Using handheld pXRF to study medieval stained glass: A methodology using trace elements
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Using handheld pXRF to study medieval stained glass: A methodology using trace elements Laura W. Adlington1 and Ian C. Freestone1 1
UCL Institute of Archaeology, 31-34 Gordon Square, London, WC1H 0PY
ABSTRACT The surfaces of 30 pieces of glass from panel 3b of the Great East Window of York Minster (1405-1408 CE) were analyzed by handheld portable X-ray fluorescence (pXRF) and small samples from the same pieces were analyzed by electron microprobe (EPMA). Comparison of the two methods reveals significant divergences which are not systematic, particularly for elements lighter than Ti. Rather than a problem with pXRF calibration or correction software, the non-systematic error is attributable to the presence of a thin surface layer of weathered glass. Analysis of the depths of X-ray generation indicate that virtually all X-rays characteristic of Ca and K are generated within the top 50 µm of the glass. However, for heavier elements such as Rb, Sr and Zr, most emitted X-rays are generated below 100 µm. Using pXRF data for the heavier elements, it is possible to replicate the compositional groupings identified by quantitative EPMA. White glass in the window is likely to have originated in England, while colored glasses were probably obtained from the Continent. The alkali contents of the green and yellow glasses appear to have been manipulated to generate their colors. Glass which is medieval in technology but not original to the panel was identified. In particular, zirconium proved a useful indicator of glassmaking regions, and rubidium and strontium were more sensitive to differences between batches, which has interesting implications for future work.
INTRODUCTION Stained glass was an important element of ecclesiastical architecture in the Middle Ages, and its production was underpinned by a significant industry. However, our understanding of the chaîne opératoire of a stained glass window is limited, due to the restricted nature and poor preservation of the relevant documentation (Marks, 1993). Scientific analysis has the potential to facilitate our understanding of key aspects regarding the life history of stained glass windows, including glass sources, glassmaking technology, and the identification of evidence of conservation post-construction. Unfortunately, the analysis of windows is inhibited by their architectural context, which makes sampling the windows impossible unless dismantled. Thus, in situ techniques such as handheld (portable) Xray fluorescence spectrometry (pXRF) have great potential for the study of stained glass windows. There are considerable obstacles in the application of pXRF for medieval stained glass, in particular the deterioration of medieval glass resulting in a layer of altered composition, greatly affecting surface analyses by pXRF. This problem will be discussed along with a consideration
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