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How the price of paint is set in the hearts of dying stars

Today I’m going to try to explain the real reason that barns are painted red: nuclear fusion. And yes, this is an excuse to take a mad ride around some of the stranger corners of physics and chemistry in order to give you the real, this-is-not-BS, answer to a simple question.

This question got stuck in my head as a result of an episode of a long-forgotten sitcom called Head of the Class, about a high school class full of smart kids. (Sort of like Welcome Back, Kotter in reverse) This being an American show, it’s obligatory to occasionally emphasize the superiority of the ordinary virtue of “plain folk,” so in one episode the protagonists face off in some kind of academic contest with kids from a rural school, and end up losing because their city-slicker knowledge can’t answer the question “why are barns red?” (And this episode appears to have annoyed me enough that, several decades later when I have only the haziest memory of the show’s existence, I still remember it) The answer the show gives 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.

Paints & Pigments & The Sun

First of all, let’s think about what paint is. At a minimum, paint is a combination of a binder (some material that dries to form a film, like acrylic or oil) and a pigment, some material which gives it a color. A pigment is a material which absorbs some colors of light and reflects others; most pigments are minerals. (There are also organic pigments, such as the Imperial Tyrian purple made from the snot of the Murex snail, but not as many, and they tend to be much more expensive for the simple reason that there are a lot more rocks than there are animals and plants.) So for something to be a cheap pigment, it has to be a good pigment, and it has to be cheap. So let’s figure out what makes each of these happen.

To be a good pigment, first and foremost, something has to have a nice, bright color. 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.

The detailed calculations, or even the not-so-detailed calculations, are way beyond the scope of this post. (There are even whole books about it, like Nassau’s The Physics and Chemistry of Color) But there’s one important pattern which I can at least tell you about, which is that if you look at the various atoms which form a pigment, and you look at their outermost electrons (not the inner electrons, which are so tightly bound to their atom that they don’t participate in chemistry; all of chemistry is determined by the behavior of the outermost electrons around an atom) then it turns out that certain kinds of outermost electrons form pigments, and certain ones don’t.

The magic property is what’s called “angular momentum,” which basically measures how fast these outermost electrons are rotating around the nucleus. Electrons in atoms get angular momentum only in fixed increments (there’s that quantum mechanics again, only fixed increments allowed) and for historical reasons, the first few increments are named “s,” “p,” “d,” and “f.” On the periodic table, ( the elements whose outer electrons are “s” form the two tall leftmost columns; the “p” elements are the big square on the right; the “d” elements are the big block in the middle; and the “f” elements are the two rows off at the bottom. (If we ever make element 121, it would be the first “g” element) 

Electrons with less angular momentum spin in more spherical (rather than deformed) orbits, and when multiple electrons are trying to fly in the same spherical orbit, they repel each other pretty strongly. The result of this is that two “s” electrons meeting will have very different energies -- and it turns out that, in quantum mechanics, the amount of energy an electron can absorb is exactly the difference between these energy levels. So “s” means a big gap, “p” a slightly smaller one, and so on. And it turns out that “d” electrons are right at the sweet spot where that gap corresponds to visible light. 

Well, why are those particular colors of light visible? It’s because of the temperature of the Sun: our eyes didn’t evolve to see X-rays because there aren’t many X-rays to see around here. Instead, they’re very sensitive in the range of colors that the Sun produces, from red (around 780nm wavelength) to a peak brightness of yellow (around 600nm) all the way up to violet (around 400nm). Those colors correspond to energy gaps of about 0.3 electron volts (eV, a good unit of energy for studying atoms) which are right around the energies of chemical bonds involving d electrons. S- and p- bonds involve energies of 1-3 eV, corresponding to wavelengths around 100nm, in the far ultraviolet range.

Did we just get lucky that the Sun is yellow, and if we lived orbiting another star might the useful pigments come from p bonds? Surprisingly, the answer is no. The Sun’s color comes pretty directly from its temperature: it’s literally glowing yellow-hot, with a surface temperature of about 5,800K. The coolest stars, red dwarfs, are about 2,800K and glow red. The hottest stars, the type O stars, go up to about 40,000K, only 72nm; but it turns out that when a star gets any hotter than class F (about 7,000K, about 400nm -- blue) its lifespan starts to decrease precipitously. This is because the temperature of stars is actually fixed by the kinds of fusion reaction going on in their core, which I’ll get back to in a moment, and those hotter reactions burn through their fuel a lot faster. The net result is that any star that’s going to last long enough to have planets with life on them might be a bit redder or a bit bluer than our sun, but not radically so: and it’s those d-orbitals that are going to make the best pigments for anyone whose eyeballs evolved there.

