How billiard balls are made

Pool balls were once elephant ivory, and an 1869 substitute mixed nitrocellulose, camphor and ground cattle bone.

stations: 9
things upstream: 61
raw sources: 21
steps from the ground: 10

The short version

Modern pool balls are cast from phenolic resin, a hard thermoset plastic. Liquid resin is poured into molds, cured with heat, stripped of its casting gate and then machined and polished until each ball hits the right weight, diameter and roundness. At Aramith's Belgian plant the whole run takes thirteen steps and up to 23 days, and every ball gets a final check by hand.

Make the resin

Phenolic resin sets hard and does not melt again. Saluc, maker of Aramith balls, formulates its own in a chemical plant on site, since the resin decides how the ball rebounds and wears. Color goes into the mix as dyes or pigments.

Goes in
Phenol Formaldehyde Pigment
Pour into molds

The liquid resin is poured at normal pressure with a sprue facing up, so that bubbles inside can rise and escape. A 1938 patent warms its molds and draws a vacuum for a few minutes to clear surface voids.
Cure with heat

Curing sets the resin to a first hardness. The 1938 patent bakes its cast stock for four days at about 80 °C before any machining.

up to 23 days for the whole process at one maker
Grind off the gate

The poured sprue leaves a knob standing on the sphere. The traditional fix is to hold the ball against an emery wheel, which is slow and can leave it out of round, and one Chinese design uses a collet cutter instead.
Machine slightly oversize

In the 1938 patent, the hardened stock is first machined to balls about 0.080 inch bigger than final size, which leaves stock to remove after the next step.
Harden the surface

More heating hardens the outer layer. That patent packs its balls in sawdust for three days. Aramith says its better grades go through several hardening steps that leave a translucent vitrified surface.
Turn to true size

Balls are cut down to final diameter and roundness with computer-controlled tools. Pool rules allow a diameter of 2 and a quarter inches, plus or minus 0.005 inch.
Polish the surface

Polishing brings up the glassy finish. Aramith says a finer resin grain holds that polish for longer.
Mark and check

Numbers and colors go on, and balls are judged on density, balance, diameter, roundness, color, polish and brilliance. Automated checks run throughout, and a person still checks every ball. Top sets are sorted so all balls fall within 1.5 grams.

Goes in
Paint

End of the lineBilliard ball

Worth repeating

Elephant ivory stayed the preferred material for billiard balls from 1627 or earlier into the early twentieth century.
One maker says nearly 80 percent of the world's billiard balls come from a single small village in Belgium.
Mass harvesting of camphor for early plastic balls devastated forests in Taiwan.