PIGMENTS 145
pared. Many of the pigments are not uniform in particle composition; hence, the
refractive indices lie over a range of values. The degree of hydration and water
inclusion are other factors that determine the light-bending properties. The purely
opaque pigments, of course, have no measurable refractive index.
Hiding power is the property of a pigment, when made into a paint, to obscure
the surface over which it is applied. In the case of white pigments, the ability
to reflect light and to obscure black is the measure of hiding power; in the case
of black pigments the opposite is true. As a general rule, hiding power of a pig-
ment is proportional to its refractive index, to fineness of particle size (down
to a certain limit), and to depth of color. Usually pigments which are compounds
of the heavy metals have the greatest hiding power, but there are exceptions like
carbon black and ultramarine. Lake pigments, especially when prepared on an
alumina base, are transparent and have very little hiding power.
Size and shape of pigment grains are important for various reasons. Pigments
are ordinarily very fine substances. To be useful, they must produce a paint that
can be applied evenly and smoothly in a uniform film. This requires fine and uni-
form particle size. In the production of pigments, that is attained in various ways.
Those pigments produced from minerals are simply broken crystal fragments.
Their particle size is governed by the ease and kind of fracture and by the amount
of grinding, but ordinarily mineral pigments are not very finely ground- Small
particles produced in this way have the appearance of broken fragments, the
edges are sharp and irregular and usually angular; the shape is, in fact, governed
by the cleavage properties of the mineral. Azurite and cinnabar vermilion are
examples. Many of the earth pigments of sedimentary origin consist naturally of
small, discrete particles which, however, are usually very uneven in size. In pre-
paration for use as pigment, the raw earth must be stirred up in large tanks of
water and let stand (levigation) to allow coarser particles to settle away from the
finer particles which are held in suspension* The supernatant liquid bearing the
finer particles is drawn off, is passed from tank to tank, remaining in each longer
than it did in the preceding one, and producing in each successive tank a finer and
finer deposit. Particles from this kind of source are usually irregular in shape but
are rounded or have rounded edges. They are often quite heterogeneous in com-
position and color. Examples are green earth and raw sienna. Many of the modern
pigments are produced as chemical precipitates by the interaction of salt solutions
which make an insoluble substance. Many such precipitates are crystalline in nature
and each particle is more or less a small, perfect crystal. Pigments that are pyro-
genetic in origin, like ultramarine blue and oxide of chromium, have variable
particle characteristics because conditions of formation differ greatly. They are
produced by complex chemical reactions that may take place between several
substances at high temperatures. Pigments made by the corroding action of
chemicals upon metals, like white lead and verdigris, are usually fairly coarsely
crystalline. Several important pigments are fume and smoke products and, hence,