Classification of Pigments

All paint pigments are manufactured products; regardless of whether they are obtained from natural or artificial sources, they will never be satisfactory unless they have been carefully prepared by expert methods.

The distinction between "natural" and "synthetic" pigments is that the synthetics are made by chemical reactions and processes from raw materials which are not in themselves pigments; the natural are refined from native ores or earths by physical and chemical treatments which convert these crude coloring matters into improved pigments with the desirable color and pigment properties. [p. 31]

Native Earths
Highly colored clays, soils, and rocks occur all over the world; however, the best types occur in limited districts where superior brightness or clarity of tone or freedom from impurities makes them outstanding, or where there are well-established facilties for mining, pulverizing, and purifying them economically. The approved pigments made from natural earth sources are of absolute permanence for all paint uses. [p. 31]

Calcined Native Earths
Upon calcining [roasting in a furnace], the earth colors tend to become warmer and more reddish and in some instances more transparent. The calcined or burnt earth colors have the same high degree of permanence as the raw earths. [p. 31]

Synthetic Organic Pigments
[The chemical term "organic" refers to compounds that contain carbon as one element.] Originally the brilliant, transparent organic pigments were made from water-soluble dyes obtained from plant and animal sources. Scarcely any of these were sufficiently lightfast to qualify as a permanent pigment in the twentieth century. In the mid-nineteenth century the invention of a rainbow of synthetic colors was started; many of these were improvements over the earlier natural products, and one of them, alizarin [1876], is still in use. Because all of them were made from derivatives of coal tar and because most of these were made from the specific derivative aniline, they became known as coal-tar colors or aniline colors, terms that in the past have been used as derogatory words. In the early twentieth century many pigments were developed that were vastly improved over the older ones; some of these are still in industrial use. Because several promising red pigments were prematurely adopted during this period and had to be withdrawn, the authoritative groups are slow to adopt new products until the most careful investigations have been made. During the 1960s more than three dozen organic synthetic pigments were considered eligible for adoption. It was not until the phthalocyanines were introduced in 1935 that brilliant organic synthetic pigments that could rank with inorganic pigments in terms of permanence became available.

As recently as 1966, the second edition of this book listed only the following synthetic organic colors: alizarin reds, phthalocyanine blue, phthalocyanine green, Hansa yellow, and, tentatively introduced Quinacridone red. The same edition also uses the words "aniline colors" in a nonpejorative way. Today, there exist over 500 organic synthetic pigments with fastness properties of acceptable permanence for artists' colors. However, only those colors listed as acceptable by ASTM are generally used by paint manufacturers. Paint manufacturers must choose the pigments they believe to be generally most useful to the artist. To include literally hundreds of colors in an oil color line, for example, would be impractical at best. [p. 32]

Synthetic Mineral Colors
This term includes a multitude of inorganic chemical pigments, both good and bad; no general statement can be made to cover their properties, and the characteristics of each must be learned separately. Some are made by the simple mixture of two solutions of salts which will combine to make an insoluble, colored precipitate; others entail complex processes in several stages; all must be made under expert control. Some of our most permanent colors are among the synthetic inorganic pigments, while others are not sufficiently permanent for artists' [p. 31] use. As a general rule, those with processes including a high-temperature furnace treatment are the most permanent of all. [p. 32]

Lakes & Toners
Formerly a widely used term but now seldom encountered is the term "lake" -yellow lake, madder lake, scarlet lake, and so on. To make a synthetic pigment, a dye in water solution was chemically precipitated onto a base, which was one of the inert pigments [see document]. Depending on the proportion of inert pigment used, a lake could be concentrated or weak. A tone is a sythetic pigment in its most concentrated form, with little or no inert material, or else a dyestuff that is naturally insoluble; the latter is the modern definition of a toner. One of the most serious defects of the old-fashioned organic pigment was its tendency to bleed, or become soluble in the paint and seep into or permeate the surrounding oil paint. Or if such a scarlet lake was used in an underpainting and a white oil paint layer was painted over it, the white would soon become tinged with pink. Modern synthetic pigments seldom have this property, but it is not impossible for a bleeder to be used instead of beng rejected.

A number of colors of exceptional brilliancy of hue, made with the older, impermanent dyestuff lakes, are still on the market; these are intended for use by artists who work for reproduction, as in illustration and commerical art where the original painting has no particular value as a unique work of art. These colors, which bear names that will not be found in the lists of pigments approved for fine-arts purposes, such as scarlet lake, peacock blue, and geranium red, are valuable for the purposes for which they are intended but should never be used for permanent easel painting. There is more on the subject of this class of artists' materials in Aqueous Paints. [pp. 32-33]