Electron Spin Resonance Studies To Explore the Thermal History of

Chapter 10

Electron Spin Resonance Studies To Explore the Thermal History of Archaeological Objects 1

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Downloaded by PURDUE UNIV on September 27, 2017 | http://pubs.acs.org Publication Date: August 15, 2002 | doi: 10.1021/bk-2002-0831.ch010

Robert G. Hayes and Mark R. Schurr 2

Departments of 1Chemistry and Biochemistry and Anthropology, University of Notre Dame, Notre Dame, IN 46556

The heating of many organic materials produces species with unpaired electrons in the sample. These species may be studied by ESR. The characteristics of the E S R spectra depend on the temperaure to which the sample has been heated and on the time of heating, among other parameters. Thus, E S R may be used to refine the characterization of burned organic archaeological materials. We review work on charred cereal grains and its application to samples of charred maize recovered from six late prehistoric sites in the Midwest. B y correlating the thermal histories of the prehistoric kernels with their archaeological contexts, we are able to control for some taphonomic biases in archaeobotanical samples. We also report E S R studies of modern bone samples heated under controlled conditions. They behave in the same way as carbonized plant remains at temperatures below about 300 °C, suggesting that ESR spectra can also be used to deduce the thermal histories of burnt bones from archaeological deposits.

© 2002 American Chemical Society

Jakes; Archaeological Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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It has been known for a long time that pyrolysis of organic materials produces paramagnetic materials (1-3), and that the paramagnetism is very persistent. If the pyrolysis is carried out below 600 °C or so, the paramagnetism is believed to be due to free radicals that are produced in intermediate stages of the process by which heteroatoms are eliminated from the material and a carbon network is formed (4,5). These free radicals are often very stable, and persist in the sample over hundreds or even thousands of years. The number and nature of these radicals can be investigated with electron spin resonance (ESR) spectra. The E S R spectra of organic materials that have been pyrolysed often consist of a single rather narrow resonance (6). The E S R parameters of the resonance depend on the temperature to which the sample had been heated and on the time of heating and, to a lesser extent, on other conditions of pyrolysis. The availability of information about the thermal history of pyrolysed organic materials has several interesting potential archaeological applications. Charred, or apparently charred, plant remains, especially wood and cereal grains, are often found in archaeological contexts. Charred bone is also often found in interments. The ability to measure the temperature to which charred bone had been exposed is especially interesting, because bone collagen is often used in paleodiet studies and it is known that charring changes the isotope ratios of bone collagen (7). Hillman and co-workers have studied the ESR of charred cereal grains, primarily emmer wheat, and have shown that the g value of the resonance depends on the temperature of charring (8, 9). More recently, we have studied the ESR of charred maize kernels (10). Our studies complement the work of the Hillman group. We have used our results to discuss the thermal histories of charred maize kernels from six archaeological sites in the Midwest. In this report, we review our work on maize, extend it somewhat, and attempt to provide some guidelines for the application of the work to archaeological samples. We also, in a separate section of the report, make preliminary report of our work on the E S R spectra of samples of heated modern bone and the correlation of the E S R spectra with C / C and N / N ratios from the collagen of some of the samples. The isotope ratio measurements have allowed us to relate the E S R parameters, in particular the g value, to heating temperature, and has also allowed us to relate isotope ratio shifts to heating temperature or, more directly, to ESR g values. ,3

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E S R and Charred Plant Remains Hillman and co-workers (8, 9) studied the ESR spectra of charred cereal grains, especially emmer wheat. They found that the g factors of the spectra depended on the temperature of charring, but were not influenced by the time of exposure to the elevated temperature. Hillman and co-workers also found that the line width of the resonance, and its intensity, depended on the heating temperature, and could be used to support the g value in deducing the thermal history of a sample. They also found that the sample need not be freed of oxygen in order to be observed and that the g value of the resonance did not depend on the amount of oxygen adsorbed or absorbed by the sample. Our work confirmed all these results. The primary quantitative result of the study by Hillman et al. (9) was a calibration curve of g values with temperature, claimed to be valid, independently of heating conditions (ranging from heating in air to heating in vacuum) and, by implication, applicable to all plant materials. Hillman and coworkers were able to obtain data over a temperature range of 20 °C to 600 °C. They found a monotonic decrease in the g value of the resonance, from 2.0043 at room temperature to 2.0024 at 600°C (Cit. 9, Fig. 3). We prepared, and measured triplicate sets of samples. We examined three conditions of charring. These were charring in air, charring in sand and charring in evacuated sealed tubes. We also examined two times of exposure to the elevated temperature, five minutes and two hours. There was little difference between the parameters of samples charred for five minutes and those charred for two hours, except at low temperatures. We attribute this to failure of the samples to come to temperature in five minutes at low temperatures. The samples charred in sand and the samples charred in air had very similar g values, and the intensities were, on the whole, quite similar. Figures 1 and 2 show our results for samples that were charred for two hours, in air and in evacuated tubes. We show mean values and standard deviations of the parameters. The standard deviations are shown as error bars in the figures. Our results, clearly, parallel the earlier results, but there are some significant differences. Also, we performed some ancillary experiments to examine certain aspects of the results, especially aspects that might have relevance to archaeological applications of the work. We discuss these matters here. Firstly, we were never able to take a sample above 400°C because the samples burned above this temperature. Thus, we never obtained a sample with a g value as low as 2.0024 and, in fact, the g values we observed at the highest temperature we could achieve, 400°C, were considerably higher than the value that Hillman and co-workers report for 400°C (2.0026).


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Electron Spin Resonance Studies To Explore the Thermal History of

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