How the price of iron is determined in the centers of stars

So now we know what makes a good pigment. What makes a cheap pigment? Obviously, that it’s plentiful. 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.

So if we know that good pigments will all come from elements in that big d-block in the middle, the real question is, why is one of these elements, iron, so much more common than all of the others? Why isn’t our world made mostly of, say, copper, or vanadium?

The answer, again, is nuclear fusion. 

To explain this, we need to think about how fusion actually works. The basic principle is that two small atomic nuclei combine to form a bigger nucleus. Now, there are two forces at work here: there’s an electromagnetic force, which makes the positively-charged nuclei repel each other, and repel each other more and more as they get closer. And there’s the strong nuclear force, which is what holds nuclei together: it’s powerfully attractive, much stronger than the electromagnetic force, but it has the interesting property that it simply shuts off at distances of much more than about 1fm. (10^-15m, the size of a medium nucleus) So to make fusion happen, you need to somehow push two nuclei together with enough force (generally in the form of heat and pressure) to overcome their repulsion until they get within range of the strong force, at which point it will yoink them together with spectacular force and release a good deal of energy in the process.

This gives us two rules of thumb. As the nuclei involved get bigger, the amount of energy (heat and pressure, in particular) required to set fusion off gets higher, because you have more repulsion that you have to overcome before fusion can start. And second, as the nuclei get bigger, the amount of energy you get back from the fusion gets smaller: in the bigger nucleus that you would form, you still have all of this repulsion, but the strong force can only bind together the nucleons that are close to each other, so as the nucleus gets bigger you keep adding repulsion but you don’t keep adding attraction. 

This means that fusion of really small elements is very efficient; combining two hydrogen atoms is just great. (For various technical reasons, the slightly heavier isotopes of Hydrogen -- deuterium (a proton with a neutron) and tritium (a proton with two neutrons) do better than bare protons. That’s where the “D-T” of D-T fusion comes from, and it’s the kind that powers both the Sun and H-bombs.)

In fact, once the atoms get too big, you no longer get back any net energy from fusion: the last reactions which turn out to be net-positive are the ones that form atoms with 56 total neutrons and protons in them. Beyond that, fusion starts consuming more energy than it produces, and won’t light up anything. (If you go far enough beyond that, to 232 nucleons or more, you start to see nuclei that are so unstable that a swift kick will make them separate enough that repulsion takes over, and they explode with a bang: that’s nuclear fission, a subject for another time)

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 (rule 1) 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.

How do we know that this is really where the elements came from? There’s a whole field of science around this, but the classic paper is commonly known as “B2FH” for its authors -- Burbidge, Burbidge, Fowler, and Hoyle. Using only the physics and the computational resources available to them in 1957, they calculated all of the various processes by which elements would be formed in stars, in enough detail to predict the ratios of elements which would be formed, and to predict the abundance ratios of chemical elements in our solar system. Amazingly enough, they made a pretty damned good and thorough prediction, enough that even then it was clear that this was a smoking gun -- and it’s been refined considerably since. 

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. (Check out the first figure in the B2FH paper, linked below) 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.

And that’s why barns are painted red.

To learn more:
Something a lot more interesting than you would guess:
The color of the Sun:
Colors of stars, and a place to start about how they get them:
The abundance of elements in the universe, the Earth, the human body, and other places:
A nice diagram of how much energy you get from fusion and fission for various elements, thanks to +Jas Strong
The 1980’s sitcom that inspired this:

To learn a lot more about color:
To learn a lot more about how the elements are formed, the original B2FH paper:

Photo by John Christopher:
Jacob Lewis's profile photoKen Lim's profile photoGary Walker's profile photoHans-Georg Lundahl's profile photo
Wow, and here I was, hoping for an explanation which would include Celts, Nordic Gods, Druids or Minoan myths.... Sigh...
Now that's a beautiful photo. The earth red paint (Finnish: punamultamaali) is also the traditional paint for houses and farm buildings in Finland.
I am again reminded why Yonatan is the chief architect of Google+, and I am a schmoe working a crappy job in East Nowheresville.
Lyndon NA
I Think someone has way too much time on their hands!

