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

The gray card reflected 18% of the light that falls on it and was used to standardize
the exposure of an image taken with a camera. Immediately after photographing
the sample, it was placed in the sample port to measure its diffuse reflectance.
Distemper and Encaustic must be applied hot, as the consistency of the ma-
terials changes completely as the temperature drops. Upon cooling, the materials
become very thick and difficult to apply. Hence, for these materials the binder
must be kept in a metal container on a hot plate to maintain fluidity. All of the
other media were relatively straightforward to apply to their respective substrates.
However, the author notes that problems will inevitably occur while creating
one’s own binders. One typically tests a binder before its widespread use. Binders
with too high a concentration of adhesive will result in a paint film that is too
strong, resulting in cracking. Binders that are too dilute will not adequately hold
the pigment to the ground, resulting in pigments that rub or flake off. The delicate
balance can be determined by brushing out a portion of a pigment-binder mixture.
If, upon drying, the former problem occurs, or if it is difficult to brush out a
consistent stroke, the solution must be diluted (water, turpentine, etc). If the latter
problem occurs, more binder must be added to strengthen the solution. Unwanted
issues may result due to inadequate dispersion of the pigments within the binder.
In this case, one might experience agglomeration or clumping of pigments, creating
an irregular surface. Also, evidence of the differences in pigment size and specific
gravity may also show in tinted samples. Pigmented mixtures are susceptible to
pigments separating out in solution if they were not adequately dispersed originally.
Examples of cracking and inadequate dispersion are shown in Figure 5.7.
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