Reddish ochers primarily contain hematite, Fe 2O 3, whereas goethite, α-FeOOH, is the main Fe-bearing phase in yellowish ochers ( p 70). The hue and intensity of its color result from its mineralogical composition, especially its contents of iron oxides and hydroxides. Geologically, natural red chalk can be classified as a type of ocher ( p 99). FORS presents an interesting option because it allows rapid measurements, is portable, in our setup was contact-free, and could be used for the comparison of the Karlsruhe drawings with authenticated drawings by Piranesi in other collections.įull size image Factors influencing color variation in red chalk These variations are visually differentiable, and can be photographically documented, but those methods are insufficient for the systematic color-oriented comparison of a large number of drawings in numerous collections located far apart from each other. Smudging, whether intentionally performed, or due to wear, lightens the color a darker color and a smoother line are produced as a result of contact with oiled paper when tracing a design, or by the moistening and pressing of a drawing during counterproofing, as has been shown in a study of the 18th-century artist Daniel Chodowiecki (1726–1801). Of special interest in FORS analysis are color variations that result from the compositional differences of the chalks and the additional color variations that result from the artists’ application techniques. This makes the drawings an ideal group for the study of variations in color, especially since they were kept undisturbed in albums since the early nineteenth century. In the Piranesi workshop, where several different draftsmen were employed and closely cooperated, this wide range of red-chalk tonalities appears in a narrowly defined chronological and geographical context. 1), ranging from the bright orange prevailing in many 16th-century drawings to the dark reddish tones of the eighteenth century, found especially in French drawings, as already identified by Meder in 1919 ( p 121). The Karlsruhe drawings illustrate the rich variety of red chalk tonalities (Fig. It was a medium for masters in particular, since the tendency of its fine particles to adhere to the fibers of the paper surface (, p 110) precluded erasure and correction. Red chalk has a high and varied color intensity and is exceptionally useful in the etcher’s workshop for copying and transferring drawings mechanically or via counterproofing. Since this was one of the most versatile drawing mediums used by 18th-century artists, draftsmen, and etchers alike ( p 128, ), it is not surprising that it was favored by Piranesi and his circle. Among the drawings are many that were executed in natural red chalk. These drawings have been reattributed to the Roman workshop of the Italian architect, etcher, and dealer in antiquities Giovanni Battista Piranesi (1720–1778) in 2014. Our work was carried out in the context of a research project funded by the German Research Foundation (DFG), in which three international partners are studying a group of about 300 drawings at the Staatliche Kunsthalle Karlsruhe (see Funding and Acknowledgements). In this study, we examine the viability of fiber optics reflectance spectroscopy (FORS) as a method for differentiating red chalk drawing media on paper. These similarities distinguished the historical drawings from the red chalk mock-up drawings, except for the drawings made with red chalk samples from the area near the town of Theley, Germany, which were shown to bear close similarities to those in the cluster of historical samples. Subsequent FORS analysis of selected original drawings revealed the existence of several closely grouped clusters, implying similarities on the basis of the underlying spectral features among the historical red chalks used in Rome. The compositional differences of these reference chalks were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was possible to sort these mock-up drawings according to chalk type and application technique. The method was tested on mock-up drawings made with recently acquired natural and synthetic red chalks of known origin. The evaluation of spectra was supported by principal component analysis (PCA). The viability of fiber optics reflectance spectroscopy (FORS) for the differentiation of red chalk drawing media was investigated, focusing on the group of drawings from the workshop of Giovanni Battista Piranesi (1720–1778) at the Staatliche Kunsthalle Karlsruhe, Germany.
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