80 emission of secondary or fluorescent X-rays with a fixed energy that is characteristic of the respective element (fig. 2). By detecting these fluorescent X-rays, the elements in a material can be identified. It must be noted that MA-XRF imaging can only detect those elements which have an atomic number (Z) greater than or equal to that of phosphorus (Z ≥ 15). The MA-XRF scanner consists of a motorised x-y stage onto which a measuring head is mounted, containing an X-ray source and one or more detectors. The painting under investigation is unframed and secured on an easel in front of the scanner. While ensuring that no actual contact is made, the scanner is positioned close to the surface of the painting, at a distance of approximately 1 centimetre between the measuring head and the surface. By slowly moving the measuring head on the x-y stage, the painting is then scanned ‘line by line’ at a constant speed (fig. 1). The detector can read out the energy lines for each chemical element for each measuring point on the painting. As output, it generates distribution maps of the different elements, usually displayed as grey-scale images (see MA-XRF maps in the atlas). The light areas of the maps represent areas in which the element is present in relatively high abundance. Interpretation of MA-XRF elemental maps The historic pigments typically consist of certain characteristic chemical elements. If the scanner detects lead, for instance, this usually indicates the pigment lead white, a lead carbonate: the principal white pigment of Vermeer’s time (see MA-XRF, Pb maps in the atlas). Meanwhile, the presence of mercury indicates the use of vermilion, a bright-red mercury sulphide, in the case of Girl Reading a Letter used in the carpet, red curtain, and the figure’s lips and cheek (see MA-XRF, Hg and S maps in the atlas). Some pigments are made up of several detectable characteristic elements. The pigment smalt, for example, is a ground potash-silica glass coloured blue with cobalt. Apart from the cobalt signal, MA-XRF can pick up signals from nickel, arsenic, and bismuth associated with the cobalt ore (see MA-XRF, Co, Ni, and As maps in the atlas). Even if the pigment is heavily degraded, as is the case in some passages of the carpet, cobalt and the other associated elements can still be detected. The combination of lead and tin points to lead-tin yellow, an oxide of lead and tin (also called lead stannate). Here it is used in, for example, the yellow jacket and fruit bowl (see MA-XRF, Pb and Sn maps in the atlas). The black piping and trim of the young woman’s jacket and black skirt show up in both the maps of calcium and phosphorus, its co-presence is characteristic of bone (or ivory) black (see MA-XRF, Ca and P maps in the atlas). Thus, based on the identified chemical elements, conclusions can be drawn about which pigments were used and where they occur in the painting. Table 1 gives an overview of the pigments that were identified in Girl Reading a Letter. In some cases, MA-XRF can detect pigments that originate from ‘hidden’ paint layers and visualise forms that were either discarded by the artist during the painting process or covered by overpaint later in the painting’s history. Both of which are the case in Girl Reading a Letter. Although MA-XRF scanning can provide a wealth of information, the interpretation of the elemental distribution maps can be challenging, and the technology has its limitations. For example, it is not capable of mapping organic (carbon-based) pigments, such as charcoal black or the blue colourant indigo3 (identified in other paintings by Vermeer),4 since the elements they contain are too Fig. 2 Diagram showing process of X-ray fluorescence excitation and emission inside atom
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