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

while all other light is mostly absorbed. Note that the darkening of the paint is
less for the exposure under the ultraviolet filter. In order for photochemical deteri-
oration (chemical reactions induced by electromagnetic energy) to occur, radiant
energy must be absorbed, activating the molecules. Typically, the energy of pho-
tons in the long wavelength (low frequency) range of the electromagnetic spectrum
(infrared range and higher) is not sufficient to induce chemical reactions. Yet, as
frequencies increase, photons carry more energy. Ultraviolet light, with a higher
frequency (and much more energy) than that of the visible spectrum, is capable of
inducing significant photochemical changes in the pigment particles.
The Grotthus-Draper Law states that only radiation that is absorbed by a sub-
stance may cause a chemical reaction–light must be taken in by a material in order
for the energy from the light to act upon it [Fel94]. Yet, not every frequency of
light elicits a change in the material. Photochemical changes are dependent on the
molecular structure of the pigment particles. Sir William Bragg used the analogy
to tuning forks, which are set in vibration when sound waves of the appropriate
frequency pass through them [Bra59]. Correspondingly, electromagnetic energy
tends to be absorbed in molecules when they are in tune with a particular frequency
of incident light.
If a red and a blue pigment were equally susceptible to photochemical dete-
rioration, the blue pigment would deteriorate much faster. This is due to the
increased energy in the lower wavelength range of the visible spectrum (compared
to higher wavelengths). However, since pigments are made of different materials,
they absorb different portions of the electromagnetic spectrum to differing degrees.
Hence, blue pigments do not necessarily deteriorate faster than red pigments.
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Figure 4.17 illustrates Alizarin Crimson red fading to a colorless form due to