CHAPTER X IN SITU METHODOLOGY FOR COMPOSITIONAL GROUPING OF MEDIEVAL STAINED GLASS WINDOWS: INTRODUCING THE “WINDOLYZER” FOR HANDHELD X-RAY FLUORESCENCE SPECTROMETRY L. W. ADLINGTON, I. C. FREESTONE, L. SELIGER, M. MARTINÓN-TORRES, F. BROCK, A. SHORTLAND Abstract A materials science approach can illuminate our understanding of the life history of medieval stained glass windows; however, chemical analysis has been inhibited by their architectural context, preventing the removal of samples. Non-invasive techniques that can be used in situ, such as handheld/portable X-ray fluorescence spectrometry (pXRF), are thus very important to this field of study, yet this technique is also impeded by significant obstacles: the presence of deterioration and painted detail (subject of a previous paper) and the protrusion of the lead cames which hold the individual pieces together. In the present paper, we address the obstacle that lead cames present for in situ analysis and offer a simple, inexpensive and adaptable solution in the form of a 3D-printed attachment (a window analyzer, or “WindoLyzer”) that allows control over the working distance between spectrometer and sample. This simple modification to the standard pXRF system combined with empirical calibrations allows precise, reproducible results with a level of accuracy sufficient to distinguish compositional groups. We then present a case study in which this development is put to practice to answer important questions related to the history of conservation of a medieval panel from Canterbury Cathedral. This technique has allowed the recognition of previously unidentified repairs to the original panel, and shown that late eighteenth century craftsmen were recycling medieval glass to execute their work. Particularly promising is the ability to distinguish different groups of glass which are clearly of medieval origin, opening the way for future studies on the movement of glass in the medieval period. Introduction The later medieval period saw a tremendous rise in the construction of cathedrals and churches, underpinning an immense industry and resulting in a sharp increase in the demand for glass to fill the windows. Stained glass windows were increasingly important to medieval culture as symbols of iconography and prestige, and they represent a complex chaîne opératoire. A materials science approach can illuminate our understanding of the life history of medieval windows, including the identification of different sources of glass and their provenance 1 , the investigation of glass- 1 Laura W. Adlington et al., “Regional Patterns in Medieval European Glass Composition as a Provenancing Tool,” Journal of Archaeological Science 110 (2019): 104991, https://doi.org/10.1016/j.jas.2019.104991.