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What Are Baseballs Made of?

By Jennifer Mueller

In 1934, the American and National Leagues released the standard components of a regulation baseball for the first time to the public. Despite scientific and technological advances since then, official MLB rules still require a ball made of a rubber or cork core wrapped with yarn and covered with two pieces of animal hide.

Stitched by Hand

A baseball begins with a small, spherical core of cork and rubber encased in thin layers of rubber. Hundreds of yards of wool yarn are machine-wound around this core, followed by more than a hundred yards of white cotton twine. The finished interior of the ball is coated with a layer of rubber cement to seal the twine in place. Workers then take two pieces of white leather and wrap them over the ball before sewing the pieces together by hand with thick, red thread. The finished ball weighs between 5 and 5.25 ounces and has a circumference of between 9 and 9.25 inches.

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