Arsenic sulfides, ranging from bright yellow to orange in color, have been used as pigments since antiquity [1, 2]. The two arsenic sulfides most commonly identified in works of art are orpiment (As2S3) and realgar (α-AsS or As4S4), a yellow and an orange/red pigment, respectively. Orpiment in particular was highly desirable as a pigment because of its bright, warm, yellow color, mimicking that of gold. Despite its striking visual appearance, the micaceous, flaky character of orpiment makes it challenging to grind for use as a pigment. The first reference in Western Europe to orpiment as an artificially produced pigment is given by Cennino Cennini, and the production process was first described in fifteenth century manuscripts [3,4,5]. Artificial orpiment was obtained by the dry process method, whereby natural orpiment was heated and recrystallized by sublimation, with or without the addition of sulfur. Arsenical ore and sulfur could also be used as starting materials. Artificial orpiment could also be synthesized by reacting H2S gas with a hydrochloric acid solution of As2O3 and collecting the resultant yellow precipitate: this preparation is known as the wet process method [1, 2]. Dry process artificial orpiment is typically characterized by smaller and more regular particle size compared to the natural mineral, as well as by the presence of colorless As2O3 particles. The wet process results in round and regular particles of finer size than the dry process, rarely used as a pigment [1, 2]. Grundmann and coworkers suggest classifying artificial orpiment obtained from heating as orpiment glass (g-As2S3) and any sublimation products as arsenic sulfide glass (g-AsxSx) [6]. For ease of discussion, this work will refer to any dry process orpiment as artificial arsenic sulfide, unless when citing historical sources explicitly using the name artificial orpiment.
Visual observation cannot accurately distinguish between natural and artificial arsenic sulfides. Polarized light microscopy (PLM), X-ray diffraction (XRD) and Raman microspectroscopy have been used to differentiate various classes of dry process artificial arsenic sulfides [2, 6, 7]. Recent work by Vermeulen et al. used PLM, Raman and scanning electron microscopy with energy dispersive spectrometry (SEM–EDS) to identify an artificial arsenic sulfide pigment in nineteenth century painted decorative panels in the Japanese tower in Laeken, Belgium [8]. The minimally invasive nature and high chemical sensitivity of analytical techniques such as XRF, Raman, and SEM–EDS makes them highly suitable as well for characterization of arsenic sulfide pigments in woodblock prints.
Large mineral deposits of orpiment and realgar can be found in Asia (particularly in Southeastern China), making natural arsenic sulfides a common painting material in Chinese art starting in the fifth and sixth centuries [1, 9,10,11,12]. Despite the longstanding use of orpiment in Chinese painting, orpiment does not appear in Japanese paintings before the mid-seventeenth century, becoming more common by the nineteenth century [13, 14]. Previously published studies discuss a shift in the usage of natural to artificial arsenic sulfides as a yellow pigment during the late Edo period, but do not define an exact time period of this shift [15, 16]. Recent work from Luo et al. demonstrates the exclusive use of artificial arsenic sulfide as a yellow pigment for Japanese woodblock prints dated between 1864 and 1895 [17]. The authors cite the earliest known reference to artificial arsenic sulfide production in Japan, dating to 1846, where Takamatsu describes heating arsenical ore with sulfur to produce a yellow pigment [18]. Prior to 1846, the source and production of artificial arsenic sulfide pigments and their use in Japanese woodblock prints has not been well studied.
In this study, we expand upon the work of Luo et al. by characterizing arsenic sulfide pigments used in Japanese woodblock prints dated to 1833–1834. The prints studied herein are from a thematic series designed by Katsushika Hokusai titled A Tour of the Waterfalls in Various Provinces (Shokoku Taki Meguri). Hokusai’s landscapes from the Tempyō period (1830–1844) are revered for their unique rendering of natural scenes and dominant blue tones. The onset of the “blue revolution” during this time, primarily due to the introduction of Prussian blue in Japan, saw a shift in the color palette used, from soft tones to striking blues, greens and yellows. The collection of Japanese woodblock prints at The Metropolitan Museum of Art consists of approximately 255 works by Hokusai and his followers [19, 20]. Among them are two complete sets of the of the eight-print Waterfalls series (sixteen prints in total). Several examples from each set were published by Keyes and Morse in a study describing how the original print series was copied in the late Edo period or early Meiji period from entirely redrawn blocks [21]. The two sets are distinguished by slightly different color palettes and by several minor variations in drawing. In keeping with Keyes and Morse’s notation we will henceforth refer to the original set as Set A and the reproduction as Set B.
A multi-technique analytical approach, including XRF, Raman, and SEM–EDS analysis, was used to characterize the arsenic sulfide pigments used in the prints. Our study showed that natural orpiment was used in Set A and the artificial pigment (with one notable occurrence of natural orpiment) in Set B. Additionally, a characteristic distribution for the co-occurrence of specific blue pigments with natural orpiment or the artificial arsenic sulfide to obtain a green color was seen for each print set. Prints in Set A consistently feature Prussian blue and indigo mixed with natural orpiment whereas in prints from Set B artificial arsenic sulfide is mixed with Prussian blue, indigo, and ultramarine blue for the green. Our experimental results, along with historic documentation describing the production and sources of arsenic pigments, provide further understanding of arsenic sulfide pigments use in Japanese woodblock printing during the late Edo period.