Technology: X-Ray Fluorescence (XRF) Spectrometry


For the last two days I went to a workshop on the use of a portable XRF spectrometry device. It is a tool to analyze the elemental composition of material without destroying the material. The device is based on principles of atomic physics. An x-ray is sent into the material and knocks some electrons from inner shells of an atom. As the atom becomes unstable, electrons from outer shells fill the gap and drop down into the space of the removed electron. As a result they release energy in the form of photons. These photons have a characteristic energy, which is measured by a detector. Each element has a different signature energy of these exiting photons. If the photons are counted and plotted on a graph, it is possible to determine the approximate composition of the analyzed material. Automatic recognition is not totally satisfactory, as the raw data still requires some interpretation. Often peaks of different elements overlap.
While the instrument can give a general indication of elements present, specific settings have to be selected to detect differences of certain elements. For example, if you think that stone artifacts from different rock formations contain widely varying amounts of strontium, you can choose to specifically study the variance of strontium to determine where the stone of the different artifacts was sourced from.

In the pictures shown here, we were analyzing some stone celts (stone axes), made out of American greenstone. If we compare many stone celts, we may be able to even link their provenance to particular stone sources. Or at least we might be able to say that it is highly likely that celts from various places were found at this one archaeological site. We can then make some inferences about trade. These sourcing studies are very common today.
One application I am interested in is the analysis of anthropogenic soil. When we excavate a house, we can test the soil of the floor and possibly be able to get further information on the activities carried out there. Human activities leave a trace. For example, in a temple in Mexico, the floor was analyzed. This analysis confirmed areas of sacrifice (elevated iron levels in the soil) and the burning of organic materials. The archaeologists were able to more closely reconstruct the different activities that may have occurred in the temple.
XRF is just one technology that can help archaeologists with chemical analysis. There are many more. The advantage of XRF is that it is non-destructive and portable, and not as expensive as some other methods. It is not necessarily the most accurate method available. Because it relies on radiation, it does carry some health risks, but these can be easily managed.
Two points to note: these technical applications are just tools. They have to be used wisely, critically and correctly. Often archaeologists analyze all sorts of things, sound highly technical and in the end do not know why they analyzed things. Sometimes they cannot draw any conclusions and the whole exercise was fairly useless. They still write complicated, meaningless reports. Sometimes, they think that the scientific methods allow them to speculate widely. It requires an imaginative and careful mind to use these technologies well. The research design must be clear and well-thought out. And we always have to be aware that interpretation of the results will be required.

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