The Making of a Tennis Ball continues with the step that turns a rubber shell into something alive: pressurisation.
A tennis ball, fresh out of the can, feels almost springy in the hand — noticeably firmer than the same ball a few weeks later. That difference isn’t wear or imagination. It’s gas, sealed inside the rubber core at the moment of manufacture, slowly finding its way back out through a material that was never a perfect barrier to begin with.
The Ball That Ages From Day One
This is the strange, slightly unfair truth about a tennis ball: it starts losing its best quality the moment it’s made. Unlike a racket or a pair of shoes, a pressurised ball doesn’t wait for use to begin degrading — the pressure differential between the inside of the ball and the outside air means gas is always trying to escape, whether the ball is sitting in a can on a shelf or being smashed across a court. Manufacturers fight this by sealing balls into pressurised cans at the point of packaging, matching the internal can pressure to the ball’s own, which is why an unopened can of balls stays lively for months while an opened one starts fading within days.
What’s Actually Happening Inside
During manufacturing, the two rubber hemispheres are fused together around a pocket of gas held at roughly two to two-and-a-half times normal atmospheric pressure — this is what physically pushes the rubber outward into its taut, responsive shape. As that gas slowly permeates out through the rubber wall over time, the ball loses internal pressure, the rubber shell softens, and the bounce drops — which is exactly why the ITF’s bounce-height test exists: it’s effectively a proxy measurement for how much pressure a ball still has left inside it.
Next in the series: The Bounce.
Leave a Reply