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The history and usage of pigments are intimately related to the development of technology, physics and chemistry. This chapter introduces some of these technical details, including an introduction to the nature of light, its characteristics and relationship to the three-color physiology of the human eye. Color mixing, how light interacts with matter, and how this relates to our primary interest, pigments, make up the remainder of the chapter. Some technical details are, necessarily, presented, but skipping them will not hinder you from plunging into the rest of the pigment story.

Colour is the place where our brain and the universe meet. Paul Cézanne.1 

The history and usage of pigments are intimately related to the development of technology, physics and chemistry. This chapter introduces some of these technical details, including the nature of light and how it interacts with the human eye and colored substances, and how pigments as colored substances fit in. You can easily skip the technical details of this chapter without detracting from the joy of the pigment journey you are about to embark upon.

If you looked up into Earth's big blue sky on 19 October 2017, you may have sighted a potential visitor from far beyond the solar system. The object, shaped like a cigar and dubbed 1I/2017 U1, was sighted just passing through our astronomical neighborhood. Harvard astrophysicist Avi Loeb lent credence to this possible “social call” by aliens in a blog post for Scientific American, later transformed into a book.2,3  If the inhabitants of 1I/2017 U1 had decided to stop off to say “hello,” and if their point of origin were a planet tied to the closest red giant, Gamma Crucis, they would have stumbled out of their spaceship functionally blind.

Why is this so? Why couldn't these aliens see non-red objects? We need light to see and there are different forms of light – the light coming from our sun is different from the light emitted by Gamma Crucis. What is the nature of this difference? To find out, we need to back up a bit, examine the nature of light, and then its relationship to color.

All light is accompanied by heat, so we infer that they share the same nature, called energy, i.e., anything that causes a change in motion. Energy exhibits many forms, such as heat, light, and electricity; all forms are interconvertible by interaction with matter. The ancient Greeks were the first to theorize regarding the nature of light and color. Aristotle (384–322 BCE) made the first important contribution to what is now the modern theory of selective absorption (Figure 1.4).4  Then Seneca (4 BCE – 65 CE), a Roman stoic philosopher, first noted that a prism reproduces the colors of the rainbow. Leonardo da Vinci (1452–1519) noticed that when light struck a water glass that it “spread out” as a colored image, but it was Isaac Newton (1643–1727) in 1666 who formulated modern color theory by experiment.

Newton passed a ray of sunlight through a prism and noted that it dispersed into an array of colors on the opposite wall. When he placed another prism in the path of the dispersed colors, they recombined to form sunlight again, what today we call “white light.” Newton had succeeded in dissecting sunlight into what we term the “prismatic” colors because they can be generated by a prism. He initially called them red, yellow, green, blue and violet2 in that order.5,6  Newton was astounded by this revelation, a seeming paradox that prompted controversy for the following 300 years.

First Thomas Young (1773–1829) in 1801, and then James Clerk Maxwell (1831–1879) in 1865, theorized that light is a special form of energy that can travel through empty space at great speed3 generating electric and magnetic fields as it goes; these fields oscillate with a certain frequency (symbol ν) which defines the energy of the light beam. The distance traveled by a single oscillation is called its wavelength (symbol λ), measured in a unit called a nanometer (nm), a billionth of a meter. This is a handy measure because our sunlight has a maximum intensity in the wavelength range of about 400 to 700 nanometers. Wavelength ranges above 700 nm, which defines the limit of what we call the color red, are in the infrared (IR) region; wavelength ranges below 400 nm, which defines the limit of what we call the color violet, are in the ultraviolet (UV) region. All of these energies are present in the sun's rays, which we can now call the solar spectral irradiance curve, shown in Figure 1.1, left.7  The entire range of wavelengths, called the electromagnetic spectrum, highlighting the visible spectrum and its colors, is shown in Figure 1.1, right.

Figure 1.1

Left: Solar spectral irradiance curve. The height of the curve (y-axis) shows the intensity of the radiation plotted against the wavelength in nm. Note that maximum intensity is in the green region, falling off on both sides toward the red and violet regions. Reproduced from ref. 7.4 Right: The electromagnetic spectrum showing all the regions; offset is the visible region, the only part of the spectrum that we can see. Note that the other regions of the spectrum are quite important in modern life as well. Reproduced from https://commons.wikimedia.org/w/index.php?title=File:EM_spectrumrevised.png&oldid=468982208, under the terms of the CC BY-SA 3.0 license, https://creativecommons.org/licenses/by-sa/3.0/deed.en.

