79 Introduction In August 2017, at a crucial stage of its recent conservation treatment (2017–21), Girl Reading a Letter at an Open Window (c. 1657–59), painted by Johannes Vermeer (1632–1675), was examined using macroscopic X-ray fluorescence imaging spectroscopy (MA-XRF) – at that time a relatively new diagnostic technique for research on paintings. This technique goes a step further than traditional X-radiography. While the latter only shows a contrast image between light and heavy elements, MA-XRF imaging maps the separate chemical elements across the painting. From the resulting elemental distribution maps, it can be inferred which pigments were used in the painting and where. In the case of Girl Reading a Letter at an Open Window, this revealed more specific information about Vermeer’s palette, the painting process, and, most importantly for the treatment, the condition of the picture of Cupid hidden in the background by layers of later overpaint. This information supported the decision-making process behind removing the overpaint and unveiling the Cupid. MA-XRF also visualised changes, made during the painting process by the artist himself. Past research, making use of X-radiography and infrared reflectography, had already revealed that Vermeer made some remarkable changes while developing the composition. However, the MA-XRF research from 2017 provided new insights, allowing us to follow, for the first time, Vermeer’s creative process step by step by placing the numerous changes into chronological order. Macroscopic X-Ray Fluorescence Imaging Spectroscopy (MA-XRF) Principles of the technique MA-XRF is a non-invasive, analytical imaging technique that was specifically developed for the investigation of paintings. The history of its application in conservation science starts in 2007, when a painting by Vincent van Gogh, Patch of Grass (1887), was taken to the DESY Photon Science lab in Hamburg, site of a large particle accelerator. Back then, the researchers scanned an area, 15 by 15 centimetres in size, over a period of three days, producing elemental distribution maps that enabled them to visualise an underlying portrait, on top of which Patch of Grass was painted.1 The use of this technology was revolutionary at the time and received a lot of media attention. It inspired the development of mobile macro-XRF scanners, which made it possible to chemically image an entire painting directly in situ, in the museum’s gallery or conservation studio. The Bruker M6 Jetstream is the first commercially available mobile macro-XRF scanner. Launched in 2012, it has since found its way into museum conservation studios worldwide.2 How does the technique work? Like X-radiography, MA-XRF also works with an X-ray source. The X-rays can penetrate all paint layers of the painting and expel electrons from the inner shells of the material’s atoms. This brings the atoms in an excited state. To stabilise the atoms, electrons from the outer shells fill the vacancies created in the inner shells, in a process that is accompanied by the Opening a Window What Macroscopic X-ray Fluorescence Imaging (MA-XRF) Reveals about the Materials, Condition and Genesis of Vermeer’s Girl Reading a Letter at an Open Window Anna Krekeler Annelies van Loon Ige Verslype Fig. 1 MA-XRF scanning set-up of Johannes Vermeer’s Girl Reading a Letter at an Open Window, August 2017
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