Monday, May 13, 2013

Barns Are Red Because of How Nuclear Fusion Works

Yonathan Zunger offers a tongue-in-cheek, yet accurate explanation of why barns are usually colored red:
The answer ... is “because red paint is cheaper,” which is absolutely true, but it doesn’t really tell you why red paint is cheaper. It clearly isn’t because the Central Committee for the Pricing of Paints has decreed that red shall be in vogue this century, or because of the secret Communist sympathies of early American farmers. In fact, to answer this we have to go all the way to the formation of matter itself.
Stars will burn light elements in a well established order of fusion reactions, going through stages of burning hydrogen, helium, lithium, and other successively heavier elements:
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.
What has 56 nucleons in it and is stable? A mixture of 26 protons and 30 neutrons -- that is, iron.
And it's the iron that ends up in red ochre (Fe2O3, aka hematite), the pigment used in barn paint, explains Zunger.  I have to wonder about the other iron based ochres like Yellow ochre (Fe2O3•H2O aka Limonite), Purple ochre, which is like red ochre but with a coarser particle size, and Brown ochre (goethite), which is made of partly hydrated iron oxide (rust) and why they're passed over.  I'm also surprised at the number of reference books available that explain the basis of colors, like The Chemical History of Color.

The comments have a few other interesting side notes, like this one from Francisco D'Antonia:
There is a specific combination of paint colors made from raw materials that when combined can create all the natural colors of the living world. Its been awhile, but yellow ochre, burnt sienna, cadmium yellow, burnt umber, cobalt blue and titanium white are a few.  I met someone once that worked for a company that made several of the raw colored powders from metal. Fascinating process.
The final question I'm left with is whether does Red ochre based paints really have that much of a cost advantage over other colors?  I've seen green barns and a blue barn or two, which suggests that not all farmers are the rational price-optimizing paint pickers that Zunger imagines.

Nonetheless, I give two thumbs up for his explanation!