Mexican murals store magnetic data.

Tiny magnetic particles found in the pigments of some ancient Mexican murals record the direction of Earth's magnetic field when the paint dried, a phenomenon that could help archaeologists determine the age of frescoes throughout Mexico and Central America.

The red pigments in murals painted by the artists of pre-Columbian civilizations in the New World often contain bits of magnetite, an iron oxide mineral, says Avto Goguitchaichvili of the National Autonomous University of Mexico in Mexico City. When the paints were wet, those particles were free to rotate and align themselves with Earth's magnetic field, becoming locked in that position as the paints dried.

Goguitchaichvili and his colleagues analyzed 28 paint samples from murals of known ages at four temples near Mexico City. Data from these murals, which were painted between A.D. 200 to A.D. 1200, reveal the slow changes of orientation of Earth's magnetic field over that period. That knowledge, in turn, permits the age of other murals to be deduced from the alignment of their magnetite particles. The scientists describe their findings in the June 28 Geophysical Research Letters.

The paint-analysis technique may provide an inexpensive alternative to other methods of determining the age of archaeological sites, such as radiometric dating of artifacts.