Good read ... but man, you must have a degree in beating around the science bush :D
This is interesting and you gave enough information to answer lots of other questions. Thank-you for the refresher.
+Lyndon NA Nah, there are just a lot of pieces to the explanation: the origins of color, the range of our vision, the distribution of chemical elements.
And how could anyone forget Dr. Johnny Fever teaching an AP class?
+Yonatan Zunger Could you, from the distribution of elements in our solar system, determine the characteristics of the star whose explosion formed the cloud from which it coalesced?  In particular, I'm wondering about the large amount of carbonaceous material believed to be in the early solar system.
+Susanne Ramharter I don't actually know of any religious or cultural issues in the coloring of barns. From what I can see, barns are painted because paint helps to protect and preserve the wood, and ochre red is what's cheap and therefore most common.
Paint is on barns in the first place because paint keeps out water which will eventually damage the wood (keep in mind, this is pre-pressure treated lumber).
+Gary Walker Yes, a little bit -- but only a little, because interstellar matter mixes up some over time and so no star system is formed out of the remains of only one other star. The best you can generally get are the statistics for our general neighborhood.

Large amounts of carbon are common in population II stars (the ones made from the bodies of the primordial population I stars), though: the later parts of the main sequence chain, no matter what size the star is, go through a particular production phase which makes lots of C, N, and O. 
I wish Yonatan had been my chemistry teacher; as it was, the only things that helped were Star Trek (silicon as an alternate building block of life, instead of carbon), and buying a study guide of some sort and teaching myself.
Beautiful.  And there's a meta-story here too, about how human beings can latch on to topics when they are young because of almost random events (like a quiz question in a TV show) and continue to organize information and experiences around that tiny occurrence, much as how an oyster makes a pearl in response to a tiny grain of sand.
Earth's crust contains about 5% or 6% Fe, not 32%.
Great story! Do you know why camping tents are green and not yellow or red? Because otherwise it would attrack too much flies and musquitos. I can't explain that other than it is a color for food, like flowers. I guess you could explain that too.. but thanks. I like those barns in the US, but in Holland they are mostly built out of stone or painted green. 
Thanks for the good read today. Remind me to never ask you why the sky is blue, hehe. ;)
Yet another long form post where you know it's going to be worth the time. Thank, Yonatan!
This post reminds me of the type of "why" game I played with my five-year old. We didn't do this one, but it would go something like
"Why is that barn red?"
"Because it's painted red."
"Because the paint has stuff in it called iron oxide."
"Because it's a cheap and easy to use to color stuff."
"Because there's a lot of iron in the earth"
"Because iron is one of the most stable atoms."
[... etc]

Sometimes I liked trying to see how fast I could get down to quantum field theory.
+Richard Younger That's also pretty much how I came up with this post. :) "Why are barns red?" "Because they need to be painted, and painted cheaply." "Why is red paint cheap?" "Because iron oxide is the cheapest pigment." 1: "Why is iron oxide red?" 2: "And why is it cheap?" ... 
When we continue the "why" game, we will always come to unanswered questions: Why are the laws of nature such that the quantum mechanics are as they are?
+Sakari Maaranen Because if they weren't we wouldn't exist to ask the question (I forget if that's the weak or the strong anthropic principle--it's navel-gazing either way).
I wonder if it's still actually true that red paint is cheaper than other colors. It certainly isn't true of the sort of interior paint that home owners buy in small quantities at an ordinary hardware store (I bet most consumer grade paint uses synthetic pigments), but maybe the pricing is different in industrial quantities.

It wouldn't surprise me, though, if the real answer (assuming barns are still painted red) was more like: red paint was cheap in 1880, and now the color is just habit.
+Gary Walker That's the weak one; the strong principle says that they were that way in order that we be here to see them. (And is basically theology) The medium anthropic principle, which is the one with actual meat on it, says that if the universe contains many regions with different laws of physics or physical parameters in them, then we will by definition always observe parameters consistent with our own existence in our vicinity.
A superb post about my favourite subject! I hope this is the beggining of a series!
+Matt Austern I suspect that it still is, but the sort of red paint we're talking about is a simple ochre red on a fairly cheap binder. Interior paint is made from significantly fancier pigments and binders both; I don't think anyone is trying to sell this stuff for "nice" uses. 
+Autumn Ginkgo Leaves™ Series on what? (I've been thinking about doing one on light and color, with a focus on spectroscopy, but that's not quite the same subject)
+Todd Vierling If that's a joke, it's excellent.  If it's not a joke, link to paint, please!
+Yonatan Zunger yes, "the universe contains many regions with different laws of physics or physical parameters in them".

