Distinguishing the composition of medieval stained glass windows using x-rays

Original article: Chemical imaging of stained-glass windows by means of macro X-ray fluorescence (MA-XRF) scanning

When I think about stained glass windows, I usually think of admiring the windows in a church or a museum. I don’t think about how the glass was formed, colored, and assembled to depict the scenes I’m observing. Seeing how the chemical mapping by macro x-ray fluorescence (MA-XRF) of paintings has been helpful for finding underpaintings and other details, Dr. Van der Snickt et al. used a portable MA-XRF instrument to image the surface of stained glass windows to generate similar chemical mappings for glass materials. Most of the previous research on glass objects consisted of single-point analysis, which does not provide a comprehensive understanding of stained glass windows. MA-XRF collects fluorescence for different elements present in objects after they are subjected to a focused x-ray beam, allowing a greater understanding of the elemental distribution for 2D objects that can be more readily understood by a general audience. Previous bites have shown MA-XRF applied to the study of paintings, a historic book, and objects

Van der Snickt et al. applied MA-XRF to the study of the two late 15th-century stained glass panels shown in Figure 1. The windows were believed to be a piece created to showcase the skills of the Bruges guild of window makers. They were created at a time when the preferences of glassmakers were changing from using potash glass to“high lime low alkali” (HLLA) glass as well as using a new technique for glass production. Since the elemental composition of micro samples taken from the panel had been studied before using scanning electron microscopy with an energy dispersive x-ray detector (SEM-EDX), Dr. Van der Snickt et al. decided the panels would be a good test for applying MA-XRF to stained glass windows.

St. George and St. Michael stained glass windows
Figure 1. Stained glass windows of St. George (left) and St. Michael (right), c. 1490. Originally designed for the chapel of the Bruges guild of St. Luke in the Church of Our Lady, the panels are now part of the collection of the City Museum of Bruges.

HLLA glass became preferred over potash glass because of its higher stability. To determine what sections in the panels were potash or HLLA glass, the chemical ratios of potassium (a flux agent) and calcium (a network stabilizer) were compared. The general schematic for translating the MA-XRF data to spatially sectioning the different types of glass is shown in Figure 2.

Schematic of MA-XRF mapping for a stained glass window
Figure 2. Example schematic of using data from a MA-XRF scan of a stained glass window to understand the different glass compositions present. The detector scans across the glass window to collect data on the elements present in the glass, and then the data is visualized by plotting element ratios and how those ratios cluster. The clusters are assigned a color, and the locations where those clusters occur are recolored over the glass to provide a qualitative understanding of the different glass compositions.

As can be seen in the lower right panel, the data clusters into different regions depending on the ratios of flux agent to network stabilizer present in the glass, which can be visualized in colorized maps. Combining these data sets with the knowledge of where each data point came from on the glass window, the authors were able to better understand where different types of glass were used within the panel. Most of the glass used in the panels was potash, with HLLA glass being used for more sophisticated colors within the panel, such as the red, blue, and dark green.

MA-XRF was also used to determine the method of glass coloration used, information which is otherwise difficult to obtain through conventional assessments of glass. By comparing element maps of common colorants for the interior and exterior views of the panels, Van der Snickt et al. determined the glass was colored using either pot metal (colored throughout the glass) or flash (colored on one side of the glass) techniques. The monster’s dark green claw on the right side of the Saint Michael panel has an interesting dark green coloring which has high cobalt contents instead of copper (the more common colorant for green during this time period). Since cobalt is normally used for blue panels and there are no signs of yellow pigmentation within the panel, these results encourage further investigation as to what makes the coloration of glass used for the claw so distinct from the rest of the panel. 

Learning the relative concentrations of specific elements in glass panels provides a non-invasive method for obtaining a better qualitative understanding of class composition and coloring techniques. Creating visual representations of the data makes it more accessible to conservators and curators along with tailoring the questions for further technical study of stained glass panels.

Figure 1 reproduced and adapted under a Creative Commons license from Wikimedia Commons. The original creator of the images of St. George and St. Michael is Sailko. 

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