The reader can calculate calendar ages but the writer

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the reader can calculate calendar ages, but the writer does not imply that these are necessarily right say, 2.0 microns X 220 ±44 years/micron = 440 ±88 years old (68% chance that the true age is between 352 and 528 years) rim thicknesses are often used for rough age bracketing and relative chronology in short, generally thought to be much less precise than radiocarbon dates inherent imprecision of any method that is based on chemical reactions these are affected by temperature, often by humidity, and other factors it is rarely possible to assess these, or to know that any two artifacts experienced the same conditions so their rates of reaction may not have been quite the same say two piece of obsidian are flaked at the same time, but one lays on the surface in the sun for years, while another is quickly scuffed under a little soil the one that got hot a lot hydrated more quickly, and will give an older date but they may both now be buried in the same site, near each other, with no trace of their different histories it is now routine to chemically “source” the obsidian before calculating the age of the rind the chemical compositions of different sources or quarries vary although the composition within a given deposit (a single source) is relatively uniform in many places, like California and increasingly Peru, most or all the major sources are known and have been chemically characterized and the hydration rate is more or less known for each so a piece from a site can be matched to its source sometimes it is possible to do this visually; often it is not and the appropriate hydration rate used to calculate the date if you don't chemically or visually identify the source of the obsidian, you have no idea what hydration rate to use - and they vary widely another method (which has been used here at SSU, I believe) involves measuring the hydration rate of each piece experimentally called “hydrothermal induced hydration” first you measure the rind on an ancient broken surface then you break off a fresh surface and put it in a humid chamber at known high temperature and pressure it will hydrate rapidly there is a pretty solid model of how temperature and pressure affect the hydration rate
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Introduction to Archaeology F 2002 / Owen: Absolute dating: More physics tricks and historical dating p. 3 by measuring the thickness of the hydration rind after a known time at a known high temperature and pressure, it is possible to calculate what the hydration rate would be at a more typical temperature and normal atmospheric pressure then you use this rate still another method for getting the hydration rate of a given fragment is the “intrinsic water” method based on a model of the variables that affect the hydration rate in this, the main variables are the “intrinsic water” of the obsidian piece, the relative humidity, and the effective hydration temperature by measuring the “intrinsic water” of the obsidian and measuring or estimating the other
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  • Fall '02
  • BruceOwen
  • Archaeology, Radiocarbon dating, Obsidian, Radiometric dating, Geochronology, Dating methods, Dating methodology

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