Figure 1.1

Left: Solar spectral irradiance curve. The height of the curve (y-axis) shows the intensity of the radiation plotted against the wavelength in nm. Note that maximum intensity is in the green region, falling off on both sides toward the red and violet regions. Reproduced from ref. 7.4 Right: The electromagnetic spectrum showing all the regions; offset is the visible region, the only part of the spectrum that we can see. Note that the other regions of the spectrum are quite important in modern life as well. Reproduced from https://commons.wikimedia.org/w/index.php?title=File:EM_spectrumrevised.png&oldid=468982208, under the terms of the CC BY-SA 3.0 license, https://creativecommons.org/licenses/by-sa/3.0/deed.en.

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Now, getting back to our visitors from Gamma Crucis, a star that emits a range of energy that maximizes in the IR, invisible to human eyes. But the evolutionary pattern of the light receptor cells in our visitors' eyes would have necessarily maxed out in that range (because that's where all the light is on their planet) to find food or avoid becoming someone else's dinner. Our human eyes, for the same reason – by dint of biological evolution – are sensitive to the energy range of the sun's rays of maximal intensity that actually manages to penetrate our atmosphere – namely the red, orange, yellow, green, blue and violet radiation.8,9  Hence we call this range, or spectrum of energies, the visible region (for us!). If we traveled to our Gamma Crucis visitors' planet, it would be our turn to be blind – because their solar spectral irradiance curve would maximize in the infrared region.10 

All light sources are not the same, as we have seen. It depends upon the star you were born under, but also the type of lighting in your living room. Incandescent light seems “warm” to us because incandescent lamps emit a greater intensity of red light than the other colors; fluorescent lighting seems “cool” to us because it is poor in red light, but rich in yellow, green and blue light. Each type of light source has its own spectral irradiance curve that is quite different from ordinary daylight or sunlight.

Figure 1.2 (left) illustrates how human retinal photoreceptors, consisting of rods and cones, also theorized by Young,11  respond to light stimulus.12,13  The tri-chromatic system involves retinal structures called cones that contain photopigment proteins called photopsins that are sensitive to short (blue), medium (green) and long (red) wavelengths of light respectively. Light stimulus causes these proteins to undergo a structural change that activates their ability to send an electrochemical signal to the brain. The rods, which are far more numerous and sensitive than cones, have only one photopigment, rhodopsin. Rods operate in low light to give gray images.13  Prolonged gazing at a single color can exhaust the sensitivity of the respective cones, producing an afterimage of the complementary color. If you gaze at the magenta star (a color combination of red and blue) in Figure 1.2 (right) for several minutes and then shift your gaze elsewhere, you should see a faint green afterimage – green is the complementary color of magenta.

Figure 1.2

Left: Sensitivity curves for the rods and cones in the human retina. Reproduced from ref. 12, https://openstax.org/books/anatomy-and-physiology/pages/1-introduction, with permission from OpenStax under the terms of the CC BY 4.0 license, https://creativecommons.org/licenses/by/4.0/. Right: Magenta star.

Figure 1.2

Left: Sensitivity curves for the rods and cones in the human retina. Reproduced from ref. 12, https://openstax.org/books/anatomy-and-physiology/pages/1-introduction, with permission from OpenStax under the terms of the CC BY 4.0 license, https://creativecommons.org/licenses/by/4.0/. Right: Magenta star.

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The color mixing circles shown in Figure 1.3 provide the rationale for complementary colors. Figure 1.3 (left) depicts additive color mixing, which is the mixing of lights.14  Each of the three major regions of the visible spectrum can be roughly divided into the red (600–700 nm), the green (500–600 nm) and the blue (400–500 nm) regions. If all three colored lights are added together, they will produce white light, as seen in the center of the three overlapping circles (which is why sunlight appears white: it contains all of the spectral colors). We call red, green and blue the additive primary colors. When neighboring circles overlap, they produce the complementary colors: green + blue = cyan (a turquoise color); blue + red = magenta (sometimes called purple); red + green = yellow. So the complementary colors are yellow (to blue), magenta (to green) and cyan (to red). Now can you see why staring at a magenta star overwhelmed your red and blue sensitive cones for a while, and you could only see green for a time. In Figure 1.3 (right), we see the subtractive color system. Here, the three subtractive primary colors are yellow, magenta and cyan; their respective complements, shown in the overlapping neighboring circles are, respectively, blue (magenta + cyan), green (cyan + yellow) and red (yellow + magenta). A combination of all three subtractive primaries yields black. This is the system used in four-color printing.15  Check the colors of your laser printer cartridges – they correspond to the subtractive primaries plus black. We use this system when mixing pigments in a work of art: when complementary colors are placed adjacent to one another in a painting, they enhance one another producing a brightening effect; adjacent non-complementary colors, on the contrary, produce a duller effect (Chapter 13.4). Additive color mixing is good for adding lights such as theatre lights and the pixels on your television screen; it is also the principle behind photonic and structural colors (Chapter 16.3).

