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Aug. 18, 2010

Artistic Elements: Paint Pigments and the Periodic Table

by Sam Kean

Click to enlarge images

[The second in a series of posts on the elements in art, by D.C. writer and Sci-Arts guest blogger Sam Kean. Check back each Wednesday for a new artistic element.]

Biologist Edward O. Wilson once said, “In the natural world, beautiful usually means deadly.” Wilson was referring to how the most brightly colored snakes, frogs, and insects usually harbor the deadliest venoms. But the sentiment applies equally well to the periodic table: Some of the nastiest elements—lead, cadmium, arsenic, mercury—make the most beautiful paints and pigments. In fact, the artistic innovations of some of the most iconic painters, especially in the 1800s and early 1900s, can be traced back to their use of these elements.

Henri Matisse used color more boldly and effectively than almost any painter in history. But Matisse might never have created his most memorable works without newly invented pigments like cadmium red. Cadmium red is a variation of a pigment first introduced around 1820—cadmium yellow, a favorite of Claude Monet’s. (Cadmium yellow consists of cadmium plus sulfur; cadmium red uses another often-toxic element, selenium, in place of some sulfur.) Matisse added other new pigments to his palette for bold blues and greens, and constantly experimented. He once boasted he was “half a scientist.”

Henri Matisse, "The Dessert: Harmony in Red (The Red Room)," 1908

Henri Matisse, "The Dessert: Harmony in Red (The Red Room)," 1908

The vivacious landscapes of Paul Cezanne make liberal use of a pigment called emerald green (or Paris green) that contains arsenic. In fact, if not for Cezanne and other artists, emerald green—and another arsenical pigment, Scheele’s green—would be remembered today mostly for the mass poisonings they caused when used to dye dresses, socks, and other garments. But artists might not have escaped the carnage: some historians have tried to link Cezanne’s diabetes, as well as ailments suffered by Monet and Vincent van Gogh, to the toxic pigments they splashed on their canvases to capture the color so important to their work.

Paul Cézanne, "Green Apples," c. 1873

Paul Cezanne, "Green Apples," c. 1873

Artists in the 1800s owed most of their new pigments to the emerging field of synthetic chemistry, a field that found (mostly) beneficial uses for even toxic elements. Before synthetic chemistry, artists generally used pigments either from plants or from metal-rich minerals dug from the earth. Blue pigments—important for religious work depicting Mary—often came from copper- or cobalt-rich minerals. And for centuries, virtually all oil painters prepped their canvases by coating them with a base called lead white. Lead white also occasionally found use in flesh tones, though it has an unfortunate tendency to discolor in the presence of gas lamps, leaving some white angels in old illuminated books looking more like the angel of death.

Even Rembrandt—known for his somber colors—wielded bright metallic pigments when needed. For instance, there’s “Belshazzar’s Feast,” his depiction of the famous “writing on the wall” story from the Old Testament. A king was feasting with his retinue the night before a battle when the hand of God appeared and wrote a gloomily prophetic statement on the castle wall. To highlight the miracle and make the letters shine, Rembrandt used one of the brightest yellows available before cadmium yellow, lead-tin yellow.

Rembrandt, "Belshazzar's Feast," c. 1635

Rembrandt, "Belshazzar's Feast," c. 1635

Surprisingly, some pigments, especially reds and yellows, have such vibrant colors for the same reason that semiconductors are so useful in electronics. It has to do with electrons—charged bits that whirl around inside atoms. In pure metals, atoms share electrons with each other freely. This causes electricity to flow, making metals good conductors. Semiconductors don’t share electrons quite as freely: their electrons first have to jump over small energy barriers (called band gaps) before electricity can flow. This barrier lowers conductivity, but allows semiconductors to control electricity more precisely.

Engineers don’t use semiconductors in electronics for reasons related to color—they’re more interested in the electricity. But on a molecular level, the movement of electrons is also restricted inside some pigments. And one byproduct of that restricted movement is a bold color. The toxic parts of many pigments are metals, but when mixed with nonmetals—as in cinnabar, a gorgeous red mercury-sulfur mix—the combination behaves more like a semiconductor. So electrons in the mercury-sulfur molecules have to jump over energy barriers, too.

When white light hits the surface of a painting, the pigment absorbs some of the light’s colors as energy. This allows it to launch electrons over the barrier. The key is that light near the purple end of the visible spectrum is more energetic than red light. So pigments are more likely to absorb the purplish light—and let the reds and yellows reflect back to our pupils. Not all pigment coloration works this way, but it’s fascinating to think that Monet and others, in exploring the pigments of the periodic table, took advantage of the same basic phenomenon that drives computers, iPods, and digital cameras today.

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