Pencil graphite is mixed with clay, and more clay makes the written mark harder and lighter.

The longer version

The black core of a pencil is a baked mixture of graphite and clay, and the ratio sets how the mark behaves. More clay means fewer graphite flakes reach the paper, so the line is paler and the point survives longer. More graphite means a darker, softer line that smears readily and wears down quickly under the same pressure.

Grading letters record that ratio, with hard grades at one end and soft at the other, though the numbering was never standardised between makers, so one maker's grade may not match another's. Pure graphite is greasy and crumbles; clay is what turns a slippery mineral into something that can be sharpened to a point and leaned on while writing.

Where this entry sits

Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.