By Lily Rogers, Master of Maritime Archaeology student
This blog will explore a key issue relating to quantitative analysis of glass samples from marine contexts using pXRF. The purpose of quantitative analysis is to produce data relating to the exact relative quantities of major, minor and trace elements in glass samples (Shackley 2011:220). In relation to the analysis of historic glass, quantitative analysis is much more difficult than qualitative analysis and there are a range of factors that influence the accuracy of data produced (see Kaiser for an overview of these). These issues can be separated into two broad categories. The first relates to the limitations of the particular instrument used (this will not be discussed here) and the second relates to the condition of the glass samples themselves (Liu et al. 2012:2129).
Historic glass experiences surface alkali depletion due to weathering processes and this will affect the accuracy of quantitative data (Kaiser and Shugar 2012). This is because pXRF is a surface analysis technique and processes such as weathering mean that tests on the surface of the glass are likely not to provide data that is representative of the bulk composition. Liu et al. (2012), in their study of glass beads from archaeological sites across Xianjing, China, analysed the effects of unpolished surfaces in comparison to polished surfaces on pXRF analyses. Polishing involves removing a small portion of the weathered surface of the glass. Their study showed that analysis of weathered (unpolished) surfaces affected the accuracy of compositional data in terms of the quantities of all elements detected (Liu et al. 2012:2132). This study also showed that polishing a small area of the surface to remove the weathering increased the accuracy of the measurements.
Liu, S. Q. H. Li, F. Gan, P. Zhang and J.W Lankton 2012 Silk road glass Xianjing, China: chemical compositional analysis and interpretation using a high-resolution portable XRF spectrometer. Journal of Archaeological Science 39:2128-2142.
Shackley S. 2011 X-Ray Fluorescence Spectrometry (XRF) in Geoarchaeology. New York: Springer.