Red comes mainly in natural earth colors. Most of the nuances of red ochre are named after their original sources: Venetian, Pompeiian, and many more.
Iron oxide is the coloring matter in the clay. Red oxides are little more expensive than yellow oxides, and on the whole the observations on naturally colored yellow clays apply equally to the reds. In mixtures they are not very friendly, the red base remains dominant. As burnt colors - there are burnt colors derived either naturally or artifically from yellow ochre - the red ochres and Mars red are more opaque than the yellows. Laid on thickly they can be too insistent unless care is taken with the surrounding color.
One of the best known red ochres is that used in red pastels and pencils. An ochre called "red bole" was tavored by the earlier Italian masters as a ground color. They always used it as an underlay for a gold ground.
Two classic red pigments have been eliminated from the modern, trouble-free palette: mountain cinnabar and carmine. Neither is fast to light. They will be dealt with in more detail in a chapter on obsolete colors. Their colors, however, were so unique that permanent substitutes had to be found for them.
CADMIUM RED and ALIZARIN RED were the answer. All that has been said about cadmium yellow applies to cadmium red, and it should be remembered that, like cadmium yellow, the red cadmium colors must remain unmixed if they are to retain their brilliance. Unfortunately, they are totally unsuitable as glazes; although the granulation is generally finer than in the yellow sorts, the pigment will not spread in a thin layer. The artist must find the right compromise between a transparent and an opaque application.
Neither cadmium nor alizarin red gives a true primary red, but a mixture of the two comes close. This works well because alizarin red is as excellent a transparent glaze as Prussian blue, and almost equally strong in tinting power. It is a dye made from coal tar and the only organically colored pigment on the trouble-free palette. It was found while trying to produce a permanent madder red.
Red madder, which is very similar to carmine, was for centuries an important textile dye, extracted from the root of the madder plant which was
grown in large fields. Its most important coloring constituent is alizarin, whose resistance to light is destroyed by other organic substances. It can be extracted pure from coal tar, and after the experience of 50 or 60 years may now be considered fast to light.
The organic dye can be precipitated indissolubly onto white clay and thus create a workable pigment. This color is quite indispensable, for there is no other inorganic color even approximating it; whereas cadmium red and cinnabar can be replaced to some extent by Mars red (iron oxide) .
ALIZARIN RED, like the cadmium colors, can be produced in an endless variety of tones. They range from orange to violet. The darker tones are the most resistant to light. It achieves its brightest effect as a glaze over opaque red and opaque green underpaintings (red and red-brown earths, green earth, and chrome green). In a direct mixture it gives only one really distinguished color: the purest violet of the palette, a mixture of alizarin red and ultramarine blue.
Neither cadmium nor alizarin red is unaffected by lime. Venetian red (iron oxide), Mars red, and cobalt red have to be substituted for them. All of these, however, need to be surrounded by complementary colors to enhance their brilliance. Alizarin red is an example of how the technical peculiarities of a pigment are not to be ignored: in a thick layer the decidedly transparent color loses its deep glow completely.