PAINTIN-G MATERIALS
(see Coal-Tar Colors). There are many classes of dyes; classification can be based
upon the method of application (acid dyes, basic dyes, mordant dyes, etc.) or
upon their structure (azo, triphenylmethane, etc.). They consist in structure of
a chromophore (coloring) group and a salt-forming (anchoring) group. Some may
be used directly in vehicles for staining but, for pigment purposes, most of them
are precipitated or struck on inerts to form lake colors (see Lake). Although the
pure, solid dyes are frequently crystalline when dissolved in a solvent or paint
medium, they are quite without structure and, even under the microscope at high
magnification, reveal no discrete particles.
Earth Pigments, broadly speaking, are those which are derived from minerals,
ores, and sedimentary deposits of the earth's crust. More specifically, they are
those complex mixtures of minerals that comprise the clays, ochres, siennas, and
umbers. Carbonaceous pigments, like Van Dyke brown, also belong in this group.
Earth pigments were among the earliest employed, and they include many of the
highest stability.
Egyptian Blue (blue frit, Pompeian blue). The inorganic blue color most
commonly found on wall paintings of Egyptian, Mesopotamian, and Roman times
is an artificially made pigment which contains as its essential constituents copper,
calcium, and silica. Lucas, who gives a very good summary (pp. 284-285) of the
history and occurrence of this blue, says it was made by heating a mixture con-
taining silica, a copper compound (probably generally malachite), calcium car-
bpnate, and natron (natural sodium sesquicarbonate). A. P. Laurie, W. F. P.
McLintock, and F. D. Miles ('Egyptian Blue/ Proceedings of the Royal Society
of London,, Series A> LXXXIX [1914], pp. 418-429), who carried out an investi-
gation of methods of preparing it, found that the blue crystalline compound is
formed only in the rather narrow temperature range of 800 to 900° C., probably
about 830°. Chaptal appears to have been the first to call this material a 'frit'
but, although it does contain some glass as impurity, the blue is definitely a
crystalline compound. Laurie and co-workers (loc. cit.} point out that the Egyp-
tian blue pigment is closely related to the well known blue glaze of Egyptian
ceramics; that glaze was applied to a base of carved sandstone at a temperature
somewhat lower than that required to form the crystalline blue. There is con-
temporary mention of this artificial blue which includes descriptions of its method
of preparation. It is no doubt the Egyptian caeruleum of Pliny (see Bailey, I,
145 and 234). Vitruvius (VII, Chap. XI) describes its manufacture but errone-
ously states that the method for making it was first discovered in Alexandria.
Egyptian blue which is coarsely crystalline and pure blue in color is similar,
in appearance, to finely ground azurite. Unlike azurite, however, it is insoluble
in acids, is not affected by light or heat (except at very high temperatures), and
by alkalis only on fusion. Many specimens, well over 3000 years old, appear to
be little changed by time or environment. The blue is characteristic microscopi-
cally; it is birefracting (« = 1.635), and it is moderately pleochroic, the crystals