Potters at Maya cities on the Caribbean side of Central America fused volcanic ash with local limestone to form household and ceremonial pottery, because the ash made their ceramics easier to fire. The distinctive recipe was a hallmark of the Late Classic Period from A.D. 600 to 900, Ford said. With thousands of people living in cities such as El Pilar and Tikal, the Mayan potters burned through several tons of volcanic ash every year, Ford has estimated. But no one can figure out where the ash came from. The mystery begins with the fact that there just aren't any...