More information: Tomoya Kamine et al, Pencil lead etched by plasma: Novel photonic material that can control the reflection from the visible to the infrared region, Optical Materials (2024).

The pencil “lead” is actually a mixture of graphite and clay. More clay in the mixture makes the lead harder. A harder lead has a higher number. Pencils are numbered from 1-3 but most people use a ...

No.2 pencils are widely used, but you may not know what the number actually refers to. This video explains how pencils developed into their modern form and the significance of the labeled numbers.