REVEALING THE COLOR PALETTE OF AUSTRIAN 19th CENTURY WATERCOLOR PAINTERS – A NON-DESTRUCTIVE APPROACH BY USE OF COMPLEMENTARY FTIR-, UV/VIS/NIR- AND XRF-SPECTROSCOPY

IntroductionCompared to other paint techniques e.g. oil or tempera, watercolor paintings are characterized by thin, in many cases even translucent paint layers. Therefore, non-destructive examination of such objects requires analytical techniques with a high sensitivity, as it is the case for XRF and fiber optic reflection-UV/Vis/NIR spectroscopy. On the other hand, watercolor paintings are usually unvarnished and thus reflection-FTIR can be applied which only allows analysis of the uppermost layer of objects.From a large number of Austrian 19th century watercolor paintings in the Graphic Collection (Kupferstichkabinett) of the Academy of Fine Arts Vienna we analyzed 10 objects by Rudolf von Alt (1812-1905) and 16 objects by Moritz M. Daffinger (1790-1859) in order to identify the painting materials and to elucidate their paint techniques. The watercolor paintings by Rudolf von Alt showed landscapes, architecture and scenes from the sociallife whereas endemic flowers of the Austrian flora were the favorite theme in the last decade of Moritz M. Daffinger`s life. The paintings of both artists show very fine details and thus instruments with a high spatial resolution are required for their examination. Moreover, the historic literature1 shows that a variety of inorganic as well as organic pigments was available for artists in the 19th century. With regard to the intrinsic limitations of the methods applied only the combination of these complementary techniques enables a comprehensive analysis of the objects studied.

ExperimentalFor XRF and r-FTIR measurements the instruments were attached to a robust photographic tripod which allows motion in y- and z-directions whereas the paintings were fixed on an easel. In contrary r-UV/Vis/NIR measurements were performed by using a self built fiber optic measuring system for planar objects (Figures 1a and 1b). The paintings of Rudolf von Alt were not analyzed by using r-FTIR. Data processing was applied to r-FTIR spectra (Kramers-Kronig transformation, baseline correction) and r-UV/Vis/NIR spectra (1. derivative). The parameters of the utilized instruments are shown in Table 1:

Results and Discussion23 pigments were identified (Table 2). Arabic gum could be determined by r-FTIR as binding medium and protective colloid. As only the uppermost layer can be analyzed by r-FTIR, the identification of pigments from underlying layers was not possible when several paint layers were present. In contrary all layers including the support contributed to the measuring signal in case of XRF and r-UV/Vis/NIR. Identification failed in case of black and earth pigments due to the lack of characteristic measuring signals for all methods applied.The results suggested that the artists had different preferences especially concerning the use of blue and yellow pigments which might be connected with the different subjects of the paintings. The identification of cobalt blue, barium chromate and cochineal on the paintings shown in Figures 2, 3 and 4 demonstrates the complementarity of the methods as only the combination of element specific (XRF) and compound specific information (r-FTIR, r-UV/Vis/NIR) allowed to obtain reliable results.

ConclusionComplementary non-destructive XRF, r-FTIR and r-UV/Vis/NIR spectroscopy were used to analyze 19th century watercolor paintings. 23 inorganic and organic pigments could be identified at least by one of the utilized methods. On the other hand the results revealed several limitations of the methods as the identification of black pigments as well as a differentiation of earth pigments was not possible and thus the use of additional complementary non-destructive methods, e.g. fiber optic Raman or XRD would be highly desirable.

W. Vetter, M. Schreiner

Institute of Science and Technology in Art (ISTA), Academy of Fine Arts, Schillerplatz 3, 1010 Vienna, Austria

w.vetter@akbild.ac.at

Figure 1a: r-UV/Vis/NIR measurement of a watercolor painting by Moritz M. Daffinger using a self built fiber optic measuring system for large sized planar objects. It enables linear motion of the measuring head in x-, y- and z-directions.

Literature1. S. Tschelnitz: FARBEN-CHEMIE insbesondere der Oel- und Wasserfarben nach ihrem physikalischen Verhalten, ihrer

Darstellung und Verwendung, so wie ihren gewöhnlichen Verfälschungen. Carl Gerold´s Sohn, Wien [1857]2. V. Desnica, M. Schreiner: A LabVIEW-controlled portable x-ray fluorescence spectrometer for the analysis of art objects. X-

Ray Spectrometry 35 [2006] 280-286

Table 2: Pigments identified by XRF, r-FTIR and r-UV/Vis/NIR on watercolor paintings by Moritz M. Daffinger () and Rudolf von Alt (). and indicate that a reliable identification was not possible by use of the respective method.

Table 1: Parameters of the utilized instruments.

Despite of pale color application cobalt blue could be identified from the r-UV/Vis/NIR and XRF spectra below (measuring point – red line and cobalt blue reference – black line). Orpiment (As2S3) additionally was detected by XRFwhereas no yellow component was detected by r-UV/Vis/NIR.

Fig. 2: Watercolor painting by the Austrian painter Rudolf von Alt „Der Brückenturm auf der Kleinseite in Prag“, 1853, 33 x 26 cm2.

Fig. 3: Watercolor painting by the Austrian painter Moritz M. Daffinger - „Primula veris acaulis Linnaeus“, 33 x 26 cm2.

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Yellow barium chromate was identified by both XRF and r-FTIR (measuring point –red line, barium chromate reference – blue line). Additionally, Indian yellow (magne-sium euxanthate) was detected by r-FTIR(Indian yellow reference – black line) and zinc white by XRF.

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Fig. 4: Detail from “Sempervivum arachnoideumLinnaeus” by Moritz M. Daffinger.

Red cochineal was identified by both r-FTIR and r-UV/Vis/NIR(measuring point – red line, cochineal reference – black line). XRF analysis showed that no inorganic pigments were present.

Figure 1b: Front view of the fiber optic measuring head with 0°/45° geometry and laser assisted positioning (red lines).

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