PAINTING MATERIALS
are finely divided and uniform in particle size. Zinc oxide and lamp black are
examples; particles of the former may be perfectly crystalline, existing as stout
prisms. Vermilion, prepared in the dry way, is a sublimation product and each
particle is more or less a perfect crystal. Pure dyestuffs and toners often appear as
stains without any discontinuity in the film, particularly if they are soluble in the
film-forming substance. Lake colors are variable in character, depending upon the
base on which they are precipitated. The size of grains is usually expressed in
microns (i micron, y. = o.ooi mm.). Merwin (p. 499, f.n. i) calls grains very small
that are less than o.8/z in diameter; small, those that are between 0.8 and 2/^;
medium, 2 to 5/^5 large, 5 to ioju; and very large, over 10^. The most effective black
and white pigments are those which have an average particle size in the order of
iju in diameter or slightly less. White pigments, however, with average grain size
much below 0.5^ do not have such good hiding power as larger grains, they tend
to diffuse blue light more than red, and, when mixed with black, give blue-grays
(see Merwin, p. 494). Most colored pigments have grains ranging from 0.5 to
lOju in mean diameter. Prussian blue and indigo are extremely fine-grained, but
pigments like emerald green, verdigris, and cobalt blue are comparatively coarse
(see Merwin, p. 499, f.n. 2). Pigments in older paintings, in general, are coarse,'
particularly the mineral pigments. Azurite and smalt had to be used coarsely
ground because, when very finely ground, so much white light is reflected from the
surfaces of their particles that they become pale and unsuitable as coloring ma-
terials. Large particle size and graininess are characteristic of the pigments used in
early Chinese paintings. Granular, crystalline pigments give a certain pleasing
quality to paint films that can not be had from fine, well-dispersed pigments such
as are produced for the modern paint industry. Fine and uniform particle size in
modern pigments is also partly the result of modern mechanical methods for
grinding dry pigments. Control of particle size of pigments is carried out in pre-
paration or in dry grinding. The grinding of a pigment in a vehicle ordinarily does
not reduce particle size but merely effects wetting and dispersion of each individual
pigment particle.
Individual pigments vary greatly in density or specific gravity and this varia-
tion has to be taken into consideration, both in the preparation and in the prac-
tical use of paints- Some pigments, particularly the organic lakes and toners,
are light and bulky, and so are a few of the inert materials like aluminum hy-
drate (sp. gr. = 2.45). Lamp black is one of these very light materials (sp. gr.
= 1.77)* Many of the pigments, however, are compounds of the heavy metals and,
hence, have a high specific gravity. Examples are vermilion (sp. gr. = 8.09) and
red lead ^(sp. gr. ~ 8.73). Pigments with high specific gravity settle rapidly in
liquid paints. In paints that contain mixtures of light and heavy pigments, there
are sometimes indications of a slight separation of the light and heavy components
when the paint is spread thickly on a horizontal surface. Specific gravity has an
important bearing in centrifugal methods for the separation and analysis of paint