X-ray Fluorescence in Archaeology
A common misconception is that archaeological work ends in the field. However, once artifacts have been excavated, archaeologists continue their research in the lab. Artifacts are cleaned, catalogued, analyzed, and curated. When an artifact is particularly difficult to identify, we often will breakdown the information by trying to determine what material an object is made from, if it has an identifiable shape, or what else may have been found with the artifact that might provide contextual clues. Modern technology can also play a key role in helping archaeologists understand their finds.
One piece of technology that can be used to identify material types is x-ray fluorescence (XRF). XRF spectrometry is a non-destructive technology that analyzes the elemental composition of an object using fluorescent x-rays. Each natural element emits a unique fluorescent photon when the x-rays collide with the atomic structure. This fluorescent radiation has a wavelength that is measured by the machine. The XRF device calculates how many times it emits pulses of x-rays while simultaneously measuring the resulting wavelengths that are produced from the object being examined. The XRF analyzer produces a graph of the measured wavelengths. Since each element has a specific fluorescence that can be measured at a specific wavelength, XRF spectrometry can be used to determine the elements present in the artifact being analyzed. The higher the spike at a particular wavelength on the graph, the more of that element is present in the artifact. XRF instruments can also produce a chart of each element detected and the percentage of its composition within the artifact being scanned. Some XRF spectrometers even identify the carat for objects with gold present.
This technology is extremely useful for analyzing the geochemical composition of metals, glass, ceramics, and other inorganic materials. Museums use XRF spectrometry for a variety of purposes such as determining whether or not an object has been treated with arsenic in the past, whether a glass object contains uranium, or to differentiate the composition of a metal alloy (i.e. brass vs. bronze). XRF can be useful in determining where a ceramic was made based on its elemental composition compared to examples of clays from various regions. Knowing the elemental composition of an artifact can help archaeologists understand the materials that were available at the time the artifact was made, understand how the crafting processes affected the elemental structure of artifacts, and locate origin sites or link trade routes to archaeological finds.
XRF spectrometry is particularly helpful in artifact conservation. Prior to treating objects it is important to know what the material composition is so that an appropriate treatment plan can be put in place. One such object from Historic St. St. Mary’s City (HSMC) is a button found at a colonial site in the Town Center. Prior to conservation treatment, the artifact was x-rayed and photographed (Figures 1 and 2). The x-ray was particularly useful, as it brought out lettering around the edge of the button. The button was manufactured by Scovills & Co in Waterbury, Connecticut and likely dates to circa 1840-1850. It was speculated that the button might have a gold plating, and it was selected for XRF analysis to help determine it's elemental composition. Figure 3 depicts a graph of the XRF results, showing that the button is comprised of a copper alloy with a small amount of gold plating. This data provides important diagnostic information that can inform treatment options, artifact analysis, and the curation of the object.
Figure 1: X-ray image of a 19th-century button from HSMC's Town Center Site, showing details including the button’s back shank, decorative border, filigree, and letters.
Figure 2: Left – Button Front with floral design and gold plating; Right – Button Back with makers mark “Scovills & Co Waterbury”.
Figure 3: XRF chart depicting the wavelengths for copper and gold. These frequencies have been highlighted. Copper wavelengths are higher than the gold spikes.