pigmented colorants: dependence on media and time - Cornell ...

light exposure. This effect is especially noticeable in the model’s left sleeve. The
sequence of images shows a portrait exposed to increasing levels of accelerated
illumination. Five concentrations of Alizarin Crimson red were applied in glazes
to depict the sleeves of the woman’s portrait. The scale on the left of each image
corresponds to the five alizarin concentrations used in the painting.
The painting was exposed to light from a xenon lamp for a series of exposure
times. After each time period, the painting was removed for measurement of the
concentration of alizarin remaining in the glazes. The work was also photographed
at each stage. The full exposure is approximately equivalent to a hundred years on
a museum wall well illuminated by diffuse daylight. While the images only reflect
a subjective comparison to the original paintings due to gamut restrictions, the
changes in the deterioration of the Alizarin Crimson are readily seen.
The reflectances of the five concentrations of Alizarin Crimson were measured
and converted to the Munsell color space. The changes in each glaze can be seen
in Figure 4.18 as the plot describes each hue (for paint samples 1-5) in respect
to their Munsell chroma and value. The Munsell coordinates for each original
paint are plotted as white dots and the exposed sample reflectances as black dots.
Each original paint sample is grouped with its corresponding samples of increasing
exposure via a blue ellipse.
As expected, the exposed sample reflectances grew further from each original
sample as time passed. The Munsell value increased on all samples over time (the
equivalent of the hue growing closer to white). The chroma is more complicated,
however. The initially darker glazes (denoted 1,2, and 3 on the graph), increased in
chroma as the fading proceeded (the maximum chroma was achieved by the middle
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concentration (3)). In contrast, the lighter glazes (4 and 5) decreased in chroma