One of the first wooden pencils I ever spent a chunk of money on was a very old and unusual one that I had never seen before, from the Clark Indelible Pencil Company of Northampton, Massachusetts. I obtained several varieties since then.
These specialized pencils were made for writing permanently on cloth and other materials.
The active part of the pencil is integrated with a protective tube:
A patent from 1866 (http://todayinsci.com/Events/Patent/IndeliblePencilPatent56180.htm) describes in detail how the pencil was made. The indelible ingredient in the lead is silver nitrate. I remember this substance from my high school Chemistry class because it gave us permanent (until the skin wore off) stains on our hands.
At first I thought that the leads were square. But the patent describes how round leads were made from paste pressed into a mold, then placed into sawed grooves and enclosed with a strip of wood (clearly visible in the fourth pencil insert). Hence, it’s a round lead (with cementing material) in a square hole.
Some of the pencils have second insert, a soft chalky substance in a paper tube. I think this is probably gypsum. The patent talks about gypsum as an ingredient of the lead, where it forms a compound that prevents softening due to atmospheric moisture. Instructions on the label seem to describe the substance being rubbed on a dampened area of the cloth prior to writing. Maybe it is gypsum helping to stabilize the mark. —
A box with French labels, showing that Clark’s pencils were sold by A.W. Faber in Paris:
The sixth pencil in the first photo is one of the newer ones. The label suggests some new uses, including marking toothbrushes (no more disgusting mix-ups!) and umbrellas. It came with this price list: