55 0.15 millimetres thick,19 which did not, however, completely even out the structure of the canvas and was probably applied as a single layer. The elemental composition of the ground layer, as revealed from nine analyses using scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX) [samples Dr1, Dr3, Dr4, H7-Q, H8-Q, H9-Q, H10-Q, H14-Q, H20-Q] indicates lead and calcium which subsequent Fourier-transform infrared spectroscopy (FTIR) showed to be a mixture of lead white with calcium carbonate (chalk), [samples H7-Q, H22-S] (see fig. 7). Polarised light microscopy (PLM) revealed that the ground layer [sample H22-S] shows a uniform particle size below approximately 5 micrometres (µm), both for chalk (no microfossils were detected) and lead white. The lead white is mostly aggregated in lumps of up to approximately 20 micrometres in diameter (see fig. 6 c). A mixture of lead white and chalk in a ratio of 1:1 is already mentioned in 1620 by Theodore Turquet de Mayerne as “ceruse” or “cerusa”.20 De Mayerne also describes a mixture of the same substances at a ratio of 2:1.21 The observed light grey-beige hue of the ground layer was achieved by an admixture of earth pigments, as indicated by minor amounts of aluminium and silicon (from clay minerals) as well as iron and traces of titanium. These elements could be detected in cross-sections by SEM-EDX [samples H10-Q, H14-Q, H20-Q]. Iron oxide-hydroxide and needle-shaped silicates observed under PLM [sample H22-S] support this conclusion Minor traces of manganese detected by SEM-EDX in the cross-sections [samples H14-Q, H20-Q] indicate a little umber, which is confirmed from very few particles of pyrolusite (manganese dioxide) under the polarising microscope [sample H22-S] (fig. 7). Underdrawing Traces of the first steps of the creative process are seldom evident in Vermeer’s pictures. In keeping with the 17th-century concept of inventio, these preparatory steps involved drawing, usually directly on the canvas, to work out fundamental decisions about the compositional design and probably also the distribution of light and shadow. In discussing Vermeer’s painterly process, scholars often look to his The Art of Painting (c. 1666–68, Vienna, Kunsthistorisches Museum) which shows an artist sitting at his easel working on a painting. In the early laying-out stage of that depicted work, we see the lines of an underdrawing executed in white. However, we should be wary of jumping to the conclusion that Vermeer executed his own underdrawings in white chalk. At least in the case of Girl Reading a Letter this scenario is highly unlikely, since white chalk marks would have been barely visible on the light-grey ground and thus of little use to him. In Vermeer’s Girl with a Pearl Earring (c. 1665–67, Den Haag, Mauritshuis) black outlines applied in short strokes with a fine brush were detected by multispectral infrared reflectography (MS-IRR).22 A similar underdrawing in black could not, however, be detected in the painting Girl Reading a Letter. Even if there is no evidence of underdrawings or similar marks or notations to assist in a preparatory design, one of the first compositional decisions may have been the perspectival arrangement of objects in the pictorial space, in particular the window jamb. It was necessary to define the central main point, which denotes the horizontal axis between the artist’s eye and the back wall, which is assumed to be parallel to the picture plane.23 Unlike many of his other compositions, which the painter laid out with architectural exactitude, using needles pinned into the canvas and coloured threads to make perspectival constructions,24 the lines of recession in Girl Reading a Letter do not converge on a single point and instead form a row of points near to the left-hand edge of the green curtain. These main points divide the horizontal line approximately in the ratio 2:1.25 The perspectival construction and calculated spatial relationships between objects within the pictorial space26 are evidence of a precise planning of the pictorial design, for which a ruler and compass were more likely to have been used than a camera obscura. Fig. 7 Microscopic image (PLM, 1 polariser), sample H22-S from the upper edge of the image, primer: lead white, calcite (no coccoliths detectable), iron oxide hydroxide, silicate (needles), very little pyrolusite (center of image)
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