YELLOW is the most frequent color among natural pigments. The yellow coloring of earths is always iron hydroxide, or rust, precipitated on clays of varying degress of purity. These yellow or red-colored clays are called ochres. The color is purer the less the iron hydroxide is dulled by other additions. Manganese combinations often occur in nature and to a varying extent change the yellow into brown.
Natural ochres are the cheapest of all good pigments. Synthetic ones are dearer; they are yellow iron oxides and are sold as "Mars colors." They are purer in color than the natural earth pigments, but they are still classed with the dull pigments. Ochres are the most widely used colors in painting, as they are often employed as an underpaint for other dull color tones. They mix with all binders and pigments, and as colors constifute to some extent a reconciling element, quiet and soft, amid the strong, bright colors, like a calm person among a group of excitable temperaments.
The same applies to any mixtures made with ochre or Mars yellow. They bring a harmony because of their dulling quality, a pervasive ground tone to the picture, especially when the ochres are used for monochrome underpainting. They are of medium strength, and can be used equally well for glazing and covering. They are chemically neutral and are found in every paint box.
CADMIUM YELLOW is the purest of all light-fast pigments. It corresponds, in its medium tone, to the primary yellow of the color circle. It can be produced in an endless number of tones, from the lightest lemon yellow to orange and even a deep red. Combination of cadmium with sulphur does not alter its chemical structure, but only its physical character, from crystalline to amorphous. The best of all yellows to be bought are light, medium, and dark cadmium.
All cadmium colors are considerably more expensive than ochres and all are affected by lime. They must not be brought together with lead colors, such as white lead, either, as this darkens them. The only whites suitable for mixing with them are the completely neutral zinc white or titanium white. For painting on lime, uranium yellow is substituted for cadmium as far as its high price allows, uranium being the most expensive of all pigments. Cadmium loses its original lightness and transparency as it loses its crystalline form. Deep cadmium and chrome green are the most opaque of all pigments on the painter's palette. Cadmium yellows are pure and can be used in many bright color mixtures. They give the most radiant warm greens when mixed with Prussian blue and viridian. They also mix well with all red pigments. Cobalt yellow, which often has a slight green tinge, can be used as a substitute for cadmium lemon on lime.
NAPLES YELLOW, or antimony (lead antimoniate), should have its place in every well-ordered palette, although it and its reddish variant are not classed among the pure colors. It has another limitation: having strong covering power, it can ruin the transparent character of a rapid watercolor painting, and should therefore only be used very sparingly or not at all in this medium.
On the other hand, both in color and consistency it is very attractive in tempera and oil, giving the whole coloration of the picture a misty solidity, something of the tangibility of air, as though a bright yellow were shimmering through fog and cloud. Its stability in lime makes it a valuable addition to the palette of the mural painter. Naples yellow has a very different consistency from other pigments, showing how individual a character a pigment can have. The use of the tactile quality of pigments, apart from their color, is one of the aspects of painting technique which should form part of the equipment of every able, experienced artist.