PIGMENTS 129
been first prepared by E. Leykauf in 1868, and named by him 'Nurnberg violet*
(Rose, p. 254).
Mars Colors (Mars yellow. Mars orange. Mars reds Mars violet). The Mars
colors, so-called, are artificial ochres which are made by precipitating a mixture
of a soluble iron salt (ferrous sulphate) and alum (or aluminum sulphate) with
an alkali like lime or potash. The depth of the yellow color of the primary product
can be controlled by the proportion of alum used. The product is a mixture of
ferric and aluminum hydroxides with gypsum (if lime is the precipitant) and, if
simply worked and dried, is Mars yellow. When this Mars yellow is heated,
various shades of orange, red, brown, and violet result, depending upon the
degree and duration of the heat. The product must be thoroughly washed free
from soluble salts to be useful as an artist's pigment. The preparation of artificial
iron oxide colors of this nature from iron vitriol was described in the middle
XIX century (Rose, p. 222). Although these Mars colors are very homogeneous
and fine, they possess no advantage over the natural iron yellows and reds. They
are sometimes sold for the natural iron oxides.
Massicot (litharge). Both 'massicot' and 'litharge* are names which have
long been used for the yellow monoxide of lead (PbO). Some writers have used
them as synonyms but, on better authority, they are separated in meaning to
indicate lead monoxides that are derived from different sources and have slightly
different properties.
Massicot is understood to be the unfused monoxide of lead that is made by
the gentle roasting of white lead. At a temperature of about 300° C., white lead
gives off carbon dioxide and water, and the oxide is left as a soft, sulphur-yellow
powder. It is not an intense yellow but it has good hiding power and is similar
to white lead in pigment properties. Microscopically, it is not characteristic; it
appears to be nearly amorphous, but Merwin states (p. 519) that natural massi-
cot occurs in orthorhombic, thin plates or scales. Chemically, it has properties
like white lead; it dissolves in nitric and acetic acids, and may even give off car-
bon dioxide from undecomposed white lead. It melts in strong heat and is changed
to litharge or red leads depending upon the temperature. It is unaffected by
strong light but may revert to white lead on long exposure to damp air.
Litharge or * flake litharge* is the fused and crystalline oxide which is formed
from the direct oxidation of molten metallic lead. The molten lead, in rever-
beratory furnaces, is stirred from time to time to expose fresh surfaces of lead
to the oxidizing action of the hot air above (see Beam, p. 114). A more modern
way is to atomize molten lead by whirling propellers and allow it to oxidize in
contact with hot air. It has long been a by-product of the refining of silver by the
'cupellation* process. Litharge is more orange in color than massicot, caused by
the presence of some red lead (Pb304). It is not used as a pigment but is exten-
sively employed as a drier in paints and varnishes; it is important as an inter-
mediate step in the preparation of red lead (see Red Lead).