Journal of Archaeological Science (1999) 26, 735–752 Article No. jasc.1988.0342, available online at http://www.idealibrary.com on The Failure of Obsidian Hydration Dating: Sources, Implications, and New Directions Lawrence M. Anovitz Chemical and Analytical Sciences Division, MS 6110, P.O. Box 2008, Building 4500S, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6110, U.S.A. J. Michael Elam Department of Anthropology, University of Tennessee, Knoxville, Tennessee 37996, U.S.A. Lee R. Riciputi Chemical and Analytical Sciences Division, MS 6365, P.O. Box 2008, Building 5510B, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6365, U.S.A. David R. Cole Chemical and Analytical Sciences Division, MS 6110, P.O. Box 2008, Building 4500S, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6110, U.S.A. (Received 6 January 1998, revised and accepted 20 August 1998) Obsidian hydration dating (OHD) originally promised to be a rapid, inexpensive, simple and reliable method for dating obsidian artefacts. The perceived elegance of OHD resulted in rapid acceptance and widespread application despite questions concerning its theoretical rigor. With increased usage it has become evident that, while economical, simple and fast, OHD is unreliable. Here results of a secondary ion mass spectrometry (SIMS) study of obsidian and synthetic glass artefacts are presented that explain why OHD has not lived up to expectations. The results suggest that the standard OHD equations are inappropriate and that traditional optical measurements are inherently flawed. Although significant refinement will be needed, the results suggest that both chronological and palaeoclimatic data might be obtained from glass hydration rims through an improved analytical methodology and more rigorous treatment of the data.  1999 Academic Press Keywords: OBSIDIAN, SECONDARY ION MASS SPECTROMETRY, SIMS, DIFFUSION, HYDRATION DATING, GLASS, BECKE LINE, ANISOTROPY, FINITE DIFFERENCE MODELLING, CHALCO, COPAN, XALTOCAN, MESOAMERICA, MATANO INTERFACE. ‘‘Nor can they follow the atoms one by one To see the law that governs all events’’ Lucretius De Rerum Natura, Book II Introduction R idings (1996), in a review of obsidian hydration dating (OHD), raised the pointed question, ‘‘Where in the world does obsidian hydration dating work?’’ This statement reflects the frustration of many archaeologists and geologists over a dating technique that has been long on promise but short on results. This report demonstrates that the prob- lems with OHD are due, in part, to use of both inappropriate analytical techniques and an improper model of the hydration process. The implications affect not only archaeological and geological applications of OHD, but also modelling of the decomposition of nuclear waste storage glasses. Our results suggest, however, that refinement of the OHD technique is possible in a manner which will both improve its accuracy and precision, and potentially expand its 735 0305–4403/99/070735+18 $30.00/0  1999 Academic Press