Ancient Chemists Had a Ball

When 16th century Spanish clerics came to the New World, they were enthralled by a fast-paced and sometimes bloody sport. Apart from the occasional postgame human sacrifice, what most astonished the Spanish were the ricocheting rubber balls. Now the recipe used by the Olmec, Maya, and other ancient cultures to turn raw latex into rubber has finally given up some of its secrets. Their feat of chemistry, described in tomorrow's issue of Science, was not duplicated until the mid-19th century.

The ball game, invented at least 3400 years ago, was an important ritual for many Mesoamerican societies. To the Maya, for instance, the game--called chaah--reenacted portions of their creation story. By the 5th century A.D., many towns had central stone courts, some of which could hold thousands of spectators. These societies also used rubber for a host of other products, including religious figurines, incense, and even lip balm.

Last summer, Massachusetts Institute of Technology (MIT) archaeologist Dorothy Hosler and undergrad Michael Tarkanian traveled to Chiapas, Mexico, to gather the raw materials for rubbermaking mentioned in ancient documents. To their surprise, they saw farmers collecting latex by slashing the bark of Castilla elastica trees, then mixing in juice from pulverized morning glory vines that wrap around the trees--just as the 400-year-old texts described. "It was amazing," recalls Tarkanian. "After about 10 minutes, a mass of rubber rose to the surface. We formed it into a ball that would easily bounce over your head."

A battery of lab tests showed that the homemade rubber was about twice as elastic as dried latex, which cracks when handled. With MIT materials scientist Sandra Burkett, the researchers probed the material with nuclear magnetic resonance spectroscopy. They discovered vine juice components--traces of sulfonyl chlorides and sulfonic acids--that can react with polymers, stiffening segments and making them more likely to interact. The team says that only a few such entanglements would be enough to give the rubber its spring.

"It's a marvelous example of technology demonstrated at an incredibly early stage," says Frank Bates, a polymer chemist at the University of Minnesota, Minneapolis. The ancient Mesoamericans, he quips, "probably had a pretty good R&D team."