More on the data

Visual inspection of each print was carried out under a stereobinocular microscope, as well as examination under ultraviolet illumination, followed by examination using one or more non-invasive analytical techniques. These techniques were used to were used to 'identify' the colorants based on their physical and chemical characteristics. Reference samples for each of the colorants as well as the mixtures were formulated and printed using organic and inorganic materials traditionally believed to have been used during the Edo period (1603–1868). It is important to note that this database contains our hypothetical identifications of the colorants for locations within each given print, but strictly speaking, we cannot provide certain identifications since no samples were removed. This database is meant to be a beginning point for continuing targeted research on the specific types of colorants found in Edo period Japanese prints.

Non-invasive and analytical procedures

• Stereobinoular microscopy at relatively low magnifications, is used to assess condition and provide information on paint or printed layers, larger pigments particles, etc.
• Elemental analysis by X-ray fluorescence spectroscopy (XRF) can be used to hypothesize the presence of certain inorganic pigments (for example, the detection of mercury suggests vermilion or cinnabar).
• Fiber-optics reflectance spectroscopy (FORS) can help to identify some colorants, particularly organic ones that XRF cannot detect.
• Excitation emission matrix fluorescence spectrophotometry (EEM) can often identify organic compounds that fluoresce (such as turmeric and safflower) and suggest the identification of certain others.

XRF, FORS and EEM were applied at multiple points on all prints. Some prints were also examined with a hyperspectral camera, as described below.. There are other non-invasive techniques that are available, but were not routinely used in the research for this database, unless noted in individual object entries.

What is XRF?

XRF analysis using an open architecture Bruker ARTAX

XRF spectrum of yellow area exhibiting major Arsenic peak

X-ray fluorescence spectroscopy (XRF) is a nondestructive analytical method used to determine the elemental composition of materials. When excited by X-rays, each element emits secondary or fluorescent X-rays that have energies unique to the element. The XRF instrument used in this research, a Bruker Artax micro-XRF spectrometer, cannot detect elements lighter than silicon, thus organic colorants (which mainly consist of carbon, oxygen, and hydrogen) cannot be identified. For this database, XRF was used to tentatively identify inorganic colorants and metals based on the element(s) detected. While chemical elements can suggest specific pigments, from a scientific point of view other techniques not yet used in this study are required to identify pigments with certainty (such as Raman spectroscopy or X-ray diffraction). In the case of yellows that contain arsenic, orpiment is the most likely pigment, but there are two other possibilities. In the later part of the Edo Period, a synthetic pigment of the same chemical composition was manufactured, and this artificial pigment cannot be distinguished from the natural mineral by XRF alone.

For more info on XRF, please click here

What is FORS?

Ocean Optics FORS components for reflective analysis

FORS spectra blues

Fiber Optics Reflectance spectroscopy (FORS) measures the reflectance of a point on an object across the visible and near infrared region (about 400-1000 nm). FORS uses a fiber optic probe and a high intensity halogen light source to examine very small spots, minimizing the interference from ambient light. For this research, an Ocean Optics miniature spectrometer was used.

Reflectance spectra can distinguish between some colorants. For example, the three common blue colorants used in Japanese woodblock prints (dayflower, indigo, and Prussian blue) can be easily identified by FORS. Since the green colors were usually made from a combination of a blue and a yellow colorants, while purple colors were usually made from a combination of a blue and a red colorant, the blues in both of these mixtures can usually be determined by FORS analysis. FORS is not usually useful for identifying yellow colorants, but can sometimes be useful for the organic reds.

For more info on FORS, please click here]

What is EEM?

EEM analysis using an Agilent Cary Eclipse Spectrometer with a fiber optic probe

Excitation and emission maxima of the three red colorants

3D EEM plots of reds

Some organic colorants used in Japanese woodblock prints can be characterize based on their ability to fluoresce when exposed to ultraviolet (and even visible) illumination. Excitation emission matrix (EEM) fluorescence spectrophotometry measures emission (fluorescence) of a material (in the visible range) as a function of the wavelength to which the material is exposed (in the ultraviolet as well as visible range). The typical display of results is a color contour map with excitation wavelength on the y-axis and emission wavelength on the x-axis. These maps can allow for (at least tentative) identification of some materials if the contour maps are visibly different for each of the potential materials. Even the lack of fluorescence, which indicates the material is non-fluorescent, may provide a key piece of information for identification of certain colorants. For this research, an Agilent Cary Eclipse Spectrometer with a fiber optic probe was used.

The excitation and emission maxima of the red colorants, safflower, madder, and sappanwood are similar but not identical (see left). The emission maxima for safflower is in the yellow-orange region, madder is in the orange region, and sappanwood is in the orange-red region. (Viewing these colorants side by side under ultraviolet illumination would show the subtle differences in tone.) The relative intensity of the absorption or emission is indicated by the color with dark red representing highest intensity. The oblique bar on the plot is where the excitation and emission wavelengths are identical, thus the detector is saturated at these points.

Some of the yellows in ukiyo-e prints also fluoresce. For example, the strongly-fluorescing turmeric is easily identifiable by EEM. However, other organic yellow colorants such as Japanese pagoda tree (enju) and gardenia (kuchinashi), which belong to a general chemical class known as flavonoids, exhibit relatively little fluorescence, and while their presence can be hypothesized by EEM, they are impossible to differentiate from one another.
For more info on EEM, please click here

What is RIS?

