Remagnetization dating of Roman and mediaeval masonry

Abstract The Earth's magnetic field may partially remagnetize rocks, often by the process of viscous remanent magnetization (VRM). Thus, when masonry is turned and placed in a monument it magnetizes parallel to the Earth's field, partly overprinting any older geological magnetizations. The intensity of this new overprint will increase with time, if the masonry is undisturbed. Older magnetizations require higher demagnetization temperatures. Thus, the unblocking temperature (T UB ) relates directly to the age of the remanence and, therefore, the age of stabilization of the masonry. For suitable rocks, we can calibrate the remagnetization time scale for the rock using historically dated structures. The scale then permits us to estimate the age of more enigmatic structures by interpolation, or modest extrapolation. Some samples reveal multiple episodes of remagnetization, showing that masonry was recycled, acquiring a differently oriented remanence each time that it stabilized in a new architectural configuration. Masonry from the Bishop's Palace (Lincoln, U.K.) includes Roman ( c . ad 300), mediaeval (1160–1450), post-Civil War (1650–1720) and Victorian phases that have characteristic T UB , consistent with their ages. Some mediaeval masonry recycled Roman building materials.