Experimentation in Manufacturing Zinc Orange Pigment
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Experimentation in Manufacturing Zinc Orange Pigment Kathryn Harada1, Dr. Aaron Shuger1, and Dr. Rebecca Ploeger1 1 Art Conservation Department, State University of New York at Buffalo, 1300 Elmwood Avenue Buffalo, New York 14222 U.S.A. ABSTRACT Zinc orange is a rare and relatively undocumented pigment. Mention of this pigment in the literature indicated its short lived availability to artists in the nineteenth century and that it is unstable, but does not address any specific issues related to its degradation. The synthesis of the original pigment is discussed. In addition, a full characterization of the pigment is presented including XRF, FTIR, Raman and optical microscopy. Research into its degradation mechanisms was also undertaken, and the light fastness of the pigment is presented. INTRODUCTION Zinc orange is a rare and generally undocumented artists’ pigment. Very little information exists in the literature about this pigment; only one sentence is devoted to it in George Field’s 1869 Chromatography which states: “…when hydrochloric acid and zinc are made to act on nitro-prusside of sodium, a corresponding zinc compound is formed of a deep orange colour, slightly soluble in water, and not permanent” [1]. This statement is echoed in the Pigment Compendium [2]. This project began when two orange paint samples retrieved from the family home of Henri de Toulouse-Lautrec in France. Samples were taken from paint tubes labeled orangé de zinc, manufactured by the Parisian color merchant, Dubus (1877-1897). Over the course of this project, the samples were characterized in an attempt to understand how to synthesize zinc orange, producing some unexpected results. X-ray fluorescence analysis showed evidence of zinc in these pigments, however, pyrolysis gas chromatography mass spectrometry (py-GC-MS) analysis detected triphenylmethane, a base structure of many synthetic dyes [3]. These dyes were first produced in the late 19th century and are generally quite fugitive, especially when exposed to light [4]. Findings show that the samples obtained from Toulouse-Lautrec are not “true” zinc orange as described by Field’s Chromatography, but a substitute, where the manufacture maintained the original name. The pigment manufacture was probably knew about the permanence of true zinc orange and substituted a pigment of similar color, unfortunately, using a dye that was just as fugitive. While the Toulouse-Lautrec samples formed the initial inspiration for this study, the main focus of this project was to investigate how to manufacture zinc orange, as described by Field’s Chromatography [1]. Four ratios of the listed ingredients; metallic zinc, hydrochloric acid, and sodium nitroprusside were attempted to synthesize the pigment. Several processing methods were tested, including varying heating, grinding, and washing. The pigments were tested using artificial ageing to test the light stability of the pigment.
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