That's the best we can think of. Essentially it says: Everything is possible. That's pretty much synonymous with we don't know. Stating something like "everything that is possible really is so in various regions of the universe" is logically equivalent to: G = universe. G is omniscient, omnipotent and omnipresent. Here, omniscient would mean G contains all the physical information of the universe. Omnipotent and omnipresent would mean that the universe contains everything that is possible in its various regions. Yes, I know this version is called the pantheist interpretation. It's interesting that simply using a synonym for nature has an -ism of its own.

In any case, this enters a metaphysical realm where we can't test our hypotheses. So, the current scientific answer is we do not know. If I would have to throw a guess, less scientifically, I would make the guess that it's the probability distribution explained above—because it has the least assumptions. Some people use the G to label it and that doesn't really change the conclusion at all, nor does it make the explanation any more useful.
Maybe one about why when I paint and mix all colors together I end up with black but when I could paint with energy in different spectrums and put them together it would turn out white. I am not physician btw.. so pardon me for questioning something that might be obvious for you. ;)
+Ellen Molenaar You're dealing with additive and subtractive color.  When you use pigments you're subtracting segments of the spectrum.  When you use them all together you wind up with black.  When you use light you're adding segments of the spectrum.  When you have them all together you wind up with white.

This is why printing uses CMYK instead of RGB.
+Sakari Maaranen The medium principle is nontrivial because, for it to apply to anything, you first have to show that different values of the parameter (or law, or whatever) do occur in different parts of the universe. If you can show that, then you can have an argument for why we observe some particular value. 
+Ellen Molenaar What +Gary Walker said. We see light as being made of red, green, and blue because of the way our eyes work: the cones (color sensors in the retina) come in those three colors, and are sensitive to those patterns, so when we see particular patterns of those our eyes interpret those as being white. Since our eyes build everything out of those, they act as "primary colors" and mixes of them look like other colors to us, even though they actually aren't at all.

Cyan, magenta, and yellow are the three primary pigments, and they're the duals of the three primary colors: something which absorbs red and reflects green and blue will look cyan, something which absorbs green will look magenta, and something which absorbs blue will look yellow. So when you're working with pigments, which produce color because of the lights they absorb, you want to mix together the pigments which absorb the primary colors -- thus, C, M, and Y. 

(Black -- the K of CMYK -- is added to the mix because with real pigments, the mixture of equal parts CMY looks like a kind of muddy brown, and it's often important to have a nice, true black, so we add a separate pigment for that)
This has to be one of the best posts I have ever read. Thanks for the informative chuckles!
Oh thanks! Will look that last sentence up..
Also, printer brat trivia:  The K in CMYK stands for black because the black plate is always printed first and is the Key plate to which the other colors are aligned.
+Yonatan Zunger Common misconception, but my dad would be quick to correct you (although polite about it).

Black is printed first because, unlike the other pigments, it is not transparent.  We don't usually consider that pigments used in offset printing are transparent, but they are.  They function more like filter elements than like paint (except for black).
+Yonatan Zunger You lie.  Your barn is red with the blood of your enemies.  Don't think we don't all know it despite your most excellent shaggy dog physics post.
You hired your architect from a Python sketch, didn't you?
So all that physics is to say that red paint is cheap? And farmers parking barns red because its the cheapest? Wanna make sure I caught all that correctly Haha.
Also why asteroids contain a lot of iron, why Mars is reddish, etc. etc. etc.
Also the secret code to +Yonatan Zunger's underground abbatoir/lair.  But don't worry if you missed it in the text, because I'm sure it'll be revealed more readily in the Tom Hanks movie adaptation.
up, up, down, down, left, right, left, right, a, b, start.
+Kimberly Chapman and +Yonatan Zunger, while I was unfamiliar with the word abattoire when I saw that Python sketch decades ago, and had forgotten the word immediately, I finally looked it up after listening to a beautiful Nick Cave song years ago, entitled, oddly enough, The Abattoire Blues. Imagine my surprise. But then, it being Nick Cave, my surprise was more amusement. In any case, I think you might enjoy it.
Excellent and beautiful lecture, +Yonatan Zunger. While this is not what you lecture on, I positively love the way you remove money from the explanation.

