Synthetic organic pigments are colorants that have been manufactured starting in the latter half of the 19th century. These pigments belong to different chemical classes, and hundreds of them have been introduced at different time periods in the last 150 years. Identification of them in works of art would help with issues of display, conservation and attribution. The collaboration between the National Gallery of Art and the United States Naval Academy began with undergraduate research students characterizing a collection of reference pigments by Raman spectroscopy and laser desorption ionization mass spectrometry (LDI-MS). Occasional samples from works of art were also identified. Historical pigments are synthetic organic pigments that have been manufactured and subsequently removed from manufacture. They often have no modern analytical data. Thus if they are suspected to be present in a work of art, it would be difficult if not impossible to identify them. Without their identity, the art history of the object would be less comprehensive and deep, and display and conservation decisions would be less informed. For example, we identified PO2 in a painting by Mark Rothko (Red Band, 1955, NGA Accession No. 1986.43.155) by tracking down an archived historical sample and comparing the Raman and LDI-MS spectra of the two samples (J, of Raman Spectroscopy (2014), 45(6), 448-455). This indicated to us that the synthesis of these historical pigments would be of benefit to the conservation community. We have targeted a series of historical Napthol AS (including PR7, PR10, PR11, PR18, PR19) pigments, Hansa Yellow (including PY49, PY98, PY203) and Diarylide Yellow (including PY106, PY113 (Fig. 1), PY114, PY124 and PO15) pigments. Synthesis from commercially available chemicals will be described. Analysis of the product pigments by FT-IR, Raman, and mass spectrometry (GC/MS, MALDI) will be presented as appropriate. The utility of this method to assist in art conservation and provide opportunities in undergraduate research will be highlighted.

Historical azo pigments: Synthesis and characterization

254th National Meeting (2017)

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