A blue can conceal another! Noninvasive multispectroscopic analyses of mixtures of indigo and Prussian blue

Abstract

Blue colors were sparsely used in the first colored Japanese ukiyo-e prints but became predominant during the 19th century, mainly due to the integration of the synthetic Prussian blue in the palette of the printers around 1830. For a long time, researchers have assumed that the traditional Japanese organic blue colorants such as indigo were substituted by cheaper Prussian blue. Some analytical studies conducted on such artworks showed evidence of the common use of indigo and Prussian blue in Japanese paintings, alone or mixed together. Recent measurements carried out on bluish areas of an ukiyo-e designed by Utamaro showed the simultaneous use of the two pigments. However, if visible reflectance spectroscopy suggested the single use of Indigo, Fourier transform infrared spectroscopy (FTIR) also indicated the presence of Prussian blue.These results question about the possible identification of all the pigments of a colored mixture by using a single analytical technique. Without using FTIR spectroscopy, the Prussian blue would not have been detected, whereas on infrared spectra the expected specific bands ascribed to indigo are not identified. The objective of this work is to investigate mixtures of pigments, using noninvasive spectroscopic techniques in order to assess their limits of detection. Fluorimetry, hyperspectral imaging, and infrared spectroscopies have been performed on three color charts made of indigo, Prussian blue, and mixtures of them at different proportions of matter. The results emphasize the systematic identification of Prussian blue thanks to infrared spectroscopy, whereas the identification of indigo, mixed with Prussian blue, appears to be more challenging. K E Y W O R D Sblue pigments, fluorescence, hyperspectral imaging, infrared spectroscopies, Japanese ukiyo-e prints

Multi-Modal, Non-Invasive Investigation of Modern Colorants on Three Early Modern Prints by Maria Sibylla Merian

“…The analysis of hyperspectral data suggests the use of Prussian blue in plate 9. The characteristic spectral curve in blue regions (Figure 9), in particular the local minimum around 675 nm and the inflection point around 554 nm, are consistent with characteristic spectra for Prussian blue published in other identifications of Prussian blue [43][44][45]. Prussian blue did not become commercially available until 1724, seven years after Merian's death, confirming that the coloring could not have been applied during Merian's lifetime [29].…”

Beyond the connoisseurship approach: creating a chronology in Hokusai prints using non-invasive techniques and multivariate data analysis

“…Previous studies of these prints using fiber optic reflectance spectroscopy (FORS) highlight the use of Prussian blue/indigo mixtures in the blue outlines [9][10][11]. As demonstrated in these studies [9,12], FORS is well suited to differentiate Prussian blue from indigo as the two colorants show different reflection spectra. Indigo shows a maximum absorption around 660 nm and reflects most of the light in the 700-950 nm range, while Prussian blue absorbs most of the radiations in both the visible and near infrared ranges.…”

“…Indigo shows a maximum absorption around 660 nm and reflects most of the light in the 700-950 nm range, while Prussian blue absorbs most of the radiations in both the visible and near infrared ranges. When mixed, the contributions of both colorants is observed as follow: a maximum absorption at around 660 nm and an increased absorption in the 700-950 nm range compared to pure indigo, due to the absorption of the Prussian blue in this range [9,12].…”

Integrating liquid chromatography mass spectrometry into an analytical protocol for the identification of organic colorants in Japanese woodblock prints

“…Imaging techniques are well-suited for the study of Japanese prints due to their planarity and their relatively simple pigments and colorants composition, which are often applied in one single layer and, regularly, not mixed 16 . As a result, multispectral imaging (MSI) has been applied several times to the study of Japanese woodblock prints 6,14,[17][18][19] . Combined with spectroscopic single point analytical techniques, this approach generally produces reliable results in terms of identification of mineral pigments 4,5,14,15,17 and, to some extent, organic colorants 14,[19][20][21] .…”