There is I believe some sort complementarity between the goals of making (or saving) money and that of re-designing money, and the removal of - the spirit of removing - money or price-based explanations (in favor of quality money-free explanations) helps the latter.

On the (medium) anthropic principle as you present it, modulo "ecological niche" for "physical laws", does it not reveal an origin in Darwin's thinking ? (although, to be sure, the concept of niche was coined decades later).

Niches are nature's version of divine providence, and can't the medium anthropic principle simply stand as the admission by physics that the analogy has positive content?
Curiosity is such an important trait.
+Yonatan Zunger this was fascinating. Thanks for the work that obviously went into writing it. Good stuff.
I had always thought barns were red due to the plentiful supply of blood from slaughtered animals. All the other nuclear fusion still applies though -- vertebrates likely have red blood due to the abundance of iron. That said, invertebrates commonly use copper for blood, which is startling because it's relatively rare. Also, iron is far less abundant in the crust than in the core, and also why platinum and iridium are so extremely rare in the crust, since they are iron soluble and migrate to the core.
Alan H
Wonderful. :-) 
Haemoglobin (as a pigment) makes up a pretty small amount of the blood and is a lot more work to make into a useable pigment.  Look at the price of haemoglobin oil paint some time.
Thanks +Gary Walker and +Yonatan Zunger for your kind explanation. About black: as artist I am told never to use solid black, but always a mix of the colors turning black. Ofcourse that's not a law you always have to obey, but it has to do with transparency since we mostly work layer by layer. With black you make the painting 'dead' as we call that.
Well, it really is a pretty, fire-engine red, too.  Much brighter and redder than something like red ochre.
Sorry, .........come again. Didn't quite get that........
I knew that red paint was the cheapest sort back in the old days, not so much because all barns were red (which they were), but because it was very curious how the mansions of the rich (priests and owners of trade stations in the case of where I come from) often were white - when looked at from the front - and red when seen from behind. - The white house to the right here has two white and two red walls, now a museum.
I'm glad an article about nuclear fusion and stars made it into “what’s Hot” :D Thanks for a really interesting read!
+Yonatan Zunger  that's certainly correct, but there's still that one big if about the medium principle, i.e. the hypothesis seems to be only semi-testable:

If we can't seem to find observable differences in the fundamental "parameter or law", that still does not prove such differences didn't exist somewhere else that we just can't observe.

If we do find such differences then indeed we can have an argument, but until then we can't know whether we are ever going to find them.

So, it seems it cannot be falsified, only verified, unless someone has an idea how we could conclusively show that the fundamental laws or parameters of nature do not vary somewhere outside our observable universe.
Lets not get into supply and demand. Okay so there is a lot of supply of iron and oxygen. Hmmm...... :) Lets touch on every single academic topic shall we? How can we tie medieval literature? Actually that seems pretty easy. :P
Maybe barns are red because there are more red receptor cones in our eyes and that makes it our favourite colour.
+Boris Borcic The medium anthropic principle doesn't really talk about niches. It's a much simpler statement, and it's almost entirely free of metaphysics -- metaphysics tends to show up in the strong anthropic principle, instead. 

To give an example of how the medium anthropic principle might work, it turns out that the relative masses of the up and down quarks have to be tuned to within a pretty sensitive range; if the down mass changed by a few percent, the spectrum of stable elements would be completely different, stars wouldn't ignite, fusion would never happen, and our universe would basically be a big cloud of neutrons. Optimally, we'd like to be able to predict the up and down masses from first principles. But it may well be that there are no first principles which control these; instead, the universe might have several giant "regions" with different masses. (For example, if we live in a cyclic universe that goes through multiple big bang / big crunch pairs, and the values of these aren't fixed across cycles; or if the universe has large bubbles in it where these values are fixed; etc) Then we would observe these particular favorable-for-life versions of the masses, not because the masses are predicted by some fundamental law that meant they had to be this way, but because in the parts of the universe where they're set some other way, we would be dead.