Figure 1.3

Left: Additive primary color circles; Reproduced from ref. 14. Right: Subtractive color primary circles. Reproduced from https://commons.wikimedia.org/w/index.php?title=File:Subtractive_color_mixing.jpg&oldid=464911059 under the terms of the CC BY-SA 3.0 license, https://creativecommons.org/licenses/by-sa/3.0/deed.en.

Figure 1.3

Left: Additive primary color circles; Reproduced from ref. 14. Right: Subtractive color primary circles. Reproduced from https://commons.wikimedia.org/w/index.php?title=File:Subtractive_color_mixing.jpg&oldid=464911059 under the terms of the CC BY-SA 3.0 license, https://creativecommons.org/licenses/by-sa/3.0/deed.en.

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The seemingly contradictory perceptual results obtained from mixing light stimuli and mixing colored materials such as pigments has been a source of confusion for at least 2000 years. The system we describe here is based on color mixing. Other systems based on color perception and on color matching are very useful in other applications, particularly in the fashion world and computer color measurement techniques. The various color order systems that artists and color technologists have dealt with over the years to measure and explain these perceptions have great consequences in areas such as color matching, retouching, and art conservation.16,17 

When light strikes a surface, many things happen to it simultaneously depending upon the nature of the surface. It can be reflected; that is how you can see the surface. Some light can be absorbed. If all the wavelengths of visible light are absorbed, you will see the color black; if almost all of the wavelengths of visible light are reflected, then you will see white. However, if only some of the wavelengths are absorbed, then you will see only the wavelengths that are reflected to your eye. For example, if all of the blue and green wavelengths (400–600 nm) are absorbed, you will see only the wavelengths not absorbed, 600–700 nm, corresponding to red light; you perceive that object as red. The blue–green combination, cyan, is the complementary color to red because when added back into red, it completes the visible spectrum. This effect is called selective absorption, a very important pigment property (Figure 1.4). The degree of selective absorption depends on the intrinsic chemical structure of the pigment: electrons undergo transitions which correspond to discrete energy levels from 400 to 700 nm in the pigment molecule.

Figure 1.4

Illustration of selective absorption. Left: Solar spectral irradiance curve for normal daylight over the range of the visible spectrum, appearing white; Right: Selectively absorbed light (gray lines) by an object that reflects the remainder of the visible spectrum, appearing reddish-orange.

Figure 1.4

Illustration of selective absorption. Left: Solar spectral irradiance curve for normal daylight over the range of the visible spectrum, appearing white; Right: Selectively absorbed light (gray lines) by an object that reflects the remainder of the visible spectrum, appearing reddish-orange.

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The color you see also depends upon the source of the light. The sun Is our universal light source. Other light sources such as incandescent and fluorescent lighting do not have the same spectral irradiance curves as the sun, so objects do not appear the same when lighted with different light sources. Some sources are very rich in the red end of the spectrum, so their light will give a warmish glow; others contain very little red light, so will seem colder to us. In comparing colors of objects, it is very important to have the same light source, otherwise they will appear different.18 

Light can also be scattered as it travels. This phenomenon accounts for the fact that the sky is blue and that sunlight, even though we call it “white” light, has a slightly yellow tinge. The molecules in our atmosphere, mostly nitrogen and oxygen, act as scattering centers. As sunlight enters the atmosphere, the air molecules divert the various rays, and violet and blue rays are more strongly scattered than the other wavelengths. In fact, the sky is more violet than blue, but our eyes are not as sensitive to violet. Physicist Lord Rayleigh (1842–1919) found that the shorter the wavelength of light, the greater the scattering.19 

Scattering also affects the appearance of a pigment's color. Early artists knew that a pigment's particle size affected the depth of its color. Finely ground pigments are paler than coarsely ground ones because scattering power increases as particle size decreases. As the particle size decreases, the amount of light scattered increases relative to the amount of light absorbed, so the smaller particles appear lighter in color. In fact, some pigments can only be ground very coarsely because too much grinding causes their colors to fade so much that they become useless.20 Figure 1.521  shows an example of coarsely and finely ground malachite crystals; not only does the color depth change but also the hue.

