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

or darkening enables the museum personnel to develop the best conditions for the
preservation of the objects while still allowing for them to be displayed.
4.3.1 Fading of pigments
For proper care of a work over its duration, a great extent depends on the identifi-
cation of the materials used in its creation. A noninvasive technique is spectropho-
tometry, which measures the light reflected or transmitted at each wavelength in
the visible spectrum. As covered previously, a graph of the reflected light at each
wavelength of a material is spectral curve. The shape of this curve can be helpful
in identifying the <strong>colorants</strong> and media used in a work.
In studying the rate of change for a specific pigment, accelerated-aging tests are
often carried out. These methods simulate a material experiencing long exposure
to daylight by a very intense ultraviolet light exposure over a much shorter period
of time. Samples are often immersed in ultraviolet light from a source enclosed
in a carbon arc, which was has a spectral energy distribution is close to that of
natural sunlight.
Feller states that these accelerated-aging tests are carried out for three major
purposes [Fel94]: to establish in a conveniently short time the relative ranking of
materials with respect to their chemical stability or physical durability; to predict
long-term serviceability of a material under expected conditions of use; and to
speed up and study the chemical reactions which elicit the degradation.
These techniques are used to study colorant changes, although one should re-
member that these conditions do not exactly represent real world behavior. For
example, these tests are not representative of outdoor aging, as they do not in-
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clude other important factors such as the presence of moisture, temperature and