Accelerator Radiocarbon Measurements of Artistic Artifacts
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Mat. Res. Soc. Symp. Proc. Vol. 185. ('1991 Materials Research Society
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where T is the "Libby" mean-life of 4C (8033 years). For the carbon beams generally used in our instrument, the precision of a typical measurement of the ratio, F, for a sample less than 5000 years old and containing about one milligram of carbon is about 0.6%. The precision results in a standard deviation for the radiocarbon age of 50 years. Such measurements require about one hour of instrument time. A standard deviation defines the range within which two-thirds of the measurements on identical samples would be expected to fall. Better precisions and resultant smaller uncertainties can be achieved by making measurements on several samples of the same material. To deduce a calendar age for a sample from its measured radiocarbon age for artifacts less than about 10,000 years old, one uses a calibration curve compiled from measurements of the radiocarbon age of tree rings of known calendar age. Averages of measurements from several laboratories around the world have been used to construct a calibration curve, which extends from the present to about 9000 years before present [2]. Figure 1 shows a portion of this curve which covers the period from the present back to about the year A.D.1000. The precision of the curve is about 0.2% over much of its range. However, the shape of the curve (in some places it is double or triple valued) is often the determining factor in the uncertainty in calendar ages which can be obtained from radiocarbon measurements. 1200
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1 000
a-n 800
M CL W 600 0
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~0 400 a
200
Stuiver and 1000
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Pearson 1400
1600
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2000
cal AD (years)
Fig. 1. Radiocarbon age vs. calendar age of tree rings for the past 1000 years (from reference 2). As one. A makes it material quotes a
can be seen from Fig. 1, the period since A.D.1700 is an especially difficult combination of several effects resulted in an atmospheric radiocarbon content that impossible to determine the precise age of artifacts fabricated from carbonaceous that grew in the biosphere during that period. For such samples, one generally limit indicating that they were produced since A.D.1700.
Finally. to determine the limit of the method, the radiocarbon contents of targets fabricated from supposedly "dead" material (material containing no "1C). such as coal, CaCO3 , or petroleum, are routinely determined. The results of background measurements demonstrate that the oldest radiocarbon ages that we can determine in our instrument are about 46,000 years. The background 11C. at a level of about 0.4% of the 1"C contained in modern carbon, is introduced during chemical processing of the samples. APPLICATIONS TO ARTISTIC OBJECTS Over a period of several years, the instrument in our laboratory at the University of Arizona has been used to determine the age of various artistic samples, and in the remainder of this paper we describe some of these applications. Textiles Textiles are fabricated from plant or animal material
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