The imaging experiments on the painting Girl with the Pearl Earring were carried out using a laboratory scanning MA-XRPD/MA-XRF scanner, both in reflection and in transmission mode. In transmission mode, a high-brilliance low-power x-ray microsource (44 W; IμS-AgHB, Incoatec GmbH, Germany) was used to generate a monochromatic Ag-Kα (22.16 keV) x-ray beam with a photon flux of 1.1 × 107 photons s−1; a focal diameter of 112 (3) μm, a focal distance of 22 (1) cm, and a divergence of 3.8 (3) mrad. Diffraction signals were recorded using a 2D diffraction detector (PILATUS 200K, DECTRIS Ltd., Germany). The detector was placed perpendicular to the x-ray source with the painting in between. The distance between the x-ray source and the PILATUS detector was set at 22 cm to attain the best possible angular resolution. The distance between the painting and the detector was set at 11 cm, ensuring a sufficiently wide coverage of the angular range. At a distance of 11 cm between the x-ray source and the painting, a beam diameter of approximately 0.4 mm was obtained. Calibration of the setup was performed with a hydrocerussite paint layer on a mock-up panel. The diffraction data collected in transmission mode were corrected for attenuation effects.

In reflection mode, a monochromatic Cu-Kα (8.04 keV) x-ray source was used. Because of geometrical limitations dictated by the dimensions of the x-ray source, it is undesirable to use an incident angle lower than 10°. The lower primary energy of the Cu source causes diffraction signals to shift toward higher 2θ angles. Low-energy x-rays were also more easily attenuated by the sample, rendering the scanner more sensitive for superficial phenomena. This x-ray source generates a photon flux of 2.9 × 108, has a focal diameter of 142 (2) μm, a focal distance of 20 (1) cm, and a divergence of 2.4 (1) mrad. The painting was placed at a distance of 20 cm from the x-ray source, while the distance between the painting and area detector was <0.5 cm. An incident angle of 10° was chosen to maximize the sensitivity for superficial compounds, leading to a beam with an elliptical footprint of approximately 1 × 0.2 mm2. The PILATUS 200K detector was positioned on the same side of the painting as the x-ray source and at an angle of 40° with the painting surface. To account for topographical variations and curvatures on the painting surface, a laser distance sensor (Baumer Hold., CH) was used to automatically adjust the distance between the x-ray source and the painting for every point of the scan. Calibration of the setup was performed with a calcite paint layer on a mock-up panel.

In both geometries, x-ray fluorescence data can be simultaneously acquired by means of a Vortex silicon drift detector (Hitachi, Japan). A set of three motor stages (25 cm × 10 cm × 10 cm; Newport Corp., USA) are responsible for the movements during the scanning procedure. The x-ray source, PILATUS 200K detector, and Vortex detector were placed on a motorized platform, capable of moving the setup in the XY plane. The artwork was placed on a motorized easel, capable of moving the painting in the vertical (Z) direction.

The regions scanned on the painting in reflection and transmission modes are shown in Fig. 1A. In reflection mode, an area encompassing the entirety of the Girl’s face (200 mm × 224 mm) was scanned with an exposure time of 10 s point−1 and a step size of 2 mm × 2 mm, chosen larger than the actual beam size (1 mm × 0.2 mm) because of time constraints. In transmission mode, a smaller area (49.5 mm × 99.9 mm) was scanned with an exposure time of 10 s point−1 but with a reduced step size of 0.9 mm × 0.9 mm to attain a better spatial resolution.

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