138 PAINTING MATERIALS
[see Dye], which is soluble in the vehicle). Pigments are derived from a wide
variety of substances, organic and inorganic, natural and artificial. They may be
classified according to color, chemical composition, or source.
Pigments, chemical properties. Pigments comprise a wide variety of chemical
compounds; hence, they differ greatly in respect to their chemical properties.
Among the inorganic coloring materials are the oxides, sulphides, carbonates,
chromates, sulphates, phosphates, and silicates of the heavy metals. A very few
like Prussian blue and emerald green are complex metallo-organic compounds.
Carbon in the form of lamp black or charcoal and the metal pigments like gold
and aluminum are the only elements that serve in a relatively pure state. Dye-
stuffs are complex organic compounds.
For certain special purposes, a pigment should be as nearly chemically inert
as possible and be unaffected even by strong acids, alkalis, and heat. Only very
few of the paint pigments, however—carbon black, oxide of chromium, and cobalt
blue—are so resistant. A few compounds like the oxides of cobalt, chromium, tin,
and iron are so stable at high temperatures that they can serve for coloring
ceramic glazes. So far as the demands of ordinary painting are concerned, how-
ever, a pigment need only be stable and chemically inert enough to withstand
light, air, and moisture or environments in which all three agencies are combined.
Light, especially strong sunlight, is the promoter of certain photochemical
reactions which result in dimming some colors, in browning and darkening some,
and in producing a distinct color change in others. In the case of organic dye-
stuffs, light causes a definite degradation to colorless products, a change called
' fading.' The effect of light is usually accelerated by heat and moisture. The
oxidizing action of chromates, with reduction to chromic oxide, is accelerated by
light. Red lead in a glue medium has been observed to turn to brown lead dioxide
through the combined action of light and moisture. Air is the carrier of moisture
and certain obnoxious industrial gases like sulphur dioxide and hydrogen sulphide,
and the oxygen which is an important component of it may itself take part in
reactions which cause fading or discoloration. Church and others (pp. 334 and 339)
showed years ago that fugitive pigments exposed in thin washes to light faded
less when moisture is excluded and still less rapidly when both air and moisture
(in vacuo) were excluded. Many chemical reactions require a certain amount of
moisture before they can take place. The exact role of moisture is not entirely
understood, but it may be considered to act as a catalyst.
Stability and inertness enough to insure complete compatibility with others
are among the first requirements of artists' pigments because they are intermixed
or intimately juxtaposed much more than are those in house paints. Under certain
conditions, it is possible for sulphides to interact with copper and lead pigments
and produce black or brown copper and lead sulphides, with resulting discolora-
tion. Actual happenings of this phenomenon are rare, particularly when the
pigments are used in oil medium, because the oil encloses each particle in an