Types of Pigments

Excerpts from ART HARDWARE: The Definitive Guide to Artists’  Materials, by Steven Saitzyk © 1987 All rights reserved.  Reproduction  forbidden without written permission.

The older, more traditional pigments were derived mostly from substances that existed naturally in the environment. Even when some of them were later synthesized, they were not chemically altered. These older pigments produce colors that can be seen in nature and tend to convey a sense of naturalness, even when used for abstract works.

The new, synthetic pigments are still derived from such natural substances as petroleum, but they have been chemically modified to create a new substance that is unnatural to the environment and, to many, has an unnatural appearance. Many of these colors, because of their industrial applications, are now part of our cities’ visual environment, our synthetic environment.

In this comparison between modem and traditional pigments there is no suggestion that the natural pigments are better than the synthetic ones. The characteristics of both types of pigments can be exploited to a painter’s advantage to accomplish a particular effect.

I have grouped pigments in regard to whether they are inorganic or organic, and whether they have been synthesized. Organic pigments are either composed of carbon or are part of, or produced by, a living organism, such as ivory black (carbon). Natural extracts from animal or plant matter, or synthesized material originating from organic matter, such as petroleum products, have been the source of many organic pigments.

Inorganic pigments are pigments that do not have hydrocarbons (a molecular arrangement of carbon and water), such as cadmium sulfide (cadmium red), but include oxides and sulfides of carbon, like copper carbonate (malachite green, genuine). Inorganic pigments are either natural salts and minerals extracted directly from the earth or rocks or are synthesized from salts and minerals.

NATURAL INORGANIC PIGMENTS

Natural inorganic pigments are found in nature as minerals or earth and are then ground, sifted, washed, and sometimes cooked (calcinated). The tem1 “natural mineral pigment” is used to describe natural inorganic pigments that are found as naturally occurring metallic salts, such as basic copper carbonate (azurite and malachite). The terms “earth color” and “earth pigment” are used to describe natural inorganic pigments that, in addition to being naturally occurring metallic salts, have significant quantities of clay and/or silica naturally mixed into them, such as hydrated ferric oxide and silica (raw sienna).

SYNTHETIC INORGANIC PIGMENTS

Almost all of the inorganic mineral pigments used today are manufactured, or synthetic. A synthetic version of a pigment may be chemically identical to the natural form, but it is produced artificially rather than naturally. It may also be an entirely new pigment created from minerals.

An example of a naturally occurring inorganic mineral pigment would be genuine ultramarine blue, which is derived from the gemstone lapis lazuli. Synthetic ultramarine blue, on the other hand, is made by a modem process that combines silica, alumina, soda, and sulfur, the basic elements of the naturally occurring lapis lazuli. Although, chemically, they are almost identical, lapis has a crystalline structure, which gives greater depth and beauty than the synthetic pigment.

One example of an entirely new synthetic mineral pigment is cadmium yellow, which was invented in 1817. Cadmium was mined from the earth and then extracted by turning it into a man-made salt that could then be used as a pigment.

Glass pigments are synthetic inorganic pigments in an unusual package. The pigment Egyptian blue, in use from 1500 B.C. to 500 A.D., was made from small glass particles called frit, which were ground to make the pigment. The exact color made from this pigment depended upon the size of the particles of glass. When the glass frit was ground to a small particle, it was a pale blue. A coarser grind produced a bright blue. This method of making color is still used in Japan and the name given to these pigments is “new earth.” The Japanese have elaborated on this method of using the size of the glass particles to help determine a color and have taken ten to fifteen base colors and ground them into ten to fifteen different sizes, resulting in a palette of more than one hundred colors.

SYNTHETIC ORGANIC PIGMENTS

There are currently about seven thousand organic dyes synthesized from coal tar and petrochemicals, to which approximately two hundred more are added each year. A dye, as opposed to a pigment, is soluble in the medium in which it is applied, thus making it impractical to make paint directly from a dye. For example, if a dye is mixed directly into linseed oil it would readily dissolve into it. However, when this mixture is mixed with other paints, applied to a painting ground, or used with a brush, it would dissolve into them just as readily, staining and bleeding uncontrollably. To regain control it is necessary to convert a dye into a pigment, which is insoluble in the medium. This is done chemically by attaching the dye to an insoluble, inert substrate, such as aluminum hydrate. (This is like dyeing cotton threads before weaving them into a fabric.) The result is called a dye-pigment, also known as a lake. After the dye is attached to something solid and insoluble, it can be formed into a paint just like any other pigment. (However, just as dyed fabric is tested to be colorfast, so must a dye pigment be found to be bleed-resistant). The synthetic organic pigments used in paints are “dye-pigments.”

Because many of these new pigments were less costly, and because there have been dramatic improvements on the range of available colors, many of them were made available before rigorous testing for bleeding was performed. For those painters for whom bleeding of one color into another, no matter how slight, is a problem, a simple, but not conclusive, bleed test can be performed.

A watercolor or gouache can be tested by first applying a dilute watercolor to a hot-press watercolor paper and left to dry. Then, using either a sponge or a large, soft, wash brush, attempt to wash away the watercolor. Most non-bleeding pigments will be almost totally removed. Pigments that tend to bleed will leave a significant stain.

Oil colors can be tested by applying a thin coat of white paint over a semidry and a dry layer of the test color. Fast bleeders will tint the white over-paint whether wet or dry, within one day. Slow bleeders will show color in the white with the semidry test color.

There are three major groups of synthetic organic pigments used in artists’ paints-anthraquinone, azo, and phthalocyanine. The first, anthraquinone, was 1,2-dihydroxyanthraquinone (alizarin crimson) developed in 1868 in an attempt to understand the coloring properties of madder root. This discovery led to the development of several indanthrones, of which one has come into use among artists as indanthrone blue.

The second group, azo, refers to a particular molecular arrangement among nitrogen-containing organic molecules. Although azo dyes were developed as early as 1880, it was the development of naphthol AS (naphthol red) in 1912 that heralded the birth of stable dye-pigments, which today include arylides (used to make hansa yellow, cadmium yellow hue), perinones (perinone orange), and naphthols (naphthol red, naphthol crimson, and cadmium red hue).

The third major group is the phthalocyanine dyes (phthalocyanine blue and green). Phthalocyanine was first discovered in 1907. It was rediscovered several times after that until the 1930s, when it was developed for artists’ use. Out of this group came the quinacridones, of which gamma-quinacridone (quinacridone magenta) and quinacridone violet b (quinacridone violet) have become relatively commonplace.

NATURAL ORGANIC PIGMENTS

Natural organic pigments include dyes that were converted into dye-pigments (lakes) and pigments made from either animal or plant sources. All plant-source pigments, such as madder, indigo, and gamboge, are dye-pigments. Animal or plant-source pigments are all dye-pigments with the one exception of carbon from bone. Examples of animal dye-pigments are sepia and Indian yellow.