Characteristics of Pigments

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

All of us see the labeling on our packaged foods. Some of us actually read the labels, but how many of us know what any of it means or how to translate it into useful information? The same is true of artists’ materials. This section explains the essential nature of pigments and provides a working knowledge of their characteristics to help you to distinguish one pigment from another so that you may use them to your best advantage.

NAMES OF PIGMENTS

The names used for various pigments often make about as little sense as the names that automobiles are assigned by their manufacturers, and frequently convey even less information. This is because several methods are used to name pigments and, even after several thousand years, one is no less confusing or more efficient than another.

Pigments used before the nineteenth century were most often named after their discoverer, the location of their source, their appearance, or just poetically. Due to advances in technology at the tum of the nineteenth century, new synthetic mineral pigments were being rapidly discovered. Many of these pigments were given names that included part of their chemical names-cobalt blue and cadmium yellow, for example. New pigments, whose exact chemical nature was not known, continued to be referred to by the names that had been used in the past.

As confusing as the names were at this time, they did relate to a specific pigment or group of pigments. This did not last for long and things got much worse. With the development of new pigments from the end of the nineteenth century until today, it has been found that many of these new pigments have characteristics that allow them to be used as substitutes for older, less permanent, or more costly pigments. Many manufacturers simply substituted the newer pigment for the old one, and kept the old name. Since this was not a coordinated effort among manufacturers, not everyone made the same substitutions. Therefore, today, it is easily possible for a pigment to have several names, or a color to have several pigments. If the name on the container is not what is inside, then information about such characteristics as permanency, compatibility, and transparency cannot be relied upon.

Some manufacturers are attempting to clarify this confusion by adding the names and numbers assigned to specific pigments by the Society of Dyers and Colourists of London to the common name on the label. (The same information is available from the American Association of Textile Chemists and Colorists, P.O. Box 12215, Research Triangle Park, NC 27709.)

In the Table of Pigments and Colors, I have included these Colour Index (C. I.) names and numbers. It is important, however, to understand what the C. I. names and numbers do not indicate-such characteristics as purity, quality, and concentration-nor do they distinguish among the shades that may exist within a specific pigment. Phthalocyanine blue, for example, has several shades that range from a reddish-blue to a greenish-blue, although all shades have the same C. I. name and number. Phthalocyanine blue is, with rare exception, contaminated with the carcinogen PCB and is always mixed with an extender when used as a paint, yet none of this information would be reflected by the names and numbers.

C. I. names and numbers can be helpful, occasionally, in comparing the quality of different brands of paint. Cadmium red paint, for example, can be made from either chemically pure (C. P.) or chemically concentrated (C. C.) cadmium sulfo-selenide. C. P. cadmium sulfo-selenide is assigned the C. I. name of Pigment Red 108 and the C. I. number of 77196, and C. C. cadmium sulfoselenide, which is extended with barium sulfate up to a concentration of 15 percent, is assigned the same C. I. name, but a different C. I. number, 77202. Cadmium sulfo-selenide is among the most costly of pigments and is therefore often the first to be compromised in the manufacture of paint. The finest paints are made with the purer and more costly Pigment Red 108, C. I. 77196.

PERMANENCE

Permanence is defined as continued existence. All existence is conditional or relative. Nothing is truly permanent. A substance may only be considered permanent because it has a longer continuity of existence than another substance. The only thing that does seem to be permanent is impermanence. A common metaphor for permanence is, “It’s set in concrete,” but even concrete has been recently found to be good for only a hundred years before it becomes brittle and begins to crumble.

It is commonly believed that the finest quality of pigment is synonymous with the highest degree of permanence. This is not the case. In fact, the oldest and most reputable companies attempt to offer the widest possible range of colors, which includes those that are both modem and traditional. Consequently, there will be a wide range of permanency in a given manufacturer’s selection of colors.

There are many factors that influence the permanence of pigments. These include weather, ozone, visible light, ultraviolet light, acid, alkali, water, oil, solvents, detergents, humidity, air pollutants, temperature, the type of medium used, the ground on which the pigment is applied, and whether it is mixed with other pigments. Despite all the technical advances and fancy equipment for the testing of permanency, the method is still basically the same-trial and error and the examination of past artwork. Technical advances have only developed the limited accelerated-aging testing procedures.

Accelerated aging tests are heavily based on theory and speculation. In addition, there is no standardization of testing procedures among manufacturers of pigments and paints. Tests can range from the very harsh, involving exposure of pigments, or paints, to direct sunlight outdoors, or a more moderate testing procedure proposed by the American Society for Testing and Materials (ASTM) in D 4303-83, Standard Test Methods for Lightfastness of Pigments Used in Artists’ Paints. The ASTM guidelines are for lightfastness testing only. They recommend using one of three types of light-sunlight, daylight fluorescent light, or xenonarc light that is filtered through glass indoors.

