pigments	143
compared and referred to (see The Pigments and Mediums of the Old Masters^
insert after p. 112). Another type of chart (op. cit._9 p. 136, and The Analyst,, p. 176)
shows by vertical lines the occurrence of pigments in Western paintings from 800
A.D. to 1800. De Wild (see inserted table) made up a chronological chart of
pigments based on his investigations of Dutch and Flemish paintings. Although
his researches covered paintings of limited geographical origin, his data serve to
indicate some of the history of pigments in Europe from the XV to the XX cen-
tury. Eibner (JVandmalerei^ pp. 549-554) has prepared perhaps the most com-
prehensive table on the history of pigments. He uses information from several
sources, mainly literary evidence, both classical and contemporary, but, also,
to some extent, his own objective findings. Noel Heaton (' The Permanence of
Artists' Materials, Journal of the Royal Society of Arts [London], LXXX [1932],
pp. 415—416) has published a table which lists in chronological order the dates of
introduction of the important artists' pigments.
Pigments, physical properties* Pigments are materials which are useful for
painting mainly because they have some outstanding physical properties, even
when they exist in a fairly minute state of subdivision. Physical properties are
those properties inherent in a material itself and which do not involve its relation-
ship or combination with other materials.
Color is the most important physical property of a material in determining
its immediate usefulness as a pigment. A material has color because of its selective
absorption for the component colors of white light. The color of painting materials
can be treated from many points of view. One of these has for its end the com-
prehension of all possible visual tones within a single system. Morton C. Bradley,
Jr (' Systems of Color Classification/ Technical Studies,, VI [1938], pp. 240-275)
has made a brief critical review of that phase of color study. It has to do not with
pigments specifically but rather with color as visual tone. In isolated instances a
pure material itself may serve as a standard of reference in a system of visual
tones. True color characteristics are best established analytically by spectro-
photometric measurements. Barnes has recently worked out the descriptions of
some fifty artists' pigments and has given curves for them on the basis of light
reflectance from the surface for different wave-lengths of incident visible light.
Maerz and Paul treat the language of color_3 its origin, growth, and usage, and by
means of color plates showing graduations of hue, purity, and value, they offer
a quick and practical method for relating colors with the names by which they are
commonly identified. Standards of color for many of the pigments described in
these data appear on their plates.
Merwin, in treating optical properties and theory of color of pigments, has
shown that the color characteristics, the hue, purity, and brightness of the light
diffused, depend upon the color absorption, size, shape, and texture of pigment
grains. He describes the optical properties of individual pigment substances
in detail. For instance, he says of cobalt blue (p. 520) that grains vary in depth of