Evidence of early amorphous arsenic sulfide production and use in Edo period Japanese woodblock prints by Hokusai and Kunisada

This study explores the evolution of the manufacturing process of artificial arsenic sulfide pigments in Edo-period Japan through the analysis of three impressions of the same print dated from the 1830s and attributed to Katsushika Hokusai (1760–1849), and one from 1852 and attributed to Utagawa Kunisada (1786–1865). Colorants in the yellow and green areas of the four prints were investigated by means of non-invasive and microanalytical techniques such as optical microscopy, fiber optic reflectance spectroscopy and Raman spectroscopy. While the pigments in the green and yellow areas are similar throughout the set of prints—Prussian blue, indigo (for the Hokusai prints) and orpiment were identified—optical microscopy and Raman spectroscopy highlighted some variations in the orpiment used in the green areas of the prints. Two of the Hokusai prints present bright yellow particles of larger size and lamellar morphology, identified by Raman spectroscopy as natural orpiment. The third print presents an admixture of bright yellow natural orpiment particles with a smaller number of orange-yellow particles shown by Raman to be partially amorphous arsenic sulfide. Small bright yellow particles identified as fully amorphous arsenic sulfide pigments by Raman were found throughout the green areas of the Kunisada print. Although supported by Japanese historical sources, local production of artificial arsenic sulfide in the early nineteenth century was not previously documented. The simultaneous presence of both crystalline and amorphous domains in a single pigment particle in some of the Hokusai prints suggests that natural orpiment was used as primary source of arsenic for the production of a low grade artificial pigment. The pigment found in the Kunisada print, by contrast, was obtained from arsenic oxide (or arsenolite) and sulfur though a dry-process synthesis, as shown by the sulfur excess, signs of heat treatments and fully amorphous nature of the pigment. These findings set the earliest dates for both the ore sublimation process and the arsenolite dry process, and are of foremost importance to understand the evolution of the amorphous arsenic sulfide production in Edo-period Japan and its introduction in the palette of Japanese woodblock prints.