Or to take an even simpler example: imagine you lived in classical Greece, without access to space travel or the like. You would observe that the normal temperature of the universe appears to be between -10C and 40C. But coming up with a fundamental theory of the universe which explains that temperature would be very hard, because in fact there is no fundamental reason; in fact, if you travel around the universe you'll encounter everything from the 3K of deep space to the billions of degrees in a supernova. We just happened to live on a place that was hospitable to human life, and as our ability to observe more distant places improved, we realized that we were simply standing on an island.
That was a great read.
[Edit: hadn't seen your answer - but this is unrelated]

But did you not surreptitiously involve a metaphor of your employer, +Yonatan Zunger : transparent like the atmosphere ;)

Also, contemplating, came: your detailed answer to "Why are barns painted red?" appears to be neatly embeddable in a detailed answer to the related question "Why are barns the color of blood?".
Fun and smart essay, reminds me of Connections  by James Burke.
+Lyndon NA A person of exceedingly high intellect would have taken no more than 20 minutes to think about and write this piece, then had breakfast and started work at his real job... unlike us mere mortals...
This is a fun read; sadly my kids are just slightly too young for it.  I tried explaining the structure of atoms to them and it didn't quite get there - orbital types would likely throw them completely off.

I am also reminded of Carl Sagan's adage that to bake an apple pie we must first invent the universe.
Red paint is made of Iron?
I stoped reading at second paragraph,
Anyway thanks for this explanation +Yonatan Zunger 
That's a great explanation. I would have said, "Because red is a nice color."
Wow, your grocery list must look like an encyclopedia.

On a related note, Cliff Stohl said the hardest question he was asked at his PhD orals was "Why is the sky blue?"
+Gabrielle M Yes, but why is it cheap paint? That's the fun part.

+Billy Couvillion Nah, my grocery list looks like a grocery list. I only write long-form like this when I mean to. 
How cool is THIS!!?? WAAAAAY cool! Interesting article!
Sorry if my comment was negitive sounding earlier. It"s actually Awesome and something I've wondered about forever. I have an obsession with old barns so thank you. If you ever wanna go pickin in some well let me know. thanks again
The point about niches is that they imply the typical individual will come into being in environmental conditions that are more friendly to its spontaneous dynamics than a random* place and time of birth would allow to expect. Don't your explanations on the medium anthropic principle, precisely boil down to the same pattern, +Yonatan Zunger ?
Very interesting read. Too bad I'm color blind!
So barns are painted red because it's cheap?
First I thought you were being a poet, then you got all science on my ass. 
Barn Red is different than red explained by chief architect of G+.first settlers decided on red barn because cattels see the red better and run toward barns.that is why Spaniols use red flag for running of the bowels,it makes them exited and makes them run.Although now I understand why G+attrects so many bowlheaded sightes.That is why practicall and smart users of G+check everything twice before accepting everything they read.
Sorry to break it to you, +Keian Moarefi memar, but cattle are basically color-blind. The legends about bulls "seeing red" comes from the tradition of using red capes in bullfights, but that tradition doesn't have anything to do with the bulls themselves -- they just see it moving.
read mcphee's book - curve of binding energy. good story, not all deep
Yontan Zunger that is not what vetenarian in UCDavis say .Exact opposite.
Excellent essay, wish there were more of them! B2FH are widely known for their belief and support of the Steady State theory, rather than the Big Bang.
Great piece! I'd be very interested in a series on colour and spectroscopy. Is there a related explanation as to why plants are (generally) green?
Red paint is no longer cheaper.  In actual fact it is one of the more expensive colours.
Wow! Learned something new today. Excellent post about a topic that is overlooked without question. Thanks for sharing (and teaching).
+Mike Barton That's because most red paint you buy today is using fancier pigments than simple iron oxide, which give richer and more even colors, as well as fancier binders. why is a red ferrari expensive?
+Pete Fofulit It's a brand. Why are generic medications cheaper than the named brands? The list goes on... 
Great article, Yonatan! This was a pleasant reading.
There are moments in the day that I wish I was a bull [looking at G+notification box] ;)
+Yonatan Zunger So clearly explained, thoroughly enjoyed the read, thank you for this!  Added more background and understanding to all the more 'accessible' Brian Cox material I've consumed.  Cheers!
I knew all of that already so I'm even more amazed how concisely and elegantly you explained it. Good work!
No, it's a good shot but a total miss if you analyse carefully.

