Proc. R. Soc. A (2009) 465, 2407–2415 doi:10.1098/rspa.2009.0117 Published online 27 May 2009 Dating fired-clay ceramics using long-term power law rehydroxylation kinetics BY MOIRA A. WILSON 1, *, MARGARET A. CARTER 1 ,CHRISTOPHER HALL 2 , WILLIAM D. HOFF 1 ,CEREN INCE 1 ,SHAUN D. SAVAGE 1 ,BERNARD MCKAY 1 AND IAN M. BETTS 3 1 School of Mechanical, Aerospace and Civil Engineering, University of Manchester, PO Box 88, Manchester M60 1QD, UK 2 School of Engineering and Centre for Materials Science and Engineering, University of Edinburgh, The King’s Buildings, Edinburgh EH9 3JL, UK 3 Museum of London Archaeology, Mortimer Wheeler House, 46 Eagle Wharf Road, London N1 7ED, UK Fired-clay materials such as brick, tile and ceramic artefacts are found widely in archaeological deposits. The slow progressive chemical recombination of ceramics with environmental moisture (rehydroxylation) provides the basis for archaeological dating. Rehydroxylation rates are described by a (time) 1/4 power law. A ceramic sample may be dated by first heating it to determine its lifetime water mass gain, and then exposing it to water vapour to measure its mass gain rate and hence its individual rehydroxylation kinetic constant. The kinetic constant depends on temperature. Mean lifetime temperatures are estimated from historical meteorological data. Calculated ages of samples of established provenance from Roman to modern dates agree excellently with assigned (known) ages. This agreement shows that the power law holds precisely on millennial time scales. The power law exponent is accurately 1 / 4 , consistent with the theory of fractional (anomalous) ‘single-file’ diffusion. Keywords: ceramics; brick; archaeological dating; rehydroxylation Dating methods are of paramount importance in the earth and environmental sciences, palaeontology, archaeology and art history. Laboratory-based dating of any material depends on identifying and measuring a physico-chemical property, which changes in a predictable way with time, thus providing the material with an internal clock (as in radiometric dating). It is established (Cole 1962; Wilson et al. 2003) that the fired-clay ceramics produced in the manufacture of bricks, tiles and pottery gain mass and expand continuously from the time of firing as a result of chemical recombination with environmental moisture (here described as rehydroxylation). Furthermore, these ceramics can be returned to their initial as-fired condition by heating to a suitable high temperature. We have shown (Wilson et al. 2003; Savage et al. 2008) that in experiments lasting up to a few weeks both mass gain and expansion increase linearly with (time) 1/4 . There are *Author for correspondence (moira.wilson@manchester.ac.uk). Received 3 March 2009 Accepted 15 April 2009 This journal is © 2009 The Royal Society 2407