Watching Bricks Age

Civil engineers and materials scientists have long known that clay bricks and other fired ceramics expand as they age owing to the absorption of water from the atmosphere. In general, however, studies of moisture expansion in bricks have been limited to freshly fired bricks over short timescales. Now researchers from the University of Manchester Institute of Science and Technology and the University of Edinburgh have experimentally investigated expansion in bricks over periods extending back to Roman times, about 1900 years ago. They conclude that brick expansion is governed by a power law. Specifically, bricks expand in proportion to time, raised to the quarter power, as opposed to the logarithmic expansion with time predicted by studies over shorter time scales. The researchers (M. A. Wilson, 44-0161-200-4245) propose that the power law moisture expansion is consistent with the ceramics absorbing water that diffuses through atomic scale pathways in the material. The new theory should help in the engineering of brick structures intended to last a century or more by allowing designers to account for expansion that might otherwise lead to cracks. The power law may also be handy for archeological dating of bricks and ceramics. For example, archeologists could measure the dimensions of a piece of ceramic, and then bake out any moisture it may have absorbed to determine its size at the time that it was first fired. The age of the sample can be inferred from the contraction as the ceramic dries out. (M. A. Wilson et al., Physical Review Letters, 28 March 2003)