More on the data

For this research non-invasive, non-destructive techniques that do not require sampling from the prints were used to analyze the colorants. The analytical techniques used are X-ray Fluorescence (XRF), Excitation Emission Matrix (EEM), and Fiber Optic Reflectance (FORS) spectroscopies. Colorant samples were also formulated and printed using organic and inorganic materials traditionally believed to have been used during the Edo period (1603–1868). These were used to cross reference the analytical results. These methods, in conjunction with visual inspection, were then used to identify the colorants.

Analytical Methods

What is XRF?

X-ray fluorescence (XRF) is a nondestructive analytical method used to qualitatively and semi-quantitatively determine the elemental content of materials. The x-ray spectrum emitted from each element has a unique set of energies that are related to the type and number of atoms present in the sample. For this research, ARTAX open-architecture instrument was used.

XRF is used to identify inorganic colorants based on their inorganic element composition. For example, orpiment can be identified using XRF because it can detect the arsenic in orpiment, which is an arsenic trisulfide. In combination with visual inspection (yellow color) it can be identified as orpiment because it is the only known yellow colorant with arsenic.

Since most XRF cannot detect atomic elements below calcium (i.e. all elements with a smaller atomic number than calcium on the periodic table), it cannot aid in the identification of organic colorants.

What is EEM?

Excitation Emission Matrix (EEM) is a spectroscopy method that results in a 3D pattern of the fluorescence of an organic material. The pattern is produced by plotting the intensity of a sample’s fluorescence emissions occurring as a series of stepped excitation wavelengths. Each material has a reproducible 3D fluorescence pattern that allows its identification. Even the lack of fluorescence, which indicates the material is non-fluorescent, also provides a key piece of information for identification of a colorant. The reference colorant samples were printed to create a set of baseline patterns for each colorant.

EEM is used to identify some organic yellow and red colorants. For example, turmeric has a unique fluorescence or EEM pattern, making it an easily identifiable colorant using EEM.

What is FORS?

Fiber Optic Reflectance spectroscopy (FORS) is a non-invasive technique that can be used for the examination of color as well as for the identification of colorants. FORS uses a fiber optic probe and a pulsed xenon source to provide good sensitivity, examine small areas, and minimize the interference from ambient light.

Particularly of interest in this study, FORS is able to distinguish between the three common blue colorants used in Japanese woodblock prints: dayflower, indigo, and Prussian blue. This can be accomplished, not only in blue printed areas, but also in greens and purples, which are a combination of a blue and another colorant (yellow, red).

Japanese terms for color and materials

Throughout history, many of the colors and materials have had multiple names and still may have multiple names associated with them. For this database, we have tried to use the most common currently-used name for the raw material used to produce the color. For example, orpiment (石黄 sekiō) is also commonly called yūō (雄黄), which is actually the Chinese word for realgar. The Chinese term for orpiment, 雌黄 (shiō in Japanese) has been used to describe orpiment and also gamboge.

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