Figure 1.5

Malachite crystals. Left: coarsely ground pigment; Right: finely ground pigment. Reproduced from ref. 21 with permission from Natural Pigments (naturalpigments.com).

Figure 1.5

Malachite crystals. Left: coarsely ground pigment; Right: finely ground pigment. Reproduced from ref. 21 with permission from Natural Pigments (naturalpigments.com).

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Other things can happen if the object receiving the light is transparent. Light striking a clear glass window is transmitted, i.e., all the wavelengths pass right through allowing you to view what is on the other side. If the glass is red, you will see only the wavelengths of the red light. When light travels from one medium into another, e.g., from air into water, both the velocity and the wavelength change, but the frequency always remains the same. Put simply, light slows down or speeds up when it moves from one medium to another. The degree of slowing down compared to travel in a vacuum is measured by the refractive index (RI) of the medium. Each frequency of light has its own RI, which accounts for the different behaviors of light traveling through a prism and, indeed, through the atmosphere to produce rainbows and blue skies. This property is also extremely important with respect to paints; it determines whether a paint layer is transparent or opaque when formulated with different pigments and binders because both pigment particles and binders are transparent and each has its own refractive index. If they are similar, the paint will be transparent; if they are quite different, the light will be diverted many times in traveling through the particles, and not much will be reflected, so the paint will be opaque (Figure 1.6).

Figure 1.6

An incident ray of light travels from air into a paint layer. When it strikes the paint medium, it is refracted (changes direction) because the RI of the medium is different from the RI of air. It undergoes multiple reflections and refractions in the transparent pigment particles. It may be reflected back into the air in the process. If the ray does not reach the ground, it cannot reflect the ground back to the viewer; in this case the paint is opaque. Reproduced from ref. 18 with permission from Springer Nature, Copyright 2013.

Figure 1.6

An incident ray of light travels from air into a paint layer. When it strikes the paint medium, it is refracted (changes direction) because the RI of the medium is different from the RI of air. It undergoes multiple reflections and refractions in the transparent pigment particles. It may be reflected back into the air in the process. If the ray does not reach the ground, it cannot reflect the ground back to the viewer; in this case the paint is opaque. Reproduced from ref. 18 with permission from Springer Nature, Copyright 2013.

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Early humans, and even late modern humans, were ignorant of the properties of light discussed here. But, by trial and error, they developed skills in transferring pigments, the concrete substances that selectively absorbed visible light, to any desired surface. Now that we know the principles, we have to see what pigments themselves are all about.

Broadly defined, a pigment is any substance that imparts color. All materials used for coloring other objects may be classified as either pigments or dyes, depending upon the method of application. Typically, dyes are dissolved in a medium into which the object to be dyed is then immersed. A physical or chemical interaction between the dye and a substrate is necessary for the dye to become “anchored” in place. Pigments, on the other hand, do not react with the substrate and must be applied by first being suspended as insoluble particles in a medium, called the vehicle or binder, which allows the pigment to adhere to the surface. The more general term for both dyes and pigments is “colorant.”

The colorants you will encounter in this book are drawn from all three of the natural “kingdoms,” animal, vegetable and mineral. We call mineral pigments those derived from either naturally occurring rocks or other geological substances, such as ores, found in the earth, or synthesized from similar starting materials. Chemists term them “inorganic,” an adjective that applies to materials made from any element except carbon. Since none of these substances is soluble in ordinary solvents, all of them require a binder. Colorants derived from plants and animals are carbon-based or, in chemical terms, organic compounds. Many are soluble in water or in another liquid and can be used as dyes; many are also often used as pigments, but some require special treatment to render them opaque enough to be used in paints.

Just as we need three dimensions, length, width and height, to describe an object, so color, and therefore pigments, need three dimensions as well: hue, chroma and value.

  • Hue is the actual color of the pigment in terms of where it falls in the visible spectrum. For example, a green-hued pigment will reflect wavelengths largely in the green region, about 500 to 600 nm.

  • Chroma, or amount of color, is sometimes called intensity or saturation. It is not related to either the hue or the lightness or darkness of the pigment, but it is an index of how pure or intense the color is.

