|When conforming to an ASTM standard, the lightfastness rating of the color is printed on the label’s face, assuring artists of its permanence in their artwork.|
In a galaxy far, far away...
The saga of how pigments in artists’ paints are determined to be lightfast should begin like a Star Wars movie. It was a time and place that now seems far, far away.
It began in 1977 with the newly formed Inter-Society Color Council (ISCC) Project Committee #37 on Artists’ Materials. The committee’s project was “a study of the pigments used in the manufacture of artists’ paints and the labeling practices used by these manufacturers.” The committee membership and the American Society of Testing and Materials (ASTM) D01.57 subcommittee were virtually identical. It was composed of artists, art conservators, analytical chemists, color scientists, and artists’ materials manufacturers and their chemists.
ASTM Is one of the world’s largest organizations writing voluntary standards for materials, products, systems, and services. The subcommittee developed standards for artists’ oil, acrylic emulsion, watercolor, and gouache paints. Part of the standards developed was ASTM D 4303, “Test Methods for Lightfastness of Pigments Used in Artists’ Paints.”
The lightfastness test in these standards is based on the performance of a 40% reflectance tint of color. This means the color is mixed with white or white from the substrate in the case of watercolor paint. The subcommittee began testing generic pigments in generic formulations. Those that exhibited little change (less than 4Δ for a lightfastness rating of I) after exposure to both natural and artificial light were included in a table in the appendix in each standard.
Based on the tables in the appendix of the particular standard, the lightfastness rating of a specific pigment (or a mixture of pigments) is then printed by the individual manufacturer on their paint label. Manufacturers typically do not test their paint for lightfastness but rely on the table in the appendix of the standard. To bring color to market, the manufacturer purchases pigment from suppliers then looks the pigment up in the table and prints its corresponding ASTM lightfastness rating, i.e., I, II, III, IV, etc.
For the first time in history, the artists’ materials industry believed it had a “safe list” of lightfast pigments for oil, acrylic, watercolor, and gouache paint. It was an excellent time for paint making. But the euphoria did not last. The method used to test these pigments recently came into doubt as some pigments in the tables were re-tested and, to everyone’s surprise, failed the test.
Organic pigments were among those that were re-tested and failed. Due to the nature of organic pigments, it is believed that they are the most vulnerable type of pigments, so there is an urgent need to re-test them.
What is Being Done Now?
In November 2019, at a meeting of the ASTM subcommittee attended by only a few representatives of the many artists’ materials manufacturer members, it was decided to test colors under different conditions to develop a new test method. The project entails creating over 196 test samples to be tested by three manufacturers (Golden Artists Colors, Gamblin Artists Paints, and Natural Pigments). All samples will be tested in outdoor light conditions for several months at a test facility in Arizona. This enormous project will require at least two to three years to complete. The project began in 2020 with a target completion date of 2023.
Currently, the industry does not know which organic pigments are lightfast. We do not believe all organic pigments will fail. Many will pass, but we do not know which ones. The lightfast rating on the labels of artists’ colors is obsolete until each manufacturer has tested the pigment in their formulation using the new method created by the committee.
What Can Artists Do in the Meantime?
The best practice at the moment is to base your palette on inorganic pigments and use organic pigments sparingly, if necessary, and only in glazes—not mixed with white, until manufacturers have had an opportunity to test their pigments in their formulations.
What are Inorganic and Organic Pigments?
Inorganic pigments are derived from natural minerals or synthetic oxides, sulfides, or salts of metallic elements, such as iron oxides, cadmium sulfides, titanium dioxides, and lead carbonates. Organic pigments are made of carbon rings and chains from natural sources or derived synthetically. Examples of organic pigments derived from natural sources include madder lake, indigo, sap green, and carmine. Synthetic organic pigments include Hansa yellow, arylide, phthalocyanine blue and green, quinacridone magenta, pyrrole red, etc.
The specific chemical composition produces the color of inorganic pigments. In contrast, most organic pigments are chromophores in which an atom or group of atoms is responsible for the color of a compound.
Compared to organic pigments, most inorganic pigments offer relatively lower chroma, low to medium tinting strength, and moderate to high opacity. However, inorganic pigments are typically more stable in light and environmental conditions, offer excellent resistance to solvents, acids, and alkalis, and yield mixtures of excellent opacity.
Organic pigments offer high chroma, high tinting strength, and exceptional transparency. They are more susceptible to solvents, ultraviolet light, and weathering.
Examples of Inorganic Pigments
At one point in history, the variety of colors available in inorganic pigments was greater than the variety available in organic pigments. However, most of the new pigments developed today are among organic colors that surpass the variety of inorganic pigments available today.
Among the natural inorganic pigments are lapis lazuli or natural ultramarine, green earth or terra verte, yellow ocher, red ocher, brown ocher, siennas, umbers, and carbon blacks. Synthetic inorganic pigments include ultramarine blue, cobalt blue, chromium oxide green, hydrous chromium oxide green (viridian), yellow iron oxide (Mars yellow), red iron oxide (Mars red), cobalt violet, manganese violet, ultramarine violet, lead white, titanium dioxide, black iron oxide (Mars black), etc.
Organic Pigments Examples
Until the nineteenth century, organic pigments were derived from natural substances like plants and insects. These included madder lake, carmine lake, sap green, indigo, Tyrian purple, quercitron yellow, gamboge, and sepia.
Today, all organic pigments are synthetic organic compounds. The intermediate synthesis process is generally the same as that required by organic dyes. Therefore they are usually manufactured in the dye industry. However, as pigments, they are different from organic dyes. Organic pigments are a class of insoluble organic compounds of high tinting strength. Insolubility, in this case, means they have minimal solubility in water, organic solvents, and various kinds of media.
Standard synthetic organic pigments include Prussian blue or Milori blue (ferric ammonium cyanide), azo, anthraquinone, naphthol, phthalocyanine, pyrrole, and quinacridone pigments.
Organic pigments also consist of lake colors, such as madder lake, carmine lake, patent blue lake, etc. Since lake pigments are precipitated on minerals, the colors can also be classified based on the minerals, such as aluminum (Al) lakes, calcium (Ca) lakes, barium (Ba) lakes, and so on.
Note about Prussian Blue
There is debate about whether Prussian blue is an organic or inorganic pigment. It is on the borderline. Organic and inorganic is not an exclusive dichotomy and is subject to broad interpretation.
In the strictest sense, it is “inorganic” because it is a ferrocyanide, a compound of carbon and nitrogen with iron. This is similar to sodium cyanide, which is considered to be an “inorganic cyanide.” Organic cyanides are usually called “nitrile cyanides.” In this case, the carbon-nitrogen group is linked by a covalent bond to a carbon-containing group, such as methyl (CH3) in methyl cyanide (acetonitrile).
However, some consider Prussian blue an organic compound because they follow hard-and-fast rules like “if it has carbon, it’s organic,” to which I do not subscribe.
We have followed the former logic in the description of its chemical nature. Still, I have lumped it with organic pigments in this article because it shares one notable trait with all organic pigments—it produces color using a chromophore, which most inorganic pigments do not. The intense blue color in Prussian blue is a chromophore due to an intervalence charge-transfer absorption band centered at 690 nm. For this reason and its susceptibility to chemical degradation, I have classed it as an organic pigment for the purposes of this article.