How pencil erasers are made

A pencil eraser is usually synthetic rubber loaded with pumice, which does the actual scrubbing.

stations: 6
things upstream: 57
raw sources: 19
steps from the ground: 8

The short version

Rubber, natural or synthetic, is blended with pigment, vegetable oil, pumice and sulfur. The mix is pushed through a die as a long rod, heated so the sulfur cross-links the rubber, and cut into plugs. The plugs are pressed into metal ferrules at the pencil factory and fixed with glue or metal prongs.

Blend rubber and fillers

Rubber is mixed with pigments, vegetable oil, pumice and sulfur. Oil softens the mix, and pumice gives it a gentle grit that lifts graphite off paper.

Goes in
Synthetic rubber Natural rubber latex Sulfur Vegetable oil Titanium dioxide Pigment
Extrude the plug rod

The warm, still uncured mixture is pushed through a round die in a continuous rod. Flat erasers are instead injected into molds.
Heat to vulcanize

Heat makes the sulfur cross-link the rubber molecules, which turns a sticky gum into a durable rubber. Charles Goodyear discovered the process in 1839 and patented it in 1844.
Cut into plugs

The cured rod is cut into short cylinders, the plugs.
Check size and hardness

Inspectors measure plug size so they fit the ferrule, and test hardness so every batch erases the same way.
Press plugs into ferrules

At the pencil factory each plug is inserted into a ferrule, a metal cap of aluminum or brass. Glue or metal prongs hold it in place.

Goes in
Aluminum Brass Adhesive

End of the lineEraser

Worth repeating

Before rubber, people erased pencil marks with crustless bread.
An 1858 patent for attaching an eraser to a pencil was later thrown out, because it only combined two existing tools.
About 90 percent of American pencils have a built-in eraser.