2.5 Pigment types, with examples

Pigments get their colors in a few different ways, and the way decides the shape of the reflectance curve. The cadmiums are semiconductors, crystals whose absorption switches on sharply at one wavelength: they absorb every wavelength shorter than a cutoff and reflect everything longer, so their curves are a single step, placed at a different wavelength for yellow, orange and red. The iron oxide earths, the cobalts and chromium oxide owe their colors to metal ions, charged atoms of iron, cobalt or chromium held in a crystal; their absorptions are broad and their curves are rounded, so they are duller than the cadmiums. The organic pigments, the phthalocyanines, quinacridones, azo yellows, pyrroles and dioxazine, are colored molecules; their absorptions are strong and often fall in bands with clear edges, and they are transparent because their particles are small and absorb so strongly that their scattering hardly shows. Blacks absorb everything and whites absorb nothing. Figure 11 groups the curves of 55 of the 78 Golden paints this way.

Figure 11. The measured masstone reflectance of the Golden Heavy Body paints [24], grouped by the kind of pigment, each curve drawn in the paint’s own color. The cadmiums are clean steps, the earths rise gently, the cobalts have humps, and the dark organic pigments lie along the floor.

Each family has a look of its own. The cadmiums are the cleanest curves in the set, each a single step from almost nothing to almost everything; cadmium yellow light makes its whole climb between 480 and \(540\,\mathrm{nm}\). The iron oxides rise gently from the blue to the red and never reflect more than half the light, and the umbers stay low everywhere. The cobalt and chromium pigments have humped curves with two or three bands, and cobalt blue, besides its blue band, rises again at the far red end, where most eyes barely see. Ultramarine reflects a narrow band of blue and nothing else. Of the organic pigments, the phthalocyanines lie flat near the floor with a small bump in the blue or blue-green, the quinacridones rise steeply to a red plateau, the azo yellows are steps like the cadmiums but a little lower, and dioxazine and anthraquinone blue are flat and near black. The paints Golden labels as hues are mixtures of several pigments made to imitate a historical color, and their curves combine those of their ingredients.

The cadmium step comes from the crystal as a whole. A semiconductor absorbs every photon carrying more than a certain energy and none carrying less, and since blue and violet light carry more energy than red, a cadmium absorbs everything on the blue side of a cutoff and reflects everything on the red side. In cadmium sulfide the cutoff falls in the green, so it absorbs the violet, blue and green and is yellow; replacing some of the sulfur with selenium moves the cutoff through orange to red [57]. Because the cutoff belongs to the whole crystal rather than to single atoms, it is sharp, and that sharpness gives the cadmiums the highest chroma of any opaque pigment.

The earths are colored by iron, which absorbs the short wavelengths heavily and the long ones partly and so gives them their gently rising curves [68]. Yellow ochre and raw sienna owe their color to goethite, an iron oxide with water bound into it; red oxide and burnt sienna owe theirs to hematite, which has none, and heating goethite turns it into hematite, which is what “burnt” means. The umbers contain manganese as well as iron and absorb across the whole spectrum. In the cobalt pigments and chromium oxide the metal ions sit in a crystal that sets where they absorb: cobalt blue absorbs the orange and red and passes the blue, cerulean and cobalt teal are cobalt in other crystals with the bands shifted, and chromium oxide absorbs the violet and the red and passes a band of green [57]. Ultramarine’s color comes from small clusters of three sulfur atoms trapped in a cage of sodium aluminum silicate, which absorb a broad band centered in the orange and pass the blue, and since its refractive index is about the binder’s, its particles barely scatter and it is transparent [8].

The organic pigments are molecules whose electrons absorb visible light. The color comes from the extent of a network of alternating chemical bonds along which those electrons can move, and the pigment is a solid because the molecules pack into tiny crystals that do not dissolve in the binder. Phthalo blue absorbs strongly from the yellow-green through the red and weakly in the blue, and the phthalo greens are its relatives with chlorine and bromine added. The quinacridones absorb from the violet to the orange and pass the red and some violet, which puts them on the magenta side of red. The hansa and diarylide yellows absorb the blue with an edge like the cadmiums’ but less sharp. Pyrrole red has its edge in the orange and, with larger particles than the others, is nearly as opaque as a cadmium. Dioxazine absorbs almost everything and passes a trace of violet and red, so it is nearly black in masstone and a vivid violet in tints [57, 25]. The particles of these pigments are a third or less of the size that scatters best, and in the colors they absorb, their absorption is so strong that the little scattering they do is hard to see [31].