While FORS measures reflectance at a spot, Reflected Image Spectroscopy (RIS) provides a reflectance map across a selected region, which could include an entire print. The RIS data consists of individual spectra recorded at adjoining points that are usually well under 0.1 mm in diameter. The instrument used here is a Nerios HERA visible/near infrared hyperspectral camera (measuring in the 400-1000 nm range). RIS data are typically evaluated by highlighting areas of the object that was scanned which show identical or very close to identical reflectance spectra. Thus, the data can be a means of indicating where a specific colorant is present over an entire print. For example, the images below show where different colors are distributed in one print.
For more info on RIS, please click here

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  RGB projection of hyperspectral data
  MFA# 11.30331

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  RIS points similar to yellow selection
  MFA# 11.30331

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  RIS points spectrally similar to blue 1 selection
  MFA# 11.30331

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  RIS points spectrally similar to blue 2 selection
  MFA# 11.30331

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  RIS points spectrally similar to pink selection (safflower)
  MFA# 11.30331

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  RIS points spectrally similar to red selection (madder)
  MFA# 11.30331

How to look at the results

The results presented in this database are based on the three (or sometimes four) methods mentioned described above. XRF was run on all data points, but FORS and EEM were utilized according to the visual color, as well as speculated (original) color if fading or other alteration is suspected to have occurred. For example, FORS was run where blue was speculated (i.e. blues, greens, purples, and faded variations), but would not have typically been run on yellows or reds. Also, at this time other analytical or photographic methods that can also aid in the identification of colorants have not been used. Therefore, the results are not all-encompassing and/or definitive.

Results

Shown below is an example of the tables that occur on eah of the print pages. These table include images of the analysis locations as well as a list of the techniques and their resultant interpretations.

Analysis point	Image	Method	Results
Pt 1		XRF	orpiment (EEM to be run)
Pt 2		XRF, EEM	inconclusive
Pt 3		XRF, EEM	orpiment, turmeric
Pt 4		XRF, EEM	safflower (FORS to be run)
Pt 5		XRF, FORS	indigo

Tabular information:

• Analysis point: Each point corresponds to its circle on the print image, which is where the data was collected using the specified analytical methods.
• Image: This is a 10x image collected by the XRF spectrometer that shows the region of analysis.
• Method: This indicates which analytical methods were used on the region of analysis.
• Results:
  -The colorants are interpreted from the collected data in conjunction with visual inspection. For example, if arsenic is detected in a yellow area, it is interpreted as orpiment (arsenic trisulfide).

-If more than one colorant is identified, they are listed in alphabetical order.

-"Inconclusive" indicates that the data collected was not conclusive enough to identify a colorant.

-The additional comment, for example in Pt 1 and Pt 4, indicates that the specified analytical method is scheduled to be conducted in the future. Additionally, Pt 4 is visually a purple color which suggests that it was most likely created from a red and blue colorant but only a red color is listed. This is because FORS which is used to identify blue colorants has not be run; hence the comment, (FORS to be run). If FORS is run in the future and a blue colorant is identified, the result will be updated, but if a blue colorant cannot be identified, the result will only reflect the identified colorant, safflower. For more detail, please see "faded colors" below.

Raw data

The below spectra and pattern are examples of the raw data collected by each analytical method. These data are interpreted by conservation scientists experienced in the particular analytical technique to identify the colorant. As such, only the interpreted results, not the raw data, are given for each print.

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  XRF spectrum of Orpiment
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  3D EEM pattern of Safflower
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  FORS spectrum of Indigo

What colors are difficult to identify?

• Faded colors are a challenge and at times not all colors can be fully detected. Although visual observation may give a clue to the colorant or colorants present, only the results from the analytical methods are given. Therefore, if only one colorant of a mixture/overprinting was detected although visually the color is identifiable as a mixture/overprinting, only the detected color is presented in the results. In most cases, XRF, EEM, and FORS still pick up faded colors, but all three techniques are restricted by their detection limits as well as the presence of any fillers and fluorescence in the paper itself.

• Flavonoids are also a challenge. This group of organic yellow colorants containing flavonoids cannot be reliably distinguished from one another with the three analytical methods used in this research. While FTIR reflection spectroscopy has been used to differentiate the flavonoids (see Bibliography) this analytical method was not often carried out for this research.
  Dayflower which can shift to a greenish blue, yellow, or tan, also has a similar EEM fluorescence to the flavonoids once it discolors. Luckily, most times the presence of dayflower can be visually distinguished from the other flavonoids and/or FORS analysis can still detect its presence.

• Red lead/lead white is a combination which is often listed, based on the XRF identification of the element lead and the visual coloration of 'pink' in the region. Further studies using reflected FTIR or Raman would be needed to better characterize this mixture.

• Carbon black cannot de easily detected by the three non-invasive techniques used above. Infrared reflected light is most commonly used, but this examination was not always implemented. Therefore, carbon black used as a color (black, gray, or blue gray) or mixed with another color is not identified in this database.

• Orpiment can have different structural forms due to the way it is prepared; directly by grinding the mineral orpiment (natural orpiment) or by heat treating to create a mixture of crystalline and amorphorus form (processed orpiment), and synthetic orpiment. The mineral orpiment and synthetic orpiment both have been found in ukiyo-e prints (see Bibliography). The identification of these different structural forms of orpiment requires the use of Raman spectroscopy which was not often used for this research.

• Mica While XRF spectroscopy along with visual inspection is used to determine the presence of mica.

• Gamboge has been listed when a yellow colored region is non-fluorescent and its XRF spectrum does not show the presence of arsenic (indicating orpiment) or iron (indicating iron oxide yellow). In some cases, inspection of the region clearly showed a shiny resinous appearance that implicated gamboge, but this was not always the case.

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 kiō. It has also been called yūō (雄黄) which is actually the Chinese word for realgar, while the Chinese term for orpiment, 雌黄 (shiō in Japanese) has been also used to describe orpiment as well as gamboge.

Ukiyo-e Print Colorant Database