Flashed glass: chemical and structural characterization using synchrotron radiation based X-ray fluorescence and X-ray absorption near edge (XANES) spectroscopy
Josef Hormes(1,2), Yongfeng Hu(3), Qunfeng Xiao(3), Wantana Klysubun(4) & Markus Kleine(5)*
Flashed glass constituted an important component of the medieval glaziers’ palette. It is produced by covering a thick clear or tinted glass with one or several thin layers of coloured glass. Because of the colouring component “copper”, red flashed is one of the oldest known flashed glasses. Due to the fact that full copper coloured glass becomes dark black, glass manufactures invented this layering technique. Variations in the thickness of the overlay result in modulations in the corresponding colour. Variations in the glass composition allow the glaziers new options of design technique.
Synchrotron radiation based techniques, particularly X-ray excited X-ray fluorescence (SR-XRF) and X-ray absorption near edge structure (XANES) spectroscopy are “ideal” techniques for the detailed chemical characterization of flashed glass. Because of its special properties (high intensity, collimation) X-ray beams coming from a “synchrotron” can be focused down to the micrometre range using various techniques so that measurements can be carried out with high special resolution. This allows measurements in the various layers of the flashed glass and also in the transition zone between the layers. Synchrotron radiation excited X-ray fluorescence allows the determination of the elemental composition of the sample incl. minor and trace elements and “low-Z-elements” such as S, P, Si, Mg and Na. XANES spectroscopy is a technique that uses tunable monochromatic X-rays to measure the energy dependence of the photoabsorption coefficient in a narrow region around an inner shell absorption edge of the element of interest. XANES spectra provide detailed information about the electronic structure of the local vicinity of the absorbing atom species (e.g. the valency).
Most experiments (XANES as well as SR-XRF measurements) were carried out at the SXRMB (Soft-X-ray micro-characterization beamline) of the Canadian Light Source in Saskatoon using the microprobe endstation. By using a Kirkpatrick–Baez mirror system, this station provides a spatial resolution down to 10×10 µm2
in the energy range between 1·7 and 10·0 keV. Some experiments were also carried out at Beamline BL8 of the Siam Photon Source in NahkonRatchasima in Thailand. Here experiments were carried out by with an un-focussed beam and a metal aperture with a diameter <0·5 mm for the required spatial resolution.
Measurements were carried out on several different samples for investigating variations in the composition of glasses from the Romanic to the modern ages. The investigations of flashed glass are necessary because of a severe lack of records especially at the commencement of this technique. We investigated in our project a sampling spectrum from the 9th century to the 20th century. The oldest sample is an archaeological glass found in Balhorn near Paderborn. It is a green glass with a red flamed layer, dated in the 9th century and because of its tool marks it can be assigned as a part of a glass window. The condition of this sample in is very good and it shows hardly any material corrosion. The second piece is a red on white glass from the 14th century with unknown German provenience. The surface shows different form of heavy corrosion. This could be a sign for a variation of the glass composition used for developing new design techniques. The third glass is a flashed glass red on white from the Cathedral de Seville. The glazier was Arnao de Flandes. This 16th century glass is in a better condition raising the question if this is due to changes in the composition or just due to the “better” weather conditions in Andalusia? The fourth sample belongs to a private house in Hamburg. The window was made in the 19th century by the glazier Collins in England. It was used for a special etching technique.
Figure 1 shows SR-XRF spectra of two samples (marked as 4 and 10) integrated over both layers (red and white) recorded with an excitation energy of 10 keV. The spectra show quite clearly the different elemental composition of the two samples. Remarkable are, for example, the extremely high concentration of copper in sample 4 and the very high K-concentration in sample 10.
Figure 2 shows as typical examples the Cu-K-XANES spectra recorded in the red and white area of sample 4 and in the red area of sample 19. The spectra show clearly that the speciation of Cu is different at all three points.
Institute of Physics, Bonn University, Nussallee 12, D-53115 Bonn, Germany
Center for Advanced Microstructures and Devices, 6980 Jefferson Hwy., Louisiana State University, Baton Rouge, La 70806, USA
Canadian Light Source Inc., 44 Innovation Blvd., Saskatoon SK, S7N 2V3, Canada
Synchrotron Light Research Institute, 11 University Ave., Muang, Nakhorn Ratchasima 30000, Thailand
Glasmalerei Peters GmbH, Am Hilligenbusch 25, D-33098 Paderborn, Germany