing it with clay, and firing it — which not only worked without Cumbrian graphite but produced something better, because the hardness could now be chosen rather than accepted.

Every pencil since is made by Conté's method. It is a good example of a substitution forced by supply that turned out to be an improvement, and of a natural material being displaced by an engineered one that is more consistent than anything a mine produces.

The eraser came later and separately: the English chemist Joseph Priestley noted in 1770 that caoutchouc rubbed out pencil marks, which is where the material got its English name. Attaching one to the pencil was patented in the United States in 1858 and the patent was later voided for combining two existing things, which is a reasonable decision about a very good idea.