Alizarin (alizarin crimson) (see also Madder) is the coloring principle of the
madder root and it was first isolated from that source in 1826 by Colin and
Robiquet ('Recherches sur la Matiere colorante de la Garance,' Anndes de
Chimie et de Physique, 2d series, XXXIV [1827], pp. 225-253). It is 1,2 dihy-
droxyanthraquinone, and was first synthesized by two German chemists, C
Graebe and C. Lieberman, who reported their discovery in 1868 ('Ueber Alizarin
und Anthracen,' Benchte der Deutschen Chemischen Gesellschaft, I [1868], pp. 49-
51; see also English patent 3850, December, 1868). This is important in the
history of organic chemistry, for alizarin was the first of the natural dyestuffs
to be made synthetically. Its discovery caused the rapid decline and the almost
complete disappearance of the large madder-growing industry in France, The
'alizarin crimson' lake used so extensively in artists' paints is nearly all from this
source. It is made with aluminum hydrate which gives a transparent lake;
different shades of red can be made with different bases. It is more light-fast than
natural madder lake because it contains none of the fugitive purpurin associated
with alizarin from that source (see Eibner, Malmaterialienkunde, p. 202), and is
among the most light-fast of the organic red pigments. Some painters have said,
however, that synthetic alizarin does not give the pleasing, saturated, and fiery
tone that madder alizarin gives. In ultra-violet light, synthetic alizarin does not
give any of the strong fluorescence that is characteristic of madder lake. It may
not be permanent when mixed with earth colors like ochre, sienna, and umber
(see Toch, Paints, Painting, and Restoration, p. 97). Microscopically, alizarin lake
is not readily distinguished. Merwin says (p. 517) that the isotropic base with
the coloring matter has a variable low refractive index, about 1.70 for red. The
color by transmitted light is purplish red. It is soluble and turns purple in dilute
sodium hydroxide, but this behavior is hardly characteristic.
Alizarin Crimson (see Alizarin).
Aluminum Hydrate (transparent white), or aluminum hydroxide, A1(OH)3, is
a light, white material which is prepared by treating a solution of aluminum
sulphate with an alkali such as soda ash or potash. The gelatinous aluminum
hydroxide, because it adsorbs dyestuffs easily, may be used directly in pulp form
as a base in the preparation of transparent lake pigments or it can be dried to
a very light, white powder. This is used in paint manufacture, largely as a filler.
It is the most common cheapening agent for artists' oil colors and many of them
now on the market contain it. Because of its low density and low refractive index,
it lacks covering power and lends transparency to colors. It has high oil absorp-
tion and, for this reason, tends to increase yellowing in paints. Excess of it some-
times causes a rubbery consistency in prepared paste paints. Aluminum hydrate
is difficult to detect in paint films by microscopic methods because of its lack of
form and its lack of characteristic optical properties. It appears simply in clots
of fine grains showing no birefringence. The hydrate is soluble in acids and
alkalis, but is otherwise a stable material. When heated to a high temperature,
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