PIGMENTS 131
The blue paints as contrasted with the others form fairly coherent films. These
can be isolated by dissolving away the underlying plaster in dilute acid. Micro-
scopically the blue material consists of indefinite spherulitic aggregates of a bire-
fringent substance resembling the c!ay mineral beidellite. It stains like beidellite.
X-Ray powder photographs, taken by Dr. E. Posnjak, show the blue pigment,
beidellite, and a blue chromiferous clay, to be similar. Not enough of the blue clay
was available to give decisive tests for chromium, but, like the blue clay, the color
is not discharged by boiling nitric acid nor by heating much below redness. The
conclusion seems justified that this is an inorganic color.
The green paints are a mixture of the blue and yellow ochre.
The occurrence of this blue mineral or clay may have been very restricted
or known only in that single locality, but there its use seems to have been quite
general.
Mica. In mineralogy, various micaceous minerals are recognized, but the name
most commonly applies to muscovite, or hydrous potassium aluminum silicate,
H2K Al3(SiO4)3, which is found in nature in thin laminae with perfect cleavage.
It occurs widely in small deposits all over the world. Other names are * isinglass7
or 'Muscovy-glass.' Its widest present use is as an insulating material in the
electrical industry. Ground mica is used as a lubricating agent and as a re-
enforcing pigment in paints. In the Far East it had an occasional place in painted
designs where its lustrous surface gave an effect somewhat like that of metal.
Mineral Pigments are those derived from the natural minerals of the earth.
Although, broadly speaking, they may include complex mixtures and aggregates,
like earths and clays, the term is more properly restricted to those minerals which
are, on the whole, definite chemical compounds with characteristic physical form
and constant chemical behavior. Pigments derived from azurite, orpiment, and
lapis lazuli are examples.
Minium (see Red Lead).
Molybdate Orange, a pigment of recent origin, is a mixed crystal compound
of lead chromate, lead sulphate, and lead molybdate in the approximate ratio,
yPbCrOraPbSOr iPbMoC>4. This molybdate-modified lead chromate belongs to
the tetragonal system whereas lead chromate alone is usually either rhombic or
monoclinic. The particles are small, rounded, and uniform in size, have high
index of refraction, and are moderately birefracting. As a pigment, it has high
covering power and tinting strength. A. Linz has described (£ Molybdenum Orange
Pigments/ Industrial and Engineering Chemistry,, XXXI [1939], pp. 298-306)
the conditions of precipitation necessary to produce a pigment having the most
desirable properties. Molybdate orange can claim only moderate fastness to light.
Mixed crystal pigments of lead chromate and lead molybdate were first described
by E. Lederle in a German patent (no. 22F.1.52.30) applied for by the I. G.
Farbenindustrie A. G. on August 30, 1930 (see Linz, he. cit^ p. 2,98; see also
German patent 574>379> April 12, 1933, and 574,38°:> APril J3> I933; also U. S.
patent 1,926,447, September 12, 1933). Although molybdenum orange went into