TORONTO — New analyses of bricks and mortar from French buildings constructed during the 9th and 10th centuries are providing information about Earth’s magnetic field at the time, Annick Chauvin, a geophysicist at the University of Rennes 1 in France, and her colleagues reported May 25 at a meeting of the American Geophysical Union.

The researchers analyzed samples of materials used in the construction of a medieval castle and several churches in west-central France to better determine when they were built. Ages for the structures were obtained by carbon-dating bits of charcoal included in the mortar used to hold bricks together. 

Then the researchers studied the bricks, which retain a record of the strength of Earth’s magnetic field at the time they were formed. Analyses of samples of bricks from the five sites, combined with dating evidence from the mortar, suggest that the strength of Earth’s magnetic field in west-central France during medieval times peaked in 840 and measured about 70 microtesla, Chauvin reported. Today, magnetic field strength in the area is about 48 microtesla, she noted.

The new paleomagnetic data are especially welcome because scientists have only a handful of such measurements from medieval France, Chauvin said.  Archaeologists can use such data, along with other clues, to estimate the age of other magnetized artifacts unearthed elsewhere in the region (SN: 12/22&29/07, p. 392).

Although the data provide information about the strength of Earth’s magnetic field during medieval times, the measurements can’t be used to assess the direction the field because the bricks were moved after they cooled, Chauvin said.

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