PAINTING MATERIALS
color; those most deeply colored have the highest refractive index, which for blue
is 1.74^ for red a little greater than 1.78, and for green less than 1.73. Grains 10^
in diameter are practically opaque to wave-lengths between 560 and 6iom/z
whereas wave-lengths in the red longer than 65omju and in the blue-violet are
transmitted freely. Most pigment grains are minute crystals and, since many of
these crystals are anisotropic, color absorption and transmission of light are
different along different axes.
The colors of different classes of pigments cover unequally well the different
regions of the visible spectrum. There are no colors of the short wave-length region
among the common earth pigments. There is also a deficiency of bright mineral
and inorganic pigment colors in that region. The organic dyestuff colors, however,
cover all portions of the spectrum almost equally well.
The refractive index of a pigment, which is the measure of light-bending power
of particles as light passes through them, is important because the hiding power
of a transparent pigment is proportional to the refractive index of its grains.
Titanium dioxide with a refractive index of 2.55 has the greatest whiteness and
hiding power of any white pigment. Both white lead and zinc white, with refrac-
tive index approximating 2.00, have lower covering power. Merwin says (p, 497)
that the amount of light reflected from a unit area of surface of a pigment grain
increases with the refractive index. Pigment grains reflect most light when sur-
rounded with air, less light when surrounded with vehicle, and paint reflects in
proportion to the difference between the refractive indices of the pigment and the
surrounding medium* The higher the refractive index of the pigment and the lower
that of the vehicle, the greater the light reflection, and, with white pigments, the
greater is the resulting whiteness and hiding power. There is also a close relation-
ship between refractive index and color. Merwin says (p. 501):
To be most effective as a pigment when used alone, a substance should have a high refractive index
for the color which it most freely transmits. In general there are large variations of refractive
index near and through a region of color absorption. Refractive index is higher on the long-wave
side of such a region than on the short-wave side. For this reason red, orange, and yellow pigments
' usually have much higher refractive indices than blue and violet pigments. The refractive index of
lakes is largely determined by the base, and is always comparatively low. Some pigments so nearly
match vehicles in refractive index that they diffuse very little light. They become effective only
when mixed with a pigment of high refractive index which will diffuse their color, or when painted
in thin films over a surface covered with a strongly diffusing paint. For example, Prussian blue,
verdigris, and alizarine lakes.
The refractive indices for many pigments are given in the table of physical prop-
erties. Pigments may belong to the isotropic, uniaxial, or biaxial groups of
crystals and the n index, the e and o> indices, and the a, 0, and 7 indices are given for
members of each group respectively. The indices for many of the mineral pigments
and inerts are known accurately to the third decimal place. Indices for many of
the precipitated chemical pigments are not known with such accuracy because
they are too fine to be measured or do not take definitely crystal forms when pre-