GREEN can be mixed from the blues and yellows already listed to every shade likely to be required, but a few pure and cold tones are difficult to achieve; it is practical, therefore, to use independent green pigments. Moreover, there are among them two materials which both technically and as colors are unsurpassed by any other pigments: the two green chromic oxides, brilliant and opaque.
If I had to make up a serviceable palette from a minimum of colors it would include ultramarine and Prussian blue, cadmium and alizarin red, ochre and cadmium yellow, umber and burnt sienna and the two chromic greens just mentioned.
OXIDE OF CHROMIUM (hydrous green chromic oxide brilliant), or VIRIDIAN, is the most important unmixed green pigment, with a cool, very pure tone very close to a spectrum color. We have described how removal of the water of crystallization by heating results in anhydrous chromic oxide, opaque. Both pigments are unaffected by light.
The hydrous oxide, viridian, is a decidedly transparent color, which keeps much of its great brilliance even when mixed with white. Combined with cobalt blue it makes nearly the same tone as the costly blue chromic oxide, and also provides useful mixtures with ultramarine and Prussian blues. The richest scale of greens, including nearly all the warm brilliant greens, comes from combination with cadmium and Mars yellow.
CHROME GREEN (anhydrous green chromic oxide) is a pronouncedly soft and warm green and an opaque color. It is less suited to strongly altered mixtures than to slight tinting, and looks well when used pure. Being one of the colors with the greatest covering power, it is very uncertain for glazing. Terre verte, to which chrome green is otherwise similar as a color and technically superior, is better for this purpose. Both chromic greens are found in a fairly pure form even in cheap paint boxes. They can hardly be varied in tone. They are very cheap and are unaffected by lime, and thus are much used for house and industrial paints and for printing and ceramic painting and glazes.
TERRE VERTE (green earth) is produced in cool and warm tones named after the two most important early findplaces, Verona and Bohemia. (Veronese green earth is not to be confused with vert Paul Veronese, another name for emerald green.) All the variants are essentially combinations of magnesium, aluminium, and silicic acid (augite).
Terre verte used to be the most important pigment for underpainting. Being a dull transparent color, it held the lightness of the white ground for the subsequent overpainting. It was also the ideal cool and discreet basis for complementary effects with all red pigments. Terre verte is insignificant for direct mixtures, and equally so for thick application. Deep tones in unmixed terre verte are best when it is laid in several glazes one over the other-an affect which cannot be achieved with chrome green.
This example shows once again how different the characteristics of pigments can be even when their colors are nearly identical. Much subtlety can be gained by playing these different characteristics off against each other, but only if each one is known and felt on its own, just as a cook must know the separate taste of each of his ingredients, a composer the quality of each instrument, or a sculptor the tactile quality of stones and woods in themselves before he begins to carve them.
Lastly, a green should be mentioned which is found in almost every color assortment: permanent green. It is generally a mixture of viridian with zinc yellow or cadmium, and is not a separate pigment. It may be useful and give no trouble, but I would never advise you to buy ready-mixed colors. They tend to standardize your sense of color and make you more dependent than is desirable.
There is, on the other hand, an arsenic green sold as EMERALD, Schweinfurt, Smaragd, or Veronese green, which is a good, stable pigment. It cannot be imitated by mixing and is essential for cold, light green tones. It replaces the verdigris and other copper colors which were formerly used.