PIGMENTS 127
which are more orange and red than those of alizarin (Church, p. 173). This
accounts for the warm tone of madder lakes as compared with the pure alizarin
lakes. Eibner says (Malmateriallenkunde^ p. 20) that purpurin is not so light-fast
as alizarin. He gives chemical methods by which the two may be distinguished
(op. cit., p. 203). He says, also ('Les Rayons Ultra-Violet Appliques a TExamen
des Couleurs et des Agglutinants,' Mouseion, XXI-XXII [1933], pp. 32-68), that
the presence of purpurin causes madder lake to fluoresce a fiery yellow-red in
ultra-violet light whereas synthetic alizarin lakes show only a feeble violet lumi-
nescence.
Paint films colored with madder lake are nearly transparent and appear bright
red, with a definite violet hue by transmitted light. The base on which the dye is
prepared, particularly if it is aluminum hydrate, can seldom be distinguished,
even at high magnifications, because it is amorphous and transparent. Madder
lake is among the most stable of the natural organic coloring matters. The color
is turned purple by dilute sodium hydroxide but is only destroyed by much
stronger reagents. Harrison says (p. 231) that natural madder is still used in
France on a small scale for the production of fine artists' colors. Their manu-
facture is carried on by traditional methods. He further says (p. 239) that ali-
zarin lakes now far surpass lakes from the natural madder in purity, brilliance,
and range of colors.
Magenta (fuchsin) is a brilliant red-purple organic dye, C2oH2oN3Cl5 of the
triphenylmethane group of dyestuffs. It was first prepared by Natanson in 1856
(Colour Indexy p. 173). It is soluble in alcohol, acetone, and aqueous solutions.
Although a fugitive dye, it has been used for water colors and is still listed among
them by artists' colormen.
Malachite (mountain green) is perhaps the oldest known bright green pig-
ment. It is the natural (mineral) basic copper carbonate, CuC03*Cu(OH)2, and
is similar in chemical composition to the blue basic copper carbonate, azurite
(see Azurite), except that it contains a greater amount of combined water. Like
azurite, it occurs in various parts of the world associated with secondary copper
ore deposits. It is prepared as a pigment by careful selection, grinding, and sieving,
but today it is seldom used, except perhaps to a small extent in the East.
Malachite is crystalline (monoclinic) and is fairly characteristic microscopi-
cally. Particles of some varieties have a clear, faint, bottle-green color by trans-
mitted light, and show high relief, strong birefringence, and pleochroism. Prisms
with longitudinal striations are common. Since it is a carbonate, it is decomposed
by acids, even acetic acid. It is unaffected by cold dilute sodium hydroxide but
blackens when warmed with that reagent and, also, when it is heated alone. In
spite of its ready decomposition, it has remained unchanged in many paintings
for centuries, just as it has in the earth. It is unaffected by light.
The history of malachite in painting runs closely parallel to that of azurite.
It occurs on Sinai and in the eastern desert of Egypt, and was used there for