Traditional colorants used in Japanese woodblock printing may be visually and chemically altered by exposure to light, pollutants, pH extremes, humidity, water, and other solvents. It is helpful, therefore, to identify colorants in Japanese prints when considering the potential impact of conservation treatment, storage, and exhibition on their long-term preservation.
Before Conservation Treatment
Torii Kiyonaga (1752-1815), Ouka No Nibijin
(Two Beauties under a Cherry Tree), 1782-3,
color woodblock print on Japanese paper,
Library of Congress FP 2 – JPD, no. 530.
Detail of the five ink colors present in the print.
Five colored inks, including black, were used to create “Two Beauties under a Cherry Tree” (Ouka no nibijin), printed by Torii Kiyonaga in 1782-83. The inks were identified by close visual inspection and non-destructive instrumental analysis. First, a stereomicroscope at high magnification was used to visually determine if the inks appeared to be composed of dyes, pigments, or a mixture of the two. When pigment particles were present, physical characteristics such as relative particle size, edge definition, surface reflectance (matte or shiny), and transparency or opacity were noted. These observations were compared to known samples of traditional Japanese colorants to make a tentative identification. For example, the green robe on the figure at the right is a mixture of blue and yellow pigments. The yellow pigment is semi-opaque, irregularly shaped, and — compared with the blue pigment particles, which were small enough to sink into the interstices of the paper — much larger and variable in size. The yellow pigment correlated well with known samples of orpiment, an arsenic sulfide pigment, and the blue appeared similar to indigo, a plant based dye.
Green colorant under five times magnification showing yellow and blue colorant particles.
Imaging under discrete bands of ultraviolet, visible, and infrared radiation using a multispectral digital camera was used to further characterize the colorants. Because each colorant is a different chemical compound, each selectively absorbs, fluoresces, transmits, or reflects characteristic wavelengths when exposed to specific bands of the electromagnetic spectrum. These characteristic responses are captured by the digital camera and are compared to the spectral responses of known colorant samples. Because different materials may have similar responses to all wavelengths and therefore cannot be readily distinguished by imaging alone, additional analytical tools are often used to solidify a preliminary identification.
Multispectral imaging showing differing spectral responses to discrete bands of radiation. Clockwise from Top Left: Visible (400-700 nm); Ultraviolet-induced visible fluorescence (300-400 nm); Ultraviolet reflectance (300-400 nm); False color ultraviolet; Near infrared 1 (700-950 nm); Near infrared 2 (1000-1100 nm); False color near infrared 1; False color near infrared 2.
Comparison of the spectral responses obtained from the print with known samples, supported by instrumental analyses provided by the Preservation Research and Testing Division, indicated that the printing inks are composed of:
Red lead: an orange-red mineral pigment known in Japan as tan. Red lead absorbs ultraviolet radiation (appears dark in the ultraviolet images), is transparent to infrared (disappears in the infrared images), and appears yellow in false-color infrared. The chemical compound of red lead, lead tetroxide, degrades to a black product, lead sulfide, upon exposure to sulfurous compounds. In false-color infrared, the black corrosion product, lead sulfide, appears dark. (While the presence of lead sulfide could be the result of exposure to sulfur dioxide, an omnipresent environmental pollutant produced by burning fossil fuels, Japanese printmakers sometimes deliberately induced conversion of red lead tetroxide to silver lead sulfide, which also degrades to a black color. For more information about both environmentally-induced and deliberate alteration of red lead in Japanese prints, see Conservation Treatment of a Japanese Pillar Print.) X-ray fluorescence (XRF) analysis confirmed the presence of lead in the red colorant of the Kiyonaga print and Raman spectroscopic analysis positively identified the colorant as red lead.
Safflower: a pink colorant extracted from safflower petals, known in Japan as beni. The most characteristic feature of beni is its bright orange-pink fluorescence when exposed to ultraviolet radiation. Beni fades quickly upon exposure to light and may be easily solubilized in water. XRF and Raman analyses indicated only that the pink colorant in the Kiyonaga print is organic. A definitive identification of the colorant would require further analysis. The presence of safflower was indicated primarily through its bright, orange-pink fluorescence in ultraviolet light.
Indigo: a dark blue plant-based dye precipitated onto an inorganic substrate such as a metallic salt, known in Japan as bero-ai. In the image by Kiyonaga, indigo was used to print the textile pattern in the kimono worn by the left figure and was mixed with yellow to create green, seen in the kimono worn by the right figure. The colorant has altered over time so that it appears a dull green-gray. The most characteristic feature of indigo is its bright pink appearance in false-color infrared. Indigo is sensitive to high pH. Raman spectroscopy positively identified the gray colorant in the Kiyonaga print as indigo.
Orpiment: an opaque mineral pigment composed of arsenic sulfide, known as shio. The spectral responses of the yellow colorant, used alone and mixed with blue to create green, was similar to known samples of both orpiment and gamboge. However, as noted during examination under the stereomicroscope, the physical characteristics of the pigment appeared more similar to orpiment than to gamboge, a colorant obtained from tree resin that is valued for its brilliant yellow hue and transparency. XRF confirmed the presence of arsenic and sulfur in the yellow colorant of the Kiyonaga print. Raman spectroscopy positively identified the colorant as orpiment.