Posted by Steve Price on June 29, 1999 at 12:16:51:
In Reply to: Re: Why the difference? posted by Yon Bard on June 29, 1999 at 11:27:42:
Dear Yon,
Jim Allen, in one of his posts, said that Kajitani (of the Metropolitan Museum of Art in NYC) told him that washing can add several hundred years to a C-14 date. I have not run into this elsewhere, and I do not know the mechanism or situations under which it occurs.
Among the fairly well known and accepted sources of error in C-14 dating are exposure to certain sea waters, volcanic ash, and smoke. All of these can add centuries to a C-14 date. There is a concise, fairly clear exposition of the subject at
http://c14.sci.waikato.ac.nz/webinfo/corr.html
The mechanism, as I understand it, is that the smoke contains some very old carbon that gets deposited on the sample or exchanges with the carbon in the sample (it isn't clear to me which; perhaps both occur). In any case, what gets analyzed is carbon from the "real" sample plus carbon that can predate it by a long time (the table shown on the website suggests that for volcanic ash the contaminant can add more than 5,000 years to the C-14 age in extreme cases).
Now, we may assume that every real tribal textile has spent some time in a tent of less than 20 foot diameter, with a fire in the middle for warmth and cooking. The fire would give off smoke and, unless those people are much more adept than I am, so would the food. We can add to that problem the fact that many textiles were exposed to smoke and maybe even volcanic smoke in homes and in transit to them by ship or train. Please note that these errors will not go away with repetitive anaylsis.
Then superimpose the range of dates that makes up the 95% confidence interval - usually 300 to 400 years - and I have a hard time seeing any reason to pay attention to C-14 dates for rugs made during the past 500 years.
I have repeatedly requested that someone get a genuine expert into the discussion, who might be able to explain how these problems can be and were overcome in analyzing the rug samples. Jim Allen says the guys at U of Arizona, where the analyses were done, refuse to comment. This, by the way, strikes me as odd in the extreme.
I am no expert on C-14 dating, but I think I have enough of a technical background to be able to understand the method and how problems are resolved if a reasonable explanation were to be offered. Until that happens, the C-14 results don't exist as far as I'm concerned.
Steve Price