Terre Verte

Synonyms:

Terre Verte, Green Earth, Terre De Verone, Grüne Erde, Terra Verde

There are two rather indefinite minerals, probably not really distinct - namely, glauconite and celadonite - which furnish the raw material from which the artists' pigment, generally known as terre verte, is prepared. The form, or variety, celadonite is the rarer of the two: it is softer than glauconite: both minerals are probably mixtures. Green earth of very good quality is found at Bentonico, to the north of Monte Baldo, near Verona, where it occurs in cavities in an amygdaloid rock. The best samples possess a deep olive-green colour; inferior specimens are celandine or apple-green. Green earth is obtained from a large number of European and American localities, and varies much in chemical composition. From its greenish hue it has been assumed to consist chiefly of a ferrous silicate - that is, a silicate of protoxide of iron, and in this way it is represented in the older analyses. But more exact analyses have shown that green earth contains but a small part of its iron in the protoxide condition, and that it is mainly a ferric silicate.

A choice specimen of fine hue from Monte Baldo gave the following results on analysis, the numbers representing parts in one hundred:

Green earth is thus allied to the hornblendes, differing mainly in the partial replacement of soda by potash and in the presence of water. Being itself an alteration-product, it is not likely to be amenable to further change, particularly as the iron in it is for the most part fully oxidized.

Terre verte occurs in ancient Roman wall-paintings (the prepared pigment was found in the ruins of Pompeii), and was largely employed by the early artists of Italy in their works in tempera, fresco, and oil. But amongst the green pigments found in ancient wall-paintings in Rome and Pompeii there occurs also a substance of a richer and deeper hue than that possessed by terre verte. It was made by grinding into fine powder a kind of green jasper; it has proved quite permanent.

Terre verte is prepared by carefully selecting the richest-coloured and most uniform specimens of the mineral, grinding them to fine powder, and washing the pulverized material with rain-water; it is then dried. Sometimes the selected fragments are heated, and then quenched in very dilute hydrochloric acid to remove ochre and other impurities; the undissolved portion is then ground, washed thoroughly, and dried. Most samples of terre verte are found to be perfectly stable both in water-colour and oil-painting. It is a semi-opaque or translucent pigment, without much body in oil. It has no action on, nor is it affected by, other permanent pigments. When used in oil or tempera as a ground-colour or in the under-painting, terre verte sometimes becomes more conspicuous in the course of time, owing partly to the deepening of its own hue and partly to the increased translucency of the pigments which have been laid over it. Some samples of terre verte seem to be liable to become slightly rusty when brought into contact with lime hydrate in true fresco painting. This is probably due to the further oxidation of some of the ferrous oxide they contain.

Calcined terre verte is known as Verona brown.

Terre verte is rarely adulterated. A pure sample is not affected when drenched with liquor ammoniæ, becoming neither more bluish (presence of copper compounds), nor more brownish (presence of Prussian blue). But although adulterated terre verte is rarely met with I have found that a well-known firm of artists' colourmen sell, or have sold, a mixture with green ultramarine under the name of terre verte. Tubes of oil-colour of this sort have been submitted to me for analysis. This substitution is unfair to the artist, who has a right to obtain the material he demands. Green ultramarine freed from oil gives sulphuretted hydrogen when moistened with hydrochloric acid, which has no such action on terre verte.