The red barns that dot the American countryside are, more often than not, painted red. They're not red because they would otherwise be too hard to see--they're big buildings. They're not red to be fashionable. They're red because red paint is historically the cheapest. And the next logical question--if you haven't dozed off thinking about barns and red paint--is why red is the cheapest. And that question, as answered by Google's Yonatan Zunger in a Google+ post and picked up by BoingBoing, is actually fascinating. Red paint is cheap because of billions of years of nuclear fusion.
Heavy, right? Zunger titled his post "How the price of paint is set in the hearts of dying stars," but before he arrives at his explanation, he offers a crash course in how colors work and how we create paints. Nuclear fusion comes into play when he gets into the root of how and why pigments are formed. But at the high level, we have to start with paints; paints are formed by the combination of a pigment, typically formed from a mineral, and a binding agent, like an oil or acrylic, that makes the liquid stick to something when it dries.
Zunger first posed this question: what makes a good, cheap pigment? "To be a good pigment, first and foremost, something has to have a nice, bright color," he wrote. "The way pigments produce color is that light shines on them, and they absorb some, but not all, of the colors of light. (Remember that white light is a mixture of many colors of light) For example, red ochre, a.k.a. hematite, a.k.a. anhydrous iron oxide (Fe2O3), absorbs yellow, green and blue light, so the light that reflects off of it is reddish-orange. (This happens to be the pigment that’s used in barn paint, so we’re going to come back to it.) Light is absorbed when a photon (a particle of light) strikes an electron in the pigment and is absorbed, transferring its energy to the electron. But quantum mechanics tells us that an electron can’t absorb just any amount of energy: the particular energies (and therefore colors) that it can absorb depend on the layout of the electrons in the material, which in turn depends on its chemistry."
His explanation of quantum mechanics gets more involved at this point, but here's the most important part: the rotational speed of an atom's outermost electrons affect its ability to become a pigment. There are fixed increments of this property, called angular momentum, and this shows up in the periodic table in different blocks of elements. The only one we really care about is the "d" block, which is the big section in the middle. The "d" electrons produce an amount of energy that corresponds with visual light. These elements, then, tend to make for excellent pigments.
Now we get to the good stuff: Why red pigments are cheap.