Ironically, in parallel with such efforts at consistency, the core's composition and the cover's nap length and tightness are fine-tuned to create one of three speed classes of balls: fast for slow courts like clay, medium for the ubiquitous hard courts, and slow for the fast grass surfaces. U.S. players prefer a fast game and generally play on hard surfaces, so the air inside balls is pressurized to about two atmospheres to make them more responsive. Many Europeans prefer slower play, typically on softer courts, so "nonpressurized" balls, sealed at one atmosphere, are also commonly sold.
Uniform Variety—Working Knowledge for Tennis Balls
Service! How to manufacture tennis balls
Manufacturers make more than 240 million tennis balls a year worldwide. And they are surprisingly uniform, given that they begin as natural rubber and wool, which vary with every barge and bale. To be stamped "official" in accordance with the International Tennis Federation, a ball must meet rigid specifications for deformation and bounciness. Rubber, which "varies as much as lettuce," according to Lou Gagnon, technology manager at Head/Penn Racquet Sports in Phoenix, Ariz., is combined with up to 11 chemicals to create a homogeneous slurry. The mixture is pressed into molds to form the ball's center, or core. To craft a consistent cover, wool, nylon and cotton are woven into a felt that is soaked, shrunk and dried.