Carbon black absorbs nearly everything across the spectrum; mars black, an iron oxide, absorbs less evenly and scatters more, which makes it the more opaque black and the warmer one in tints. Titanium dioxide absorbs nothing in the visible, apart from a trace at the violet end where its absorption of ultraviolet light spills over, and scatters more than any other white [8]; Section 3.7 shows what that and the scattering do in mixtures. The math box at the end of this lesson adds the chemistry behind these curves. Table 1 lists all 78 paints with their pigments, opacity ratings and measured colors.

TipThe math behind this lesson

Where pigment colors come from. A semiconductor absorbs every photon with more energy than its band gap and none with less. Cadmium sulfide has a gap of 2.4 electron volts, the energy of light at about \(515\,\mathrm{nm}\), and the sulfoselenides have smaller gaps [57]. In Figure 11, cadmium yellow light rises from 5 to 90 percent between 480 and \(540\,\mathrm{nm}\) and cadmium red light between 560 and \(620\,\mathrm{nm}\). The iron oxides are colored by the Fe\(^{3+}\) ion: transitions within its \(d\) electrons, which are weak, and a charge-transfer absorption in the ultraviolet and blue, which is strong [68]. Goethite is the hydrated oxide and hematite the anhydrous one, and calcining converts the first to the second. Yellow oxide and raw sienna peak around \(600\,\mathrm{nm}\) and fall off a little in the red, red oxide and burnt sienna climb through the red, and the umbers stay below 10 percent everywhere. Cobalt blue is cobalt aluminate, with Co\(^{2+}\) in a tetrahedral site whose \(d\) electrons absorb the orange and red; cerulean and cobalt teal are cobalt in chromium and titanium oxides; chromium oxide is colored by Cr\(^{3+}\) [57]. Cobalt blue reflects a band around \(460\,\mathrm{nm}\) and rises again above \(650\,\mathrm{nm}\); cobalt teal reflects a broad band from 450 to \(550\,\mathrm{nm}\); chromium oxide reflects a modest band at \(540\,\mathrm{nm}\) and more in the far red. Ultramarine’s color is the S\(_3^-\) radical ion, which absorbs a broad band centered near \(600\,\mathrm{nm}\); it reflects a narrow band at \(450\,\mathrm{nm}\), and its refractive index is about 1.5 [8]. In the organic pigments the color comes from a conjugated system of double bonds. Copper phthalocyanine absorbs strongly from 550 to \(750\,\mathrm{nm}\) and weakly below 500, and the phthalocyanine curves have their small bump at 450 or \(500\,\mathrm{nm}\). The quinacridones absorb from 400 to about \(600\,\mathrm{nm}\) with the edge placed by the substituents; the azo and arylide yellows have their edges at 490 to \(520\,\mathrm{nm}\); pyrrole red, diketopyrrolopyrrole, has its edge at \(600\,\mathrm{nm}\) [57, 25]. These pigments have particles of about \(0.1\,\mu\)m [31], and their refractive indices, 1.6 to 2, are not especially close to the binder’s: Okumura gives 1.66 for an arylide yellow against 1.48 for the acrylic binder [59]. Titanium dioxide’s ultraviolet absorption has a tail that begins at about \(410\,\mathrm{nm}\) [8].

The full list of the 78 paints, with their pigments and opacity, is in the paint table.

Sources

[8] G. Buxbaum and G. Pfaff (eds.), Industrial Inorganic Pigments, 3rd ed., Wiley-VCH, Weinheim, 2005.

[24] Golden Artist Colors, Inc., spectral reflectance data for 78 Heavy Body acrylics (10 mil wet drawdowns over a white Leneta card, D65, \(10\degree\) observer, 400–700 nm), published by A. Glassner and E. Haines, Golden Paint Spectra, https://www.realtimerendering.com/golden.html, 2025.

[25] Golden Artist Colors, Inc., Heavy Body Acrylics: Pigment Detail Chart, https://goldenartistcolors.com, 2025.

[31] W. Herbst and K. Hunger, Industrial Organic Pigments: Production, Properties, Applications, 3rd ed., Wiley-VCH, Weinheim, 2004.

[57] K. Nassau, The Physics and Chemistry of Color: The Fifteen Causes of Color, 2nd ed., Wiley, 2001.

[59] Y. Okumura, Developing a Spectral and Colorimetric Database of Artist Paint Materials, M.S. thesis, Rochester Institute of Technology, 2005, https://repository.rit.edu/theses/4892.

[68] D. M. Sherman and T. D. Waite, Electronic spectra of Fe\(^{3+}\) oxides and oxide hydroxides in the near IR to near UV, American Mineralogist 70 (1985), 1262–1269.