Permanency ratings for pigments that have been in use for seventy-five years or more can be safely relied upon because they have been proven under actual conditions. Ratings on pigments in use for less than seventy-five years, and especially for less than twenty-five years, have to be viewed with some suspicion. Pigments tested through accelerated aging rarely include more than three variables and cannot duplicate actual conditions. Although such testing has often proven reliable, there have been exceptions. The vast number of variables involved in the natural aging process makes it impossible to guarantee the relative permanency of a pigment. Consequently, any rating given should be considered only as a summation of the best available data and not as a warranty.

When a pigment or paint is exposed to light and a change is measured, the change is then translated in a scale. The Wool Scale is a common international method of rating the lightfastness of a color or a pigment. With the help of some manufacturers I have attempted further to translate this scale, which ranges from 10 to I, into practical terms for the painter.

10: Theoretically, absolutely lightfast

9: Has shown in practice, or theoretically will show, no observable change for two thousand years

8: No visible change for two hundred years in its pure form under normal museum conditions

7: No visible change for seventy-five years in its pure form under normal museum conditions

6: No visible change for twenty-five years in its pure form under normal museum conditions Less than 6 is not considered acceptable for fine or professional artwork and is classed as fugitive

The ASTM has proposed the following rating system, which is based on their suggested testing procedures for the determination of the lightfastness of pigments in oil or acrylic. Manufacturers and researchers who have conformed to these guidelines have assigned each color in question to one of three categories:

Category I: Excellent lightfastness. Very slight to no color change after the equivalent of one hundred years of indoor museum exposure.

Category II: Very good lightfastness. Less permanent than Category I, but satisfactory for most indoor painting.

Category III: Borderline. Use with caution.

Manufacturers of paint who are not using the ASTM guidelines have condensed scales like the Wool Scale down to four or five categories and then assigned each category a symbol such as a letter or an asterisk. Most manufacturers have been reluctant to make public the correlation of their categories to actual time, such as years, under museum conditions. Since there are so many variables involved with longevity, no manufacturer could possibly supply a warranty, and it was feared that making such a correlation would imply such a warranty. Talens was the first company I could persuade to break the ice for the benefit of all painters.

Although the actual boundaries drawn between the different pennanency groups, due to the variations in testing procedures, can vary significantly from manufacturer to manufacturer, the highest rating given to a paint by manufacturers is most often equivalent to 8 on the Wool Scale. The lowest rating, other than fugitive, is equivalent to 6 on the Wool Scale. In other words, if a manufacturer’s highest rating is *** or AA, the color should show no visible change in its pure state under normal museum conditions for at least two hundred years. A color assigned the lowest rating, such as * or B, should show no visible change for at least twenty-five years in its pure state under normal museum conditions. (This rating is considered the minimal acceptable limit for an artists’ material.) Colors labeled as “fugitive” or that have no rating, will show a visible change within twenty-five years. Although the Talens Company said that its middle category ** is roughly equivalent to seventy-five years under normal museum conditions, there is too much variation among manufacturers to generalize about the ratings used between the highest and lowest.

This information applies only to artists’ paints and not to any other materials such as graphic arts materials, industrial paints, or house paints, for which the term “permanency” means fifteen to twenty-five years, or one generation.

Despite the enormous spectrum of available artists’ colors, a number of painters have experimented with pigments that do not meet minimum permanency standards (at least 6 on a Wool Scale), such as fluorescents, enamels, even vegetable juices. Artwork produced with these materials must be considered “self-destruct” artwork. As long as the customer who collects such a piece of artwork understands its nature, it seems reasonable to produce and sell it. However, there have been several situations where an artist did not do his or her homework and unknowingly produced and sold a piece of “self-destruct” artwork. Most of these situations have been resolved by the artist exchanging the artwork for another. It would be wise for any professional artist to consider that there have been successful legal actions taken against the artists and galleries in such cases.

COMPATIBILITY OF PIGMENTS

Pigments, which are chemicals, can react with one another to form new chemicals. When this occurs in artwork the results, such as changes in a painting’s appearance, are often undesirable. Chemical reactions can be avoided by not using incompatible pigments or by treating the pigments so that they can be mixed together without reacting. The only proven treatment that will prevent most incompatible pigments from reacting with one another is to grind them into a drying oil, such as linseed oil, which will coat each pigment particle. If all the pigment particles are coated they will not be able to come into contact with one another and react. This is the reason that the greatest variety of pigments and colors is found among oil paints.

LeFranc & Bourgeois has supplied additional permanency information on its oil paint tubes in regard to compatibility. Paints that can be mixed without significant reduction in permanency have a large red M painted on the front of the tube.

