Conté Pencil Lead

Graphite and dry clay make a fine dust you should not breathe. Wear a dust mask, and grind wet rather than dry where you can.

Reduce the graphite to an impalpable powder in a mortar. The finer the powder, the smoother the lead — any grain left behind scratches the paper.

Pass the graphite and the clay separately through a fine sieve. Any lump left becomes a hard spot in the finished lead.

About 30 % clay for a soft, black lead; 60 % for a hard, grey one. That single ratio is the whole range, from B to H.

Note the percentage on a slip beside the heap. Once the leads are fired, nothing tells you which one held what.

Combine the two powders thoroughly while dry, then add water drop by drop until you have a paste the consistency of stiff modelling clay.

Knead for at least ten minutes. A badly mixed paste gives a lead that writes black in places and grey right beside them.

Roll even rods about 2 mm across, or extrude them with a syringe. The diameter must stay constant along the whole length.

Leave them flat, out of draughts, for two to three days. Drying fast makes the rods split or bow.

Fire the rods in a ceramic kiln, ramping slowly to about 900 °C. The firing is what hardens the clay and sets the lead.

Do not open the kiln while it is hot. Thermal shock cracks the leads along their length.

Dip the fired leads in melted beeswax or hot oil. Without it the lead squeaks and claws at the paper: it is the fat that makes a pencil glide.

Run a groove down the length of two strips of cedar or lime, so that the two grooves together form exactly the housing for the lead.

Lay the lead in the groove, glue and clamp the halves together until fully dry. Then plane and sand the pencil to shape.

Draw a stroke with each lead on the same sheet, labelling the clay percentage. Your own scale of hardnesses appears in front of you.

A military problem, not an artistic one. Until the late eighteenth century a good pencil was solid graphite sawn into rods, and the only deposit pure enough for that was at Borrowdale in Cumbria. During the Revolutionary Wars France lost access to it. Lazare Carnot charged Nicolas-Jacques Conté — painter, balloonist, chemist, later a savant of the Egyptian expedition — with finding a substitute. Conté filed his patent in 1795.

The process changed what a pencil is. Solid graphite cannot be graded: it is what it is. Ground, mixed with clay and fired, it becomes adjustable. By varying the clay proportion Conté obtained a continuous series of hardnesses — the direct origin of the H (hard) and B (black) marks that any pencil in any country still carries. The method also made poor, powdery graphite usable, which no sawyer could ever have worked.

A parallel invention, and a common confusion. The Austrian Joseph Hardtmuth was working on a graphite-and-clay mixture at the same time, and the house of Koh-i-Noor claims it. The two lines of work were largely independent; crediting one man alone is convenient but inaccurate. What is certain is that the French process was published, industrialised, and still carries Conté's name — as does the drawing crayon he also devised.

And the famous "lead". We still say mine de plomb in French and pencil lead in English, and there has never been any lead in it. Graphite was long taken for a variety of galena, and the name outlived the chemistry that disproved it. No pencil has ever poisoned anyone — which does not stop the story going round.