Working Knowledge: Uniform Variety; April 2005; Scientific American Magazine; by Mark Fischetti; 2 Page(s)

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 [see table]. 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.