  • Value is a measure of the lightness or darkness of a pigment, usually measured from 1 to 10 on a grayscale, where 1 is more white and 10 is more black. All three characteristics are affected by the choice of a binder and how well it is bound to the pigment.22 

In addition to the refractive index (RI), described earlier in this chapter, several other pigment qualities are important.

  • Compatibility. Pigments can be compatible or incompatible with other pigments or with some binders and varnishes.

  • Undertone and Masstone. These qualities describe a pigment's difference of appearance when applied as a transparent or an opaque film.

  • Lightfastness. Fastness is a pigment's ability to resist fading when exposed to ultraviolet or visible light.

  • Tinting Strength. This quality refers to a pigment's ability to color a mixture.

  • Covering Power. A pigment's degree of opacity.

  • Toxicity. All inorganic pigments made with heavy metals (such as mercury and lead) have varying degrees of toxicity. Some organic pigments may be carcinogenic, another form of toxicity.23 

Color, at least on the visual level, is the most obvious property an object can have. Hardly a day goes by that one does not find it necessary to name a color, and we must presume that this need goes back to the first societies and the first use of human language. You will find as you read the rest of this volume that many pigments were first named according to their colors, as were many of the chemical elements. As the need developed in art, commerce, and in many other areas, color names continued to be invented so that today, there are almost 10 000 color names. The need to translate among color vocabularies so that their meaning is clear24  is a daunting task since some of them are completely unintelligible to persons working in a different field.

It would take many pages to list the entire body of colorants used as artists' pigments. Furthermore, even if they were all listed by name, ambiguities would creep in because some very different chemical substances have historically been given the same pigment name. If there was confusion and ambiguity about colorant nomenclature in general, the situation was exacerbated many-fold by the developments in the dye and color industry. Almost every day a new colorant was developed in the patent races following the 1856 discovery of the first commercially produced dye. Many of these compounds had uncertain identities, complex structures and arbitrarily bestowed names.25 The Colour Index (CI) dispenses with this uncertainty by listing identification numbers and names given to individual pigments by the Society of Dyers and Colourists in the UK and the American Association of Textile Chemists and Colorists in the United States.26  Pigments are identified by both their common names and their Colour Index Constitution Number when available. The Colour Index currently lists over 13 000 generic colorant names under more than 45 000 commercial names, although very few of them are of interest to artists. The Color of Art Pigment Database27  has sliced out the latter and provides full CI information for each pigment. Another reference, equally monumental in its scope, is The Pigment Compendium.28  It contains information and as-complete-as-possible literature sources (by “plundering” specialist surveys and “hunting down” original sources, as the compilers put it) on over 1000 separate compounds listed with a consistent naming system in almost 2600 entries, 80% of them made of inorganic materials, 16% from natural organic sources, and 4% synthetic organic pigments.

In Chapter 16, we will see that the traditional definitions and properties of pigments described here become more fluid. Their colors can be manipulated by applying various forms of energy, and their very substance can be generated by some exotic creatures. In some instances, pigments will have no substance of their own at all but will depend upon assemblies of precisely arranged structures. Meanwhile, our pigments will march on through history, responsive to the drums of cultures and civilizations.

In the next chapter, 21st century technology will give way to the cutting edge skills of humans who lived and worked 45 000 years ago and who still speak to us “through shimmering patterns [thrown] across slabs of solid rock.”29  Their voices remind us of our own vitality and mortality, echoing over a time span nine times greater than our own recorded history.

1

Glossary entries and chapter cross-references are in boldface.

2

Newton later included “orange” and “indigo” largely to bring the number of spectral colors up to seven, matching the musical scale or his version of harmony and aesthetics. In his first lecture on the topic, he admitted that the colors were continuous and that specific lines of demarcation were difficult to discern.

3

Light travels with a velocity of 1 86 000 miles s−1 (or 3 00 000 km s−1) in a vacuum. In other media, such as oil or water, it slows down, which affects the appearances of paints.

4

The Solar Spectral Irradiance (SSI) CDR used in this study was acquired from the NOAA National Centers for Environmental Information (formerly NCDC) (http://www.ncdc.noaa.gov). This CDR was developed by Judith Lean at the Naval Research Laboratory (NRL) and Odele Coddington, and Doug Lindholm at the University of Colorado Boulder's Laboratory for Atmospheric and Space Physics (LASP), in collaboration with Peter Pilewskie, and Marty Snow (also at LASP), through support from NOAA's CDR Program using the NRLSSI2 model.

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