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

over white grounds) suffer the greatest rate of color change. Finally, the fading
will appear to slow, but only after most of the glaze has been lost (absorbing very
little light). Note that the first two stages of fading are easily recognized from the
Munsell plots of the alizarin crimson glazes in Figure 4.17.
The model predicts that only absorption determines the fading rate. The fact
that the colorant concentration does not affect the colorant loss rate (pale tints
do not lose their colorant faster than that of darker colors) has been previously
observed [GED64, JFFBC84]. However, while prior fading should not affect the
loss rate, this was not the case experimentally. Whitmore and Bailie noted that
older <strong>colorants</strong> seem more resistant than fresh ones to further fading (given the
same concentrations). The authors list possible discrepancies: the pigment may
either react to form products that influence further fading, or originally have been
a mixture of components with different individual fading rates (different particle
sizes, previous fading of one of the constituents, etc).
Pigments scatter incident light, which in turn, increases the depth of which
photochemical processes occur. Johnston-Feller showed that the depth of fading
in paints from accelerated aging is decreased as the concentration of white pigment
decreases in a tint. (Figure 4.19). White pigments scatter light deeper into the
paint, and hence fading occurs deeper within the paint if there is more white
present.
Yet, the behavior of fading taking place only in the upper portion of the paint
sample is important for analysts attempting to classify materials in a work. If the
surface has faded substantially, it is possible to view relatively unfaded material
underneath via cross-sections, revealing many of the original <strong>colorants</strong> [Fel94].
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