Compatibility of pigments within oil paints is of little concern if the protective oil coating is not stripped away. However, if an oil paint is mixed with only a thinner instead of a medium, the thinner will wash away the protective coating.  Since most of the chemical reactions in this case are slow, it may be several years before the effects of this misuse become obvious. Unfortunately, the excessive use of thinners has become a common method of painting.

In watercolors, acrylics, and soft pastels, the binder does not isolate and prevent potentially incompatible pigments from coming into direct contact and reacting with one another. Consequently, such pigments are, in many cases, left out of the manufacturers’ range of colors when producing these media. This cannot be totally relied upon, and some basic knowledge of the most common incompatible mixtures is necessary.

The incompatibility of unprotected pigments containing sulfur and unprotected pigments containing lead is well known. For example, lead white, chrome red, chrome green, and Naples yellow should never be mixed with cadmium red, cadmium yellow, cadmium orange, cadmium green, vermilion, or ultramarine blue. When unprotected, these mixtures will tend to blacken. Another less common example is the incompatibility between unprotected pigments containing sulfur and some unprotected pigments containing copper, such as emerald green, malachite, verdigris, and azurite. Since only emerald green is still available as an artists’ pigment, there is little concern here.

There is another type of incompatibility that is not well understood. It has been known for some time that the mixing of any color, particularly organic dye-pigments (for example, alizarin crimson and cadmium red hue), with white can result in either a bleaching effect to the color or a staining effect·.to the white. This gave little cause for concern in the past because there were so few organic dye-pigments available. Today, more than half of the commercially available dyes and pigments are organic dye-pigments, and as more of these dye-pigments have been added to artists’ color lines so has there been an increase in reports of bleaching and staining. Until more is known, it might be wise to be conservative about using mixtures of organic dye-pigments and white.

Although it is rare, dilute watercolors and very dilute acrylics can suffer from incompatibility due to electrochemical properties. Cobalt blue, for example, when mixed with burnt sienna will form aggregates that settle to the bottom resulting in a gritty paint mixture.

TOXICITY

Indiscriminate application of powdered pigments to artwork or the manufacture of homemade paint without proper protection is simply suicidal. All pigments, particularly in their dry, powdered form, should be regarded as hazardous or potentially hazardous. Most of us would not live near a factory that produces many of the chemicals used as pigments, or a dump where these were disposed of, yet we take these same chemicals into our studios or, worse, our homes, with few or no safeguards.

The degree to which a pigment is toxic varies with the type of exposure. A pigment may be relatively nontoxic when exposed to the skin, moderately toxic when ingested, and highly toxic when inhaled. Virtually all pigments have their highest toxicity rating when either ingested and/or inhaled. The use of such solvents as turpentine, which can be absorbed through the skin, can carry pigments through the skin that would not otherwise pass. In the Table of Pigments and Colors, the highest toxicity rating given to a pigment has been used, regardless of the type of exposure. The toxicity rating also attempts to take into account the contamination of a pigment by hazardous material. Many pigments have forms in which they are less easily absorbed by the body and are, consequently, less hazardous, but it is unwise to rely on the possibility that the pigment in use is of the safer variety. It is wise to treat all pigments as hazardous or potentially hazardous. The following guidelines, which are used in the Table of Pigments and Colors, are for single exposures and are relative to the sensitivity of the individual. Carcinogens are labeled separately and allergies are not covered.

Highly toxic means that serious injury or death will result from absorption of a small amount, such as a mouthful, by a healthy adult.

Moderately toxic means that temporary to permanent minor injury will result from absorption of a small to moderate amount by a healthy adult.

Slightly toxic means that temporary minor injury will result from absorption of a small to moderate amount by a healthy adult. Larger quantities could cause greater damage.

Nontoxic means that no detectable injury will occur from absorption of small to moderate amounts by a healthy adult. Nontoxic does not mean safe or nonhazardous.

PURITY

The purity of a pigment can vary greatly. The average level of purity is the industrial grade, which is not chemically pure. The next grade is the “chemically pure” pigments, in which some trace levels of contaminates up to approximately I percent can still be found. The next level of purity is the grade used by the cosmetic and pharmaceutical industries. Most of the pigment that is readily available in bulk is of the industrial grade. For most industrial purposes, however, and for most amateur-grade paints, this grade would be considered adequately pure. In making the finest grades of artists’ paint, chemically pure pigments are preferred. Levels of purity above this in artists’ paint is unwarranted in all concerns, except where toxicity is involved.

Only the experienced can differentiate between these levels of purity. If you make small-scale paintings that will be viewed from close up, or use watercolors or egg tempera, or are involved with restoration, purity may be important. If you work very large or on murals, small impurities will not be as noticeable if you have to stand back twenty to thirty feet to see the whole artwork.