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.

Photo credit: Flickr user swainboat via Creative Commons.

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."

Photo credit: Flickr user hiddenson via Creative Commons.

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.

"The red pigment that makes cheap paint is red ochre, which is just iron and oxygen. These are incredibly plentiful: the Earth’s crust is 6% iron and 30% oxygen. Oxygen is plentiful and affects the color of compounds it’s in by shaping them, but the real color is determined by the d-electrons of whatever attaches to it: red from iron, blues and greens from copper, a beautiful deep blue from cobalt, and so on."

At this point, Zunger broke down the basics of nuclear fusion, explaining that the larger the two nuclei you're combining are, the easier they are to force together, but the less energy they create. This is why hydrogen fusion is so incredibly powerful--it has a very small nucleus. And when you hit 56 protons and neutrons in a nucleus, the fusion stops producing energy altogether.

Zunger's whole explanation of the life an death of a star, compressed into a few paragraphs, is worth reading in its entirety, and at the bottom lies the secret of red paint:

"Now imagine a star. It starts out its life as a giant ball of primordial hydrogen from the formation of the universe, and under the tremendous pressure of gravity, it starts to fuse. As it fuses, it starts to form heavier elements like helium: but it takes higher temperatures than these mere hydrogen fusion temperatures to make helium do any fusing, so the Helium basically acts as a pollutant and just gums up the works. Ultimately, it reduces the efficiency of fusion so much that power levels start to go down.

"But the only thing holding the star up was the energy of the fusion reactions, so as power levels go down, the star starts to shrink. And as it shrinks, the pressure goes up, and the temperature goes up, until suddenly it hits a temperature where a new reaction can get started. These new reactions give it a big burst of energy, but start to form heavier elements still, and so the cycle gradually repeats, with the star reacting further and further up the periodic table, producing more and more heavy elements as it goes.

"Until it hits 56. At that point, the reactions simply stop producing energy at all; the star shuts down and collapses without stopping. This collapse raises the pressure even more, and sets off various nuclear reactions which will produce even heavier elements, but they don’t produce any energy: just stuff. These reactions only happen briefly, for a few centuries (or for some reactions, just a few hours!) while the star is collapsing, so they don’t produce very much stuff that’s heavier than 56.

"If the star is small, it will end up as a slowly-cooling cinder, or as a white dwarf. But if it’s big enough, then this collapse will send shock waves through the body of the star which bounce off the star’s core, pushing the collapsing wall of matter outward with more than enough energy to escape its gravity: the star explodes in a supernova, carrying off a good ⅓ of its total mass, and seeding the rest of the universe with elements heavier than the simple hydrogen we started with. Those elements, in turn, will join the mix for the next generation of stars, as well as the accretion clouds of stuff around them which turns into clumps rather than falling into those stars: that is, the planets. And this is how all of the chemical elements in the universe were formed.

It’s because of the details of nuclear fusion--the particular size at which nuclei stop producing energy--that iron is the most common element heavier than neon.

"So how does this tie in to red paint? Well, I told you before that the magic cutoff for ordinary fusion is at 56 nucleons. Because it’s the last point in the normal reaction chain, a lot of the fusion products tend to 'build up' there before the star explodes, and so you get a lot more of isotope 56 than you do of anything except for the really light elements that didn’t fuse at all, or didn’t fuse much. And what has 56 nucleons in it and is stable? A mixture of 26 protons and 30 neutrons -- that is, iron.

"So it’s because of the details of nuclear fusion -- the particular size at which nuclei stop producing energy -- that iron is the most common element heavier than neon. And as we saw before, you have to be a d-block element to make a decent pigment, which means that iron is going to be, by far, the most plentiful pigment for any species which lives on a star that isn’t about to blow up. And it’s going to bond to oxygen, the most plentiful thing around in planetary crusts for it to bond to (only hydrogen and helium are more common, and they tend to evaporate), to form iron oxides: those rich, red ochres that we mix with oils to form a cheap, stable, red paint."