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“Get the Lead Out”: pXRF Workshop for Elemental Analysis of Lead Shot

By Stacey Whitacre, James Page, Scott Butler

LAMAR Institute pXRF Workshop

June 2017, archaeologists Scott Butler, James Page, and Stacey Whitacre attended a pXRF training workshop organized by the LAMAR Institute and the National Park Service (NPS) in Savannah, Georgia. The workshop was designed specifically for lead shot specialists and conflict archaeologists to look at the elemental make-up of lead shot collected from colonial and Revolutionary War battlefields and camp locations.

We worked with physicists from Bruker to understand pXRF technology and optimize the machine for our needs. A pXRF machine uses a beam of fluorescent light to conduct elemental analysis of an object by analyzing the number of photons emitted. Bruker experts aided us in the creation of a customized calibration for the analysis of lead shot. We looked at the relative amounts of tin and other alloys or impurities (e.g., antimony, silver, iron, copper, cadmium) used to harden lead shot during the eighteenth century. We specifically worked with the Bruker Tracer 3V+ machine, as it contains no tin or silver in its internal components. We utilized the Artax software to visually and statistically analyze the results of the pXRF data.
One of the goals of this workshop was to create a comparable collection of lead shot in the form of a collective database which we could all access. In order to maintain consistency, we collaborated with other lead shot specialists and conflict archaeologists at the workshop to create a method for pXRF use with lead shot and a protocol for uploading data to the collective database. This method minimally includes the following stipulations:

•    each lead ball should be collected and stored separately to avoid cross-contamination;
•    we are to avoid the impact area of lead shot during pXRF analysis;
•    each specimen is to be analyzed at a high voltage setting of 48kV, an anode current of 30µA, with a black filter for 180 seconds; and
•    information posted to the collective database is to include UTM, site name, weight (grams), diameter (inches), fired condition, cultural period, cleaning method, citation, machine type, date, any other pertinent comments, and a photograph for each specimen.

This data set has a wide range of possibilities within the conflict archaeology community. While there is still much to be tested, we spoke about the possibilities of lead shot variation. When assessed alongside archaeological context, size, and other characteristics, pXRF elemental analysis may be useful for sourcing lead shot. One hypothesis is that there is more tin in American-made lead shot than British-made musket balls due to the fact that American forces had limited access to lead during the Revolution. Further work is necessary to determine the accuracy of this hypothesis. Additionally, testing needs to be conducted regarding the possibility of several elemental concentrations within one artifact and soil contamination that may impact the results of pXRF analysis.

Bruker offered the Tracer 3V+ to any group wishing to borrow it for 10 days. was the first group to sign up for this use time. James Page is currently using the pXRF machine to analyze lead shot that was recovered from the Brier Creek Revolutionary War Battlefield in May of this year. The team will also use the machine to analyze several brands of modern buck shot to determine if modern shot can be isolated from historic shot (i.e., is zinc present only in modern shot?). If it can, archaeologists can utilize pXRF technology to remove modern shot from our analysis to improve our interpretation of historic battlefields and camp locations.

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