Hi Linda,
I'm attaching a link to a report for a chronometric evaluation of a prehistoric phase in the Taos Valley of northern New Mexico. Although not obviously relevant to your mission study, what is relevant is that we included examination of obsidian hydration as a dating technique in the study. What we found supported a study of obsidian hydration by Rosanna Ridings, in which Ridings found that there are MANY archaeological-record variables (depth of burial, shade, moisture, temperature) that can and do effect formation of hydration rinds and, therefore, dating the material, and they are exceedingly difficult (I'd say impossible but maybe that's just me) to control. Those are, of course, on top of inherent differences in the obsidian materials themselves that impact rind development rates. As a consequence, I am no fan of obsidian hydration for chronometric dating. Although it might have some use for relative dating, I can't see spending the money.
Further, in the case of tools (I note you mentioned 20 projectile points), flaking, retouch, and reshaping all effect the rind thicknesses, sometimes removing rinds altogether and starting the development process over, other times, thinning the rind so as to produce inaccurate thicknesses. As you might expect, as well, in those situations one can get different dates from different parts of a single artifact (that's happened to me).
I have a friend and colleague, on the other hand, who spent most of his career in the Great Basin and is a solid fan of hydration as a dating tool. He assures me that he has seen it work many times with considerable accuracy and precision. If you'd like, I can get more info from him; perhaps an opinion from the pro- side would be useful.
Here's our report: http://www.nmarchaeology.org/assets/files/archnotes/164.pdf
Here's Rosanna's 1991 paper: http://www.maneyonline.com/doi/abs/10.1179/009346991791548762
Rosanna also wrote her MA thesis (I think) at SMU on the subject.
Jeff
Jeffrey L. Boyer, RPA
Supervisory Archaeologist/Project Director
Office of Archaeological Studies, Museum of New Mexico
* The Center for New Mexico Archaeology
* PO Box 2087
* Santa Fe, New Mexico 87504
* tel: 505.476.4426
* e-mail: [log in to unmask]
"There comes a time in every rightly-constructed boy's life when he has a raging desire to go somewhere and dig for hidden treasure." -- Mark Twain, The Adventures of Tom Sawyer
________________________________________
From: HISTORICAL ARCHAEOLOGY [[log in to unmask]] on behalf of Linda Hylkema [[log in to unmask]]
Sent: Wednesday, April 09, 2014 11:59 AM
To: [log in to unmask]
Subject: Obsidian hydration on historic sites
Hi all,
We are starting the analysis phase of a major excavation on a California
mission site (ca 1780-1840's). We have a number of pieces of obsidian
(debitage and formal artifacts). My consultants are proposing hydration on
25 pieces from the assemblage. We have already sourced all of it (except
pieces that are too small) with our XRF. Sources are the same as what has
been traditionally found in pre-contact sites of the region. My questions
are:
1. What would be gained from dating obsidian from a a site with such a
tight temporal margin, and such a recent one at that? Establishing
calibration markers for comparison for other late period sites??
2. If determined to be a relevant task, would would be a statistically
valid sample from an assemblage of about 200 pieces that includes 20
projectiles? I know that's a hard, if not impossible, question without
knowing more about my site, but any insight would be appreciated. If you'd
like more details about the site to answer my questions, please feel free
to contact me off-line. It's a rich site, and we excavated fifty-six
discrete pit features full of mission trash, which is where all the
obsidian is from.
At this point, I'm not convinced that it will add substantially to our
interpretation of the site.
ANY insight is appreciated.
Linda
*Linda Hylkema*
*Cultural Resources Manager, Santa Clara University*
W: *408-554-4513* | C: *408-219-5748* | F: *408-551-1709* | 500 El Camino
Real, Santa Clara, CA 95053
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