PIGMENTS 117
Green Earth (terre-verte) has been used in European paintings since before
classical times. It occurs rather widely but that which is suitable for a pigment
is found only in restricted areas. A good quality (celadonite) is found north of
Monte Baldo, near Verona (terre de Vcrone; see Church, p. 190), and also in
Germany, France, Cyprus, and Cornwall (see Rose, pp. 205-206). Most of the
green earths seem to have originated as marine clays. They are complicated in
composition but are made up chiefly of two indefinite but closely related min-
erals, glauconite and celadonite, which are essentially hydrous iron, magnesium,
and aluminum potassium silicates. Green earth varies in composition like so
many of the complex silicates (see Clarke, pp. 519-523). Although the color may
be caused in part by a small content of iron in the ferrous state, the greater part
of the iron is ferric. The shade ranges from a neutral yellow-green to pale greenish
gray. The best quality is a neutral sage green. Green earth has a low hiding
power, especially in oil, but it works well in tempera. Microscopically, it is char-
acteristic; it consists of coarse, rounded, smoky green particles with many trans-
parent, clear, and angular silica and silicate particles. Quite frequently, scattered
bright blue particles may be seen. De Wild states (p. 74) that these blue particles
are like cobalt blue; but there is no cobalt present and, unlike cobalt blue, the
particles are birefracting. The green is turned red-brown on strong heating; other-
wise, it is a very stable pigment, unaffected by light or air or by chemical agencies
such as dilute acids or alkalis. Church (p. 192) says, however, that some samples
of terre-verte are liable to become rusty when brought into contact with lime
hydrate in true fresco painting. Although good grades of green earth are still
obtainable, it is subject to substitution by mixtures of transparent oxide of
chromium (viridian) and red earth pigment. The true green earths, which are
supplied by different dealers, usually vary in character and shade because of their
many different natural sources.
Green earth was used as a pigment on Roman wall paintings at Pompeii and
at Dura-Europos. It was widely used by Italian painters as a foundation for flesh
tones, and is the pigment that gives the greenish tone to so many of the abraded
Italian panel paintings. De Wild has reported it (p. 75) on three Dutch paintings
of the XVII century and on one of the XIX century. The green from the ceiling
of Cave I at Ajanta (India) was identified as green earth.
Green Lake is no particular compound, but a name to indicate various green
organic colors of natural or synthetic origin. Mixtures of Prussian blue with zinc
yellow or yellow lakes may be sold under this name.
Guignet's Green (see Viridian).
Gypsum (terra alba) (see also Gesso) is important among the*raw materials
that have been used in works of art. It is calcium sulphate dihydrate, CaSCV 2H2O,
and, often associated with salt deposits, occurs widely over the world; important
workings are found in most of the countries of Europe, in the United States, and
in Canada. There are several varieties: selinite is crystalline, transparent, and