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Bricks, mortar and magnetism

Medieval French castle, churches yield new data about field changes

4:52pm, May 27, 2009

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 dur

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