Some of the pigments used can be recognized by imaging the painting under a range of energy wavelengths, including visible, infrared, and ultraviolet radiation.
Detail in visible light
False-color infrared image
The lower image was taken with an infrared sensitive camera and the default RBG color channels were reprogrammed using software to produce the false-color infrared image. Areas painted in red change to yellow in the false-color image. This color shift is characteristic of certain pigments, including cadmium red and vermilion, suggesting that one of these two red pigments was used.
Detail in visible light
Ultraviolet image
Viewed under ultraviolet radiation, areas that are painted white, pink, and lavender display pink fluorescence that may be due to the colorants. Ultraviolet imaging suggests that Indian Yellow was not used, because its characteristic bright yellow fluorescence is not seen.
Students using portable x-ray fluorescence spectrometer
Using X-Ray Fluorescence Spectroscopy (XRF) students were able to determine the elemental composition of many of the colorants in the paints. XRF uses x-rays to displace electrons, releasing energy as fluorescence that can be detected and measured. This fluorescence is unique to the elements present. Recorded on a spectrum, the peaks of fluorescence represent the energy (along the x-axis) and relative abundance (along the y-axis) of the elements present.
XRF spectrum of yellow paint
Compared with the background (blue spectrum), the yellow paint (red spectrum) contains chromium (Cr) and lead (Pb), indicating the pigment chrome yellow (PbCrO4). The white paint contains zinc (Zn) and is likely zinc oxide. Zinc was found in the background areas and appears in all of the spectra. Mercury and sulfur were detected in the red, confirming the presence of vermilion (mercuric sulfide).
The pigments used help us to date the painting. Chrome yellow became available after 1818, and zinc oxide white was commercialized around 1845. Vermilion was widely used as a red pigment until the early 20th century, when cadmium red became available. Therefore, based on the pigments present, this painting likely dates to the late 19th or early 20th century. Art historians also date this painting to the late 19th century.