Iron is not the most abundant atom in the Earth crust. So even if you take that simplistic approach, things don't add up. Aluminium, Silicon, Carbon oxides are abundant and easy to come across. In actual fact painting barns in black should probably be cheaper.

There are several crucial practical aspects of the problem that the atomic/star story ignores.

* General abundance is less important than the degree of concentration of deposits. Abundant, but thinly dispersed matter is expensive to mine.

* The saturation of colour directly affects the price of paint. Whether it takes a grain of mineral to make a bucket of paint, or a couple of good shovels - that's an order of magnitude difference.

* Some mineral paints are poisonous. That immediately increases the price because of complex handling, regardless of any other qualities.

If you consider all the evidence, the key reasons barns are red are:

* ochre appears in quite concentrated deposits;
* ochre requires almost no processing to turn into paint;
* it's chemically neutral, which means no wood degradation and no fading in the sunlight.

Clearly the nuclear/supernova reasons make an indirect effect, but not as much as the three reasons above. If anything, the biological factors define most of the reasons (all iron oxide on Earth is a direct result of life after all).
Alas, my hopes of painting the foyer lanthanide-based [meta-spectral color] dashed for good!

+Yonatan Zunger  Lovely exposition, as always.  This also stands as a good pre-introduction to why life as we know it is carbon-based.  (And why, if we were to find non-carbon based life on this planet, we would most likely find it in deep oceans.)

+Billy Couvillion -- naw, I suspect Yonatan goes pure reductionist when it comes to shopping and his grocery list simply says "food".
Well i thought the cows choose the color, i will have to show them,
i hope we don't get any in herd arguments or disputes!. (Good reading)
+Oleg Mihailik It's true that abundant but dispersed matter is less useful, and there are issues of which materials are stable, and so on. That's why I started out with the discussion of pigments and d-orbitals; all of the more common elements are s- or p-group, and so don't produce any colors in the human visible range.

You're right that one could still use them (in their more plentiful forms) as pigments, if you didn't mind everything being gray or brown, or for that matter forego pigments altogether and just paint everything with a naked binder. So I suppose that the real meta-question I should have also asked is why people bother coloring things at all, to which I don't have an immediate answer.
+Yonatan Zunger P-group elements can be used as pigment, just fewer of them, and not as safely.  Lead is the prime example of a p-group pigment.

EDIT: lead is a strong pigment for red and yellow.
+D. Luria True. Yet another thing I didn't really go into... that once something is high enough in the periodic table, all of the gaps start to shrink, and anything is equally likely to form good colors. 
...and now you have, so tick that one off the list :-)
Great explanation. My geology professor had another explanation. He claimed that farmers used the blood from the cows they killed. Later they switched to using hematite to get the red color. I've never checked the feasibility of using blood for paint but it sounded good at the time.
Should I really be in this conversation?
This is easily the longest g+ posting that I have read. Great job.
Just basic red paint is still the cheapest color you can buy. Lows and home depot may not carry it but try a farm equipment store. 
Fascinating, and to a working artist like myself, very helpful; thank you. Would you care to share any thoughts about interference pigments? I work with more of them every month, and mix them with metallic pigments, so this is of practical as well as theoretical interest.
Stu M
So why is the sky blue? Why not red if its more cosmically likely?
I will never be stumped by this question again...
+Chas Williams I actually know nothing at all about interference pigments, so I don't have much to say about them.
+Stu M The sky isn't colored with paint, so this discussion really doesn't apply. :) I might do a post on why the sky is blue sometime, but it's a bit lengthy. But if you search for that you'll find a lot of good explanations. 
Very nice James Burke-like explanation!
Although if I was a farmer, I wouldn't bother painting my barn. Sounds like a lot of work.
+andrew zuo The paint protects the wood from moisture and insects; it makes barns last a whole lot longer than they would otherwise.
I painted a barn I built red...because the other inexpensive color choices that came in 5 gallon buckets were white or gray. 
Very true, +Edward Morbius. I didn't go into the whole issue of crust versus solar system frequency, because it doesn't change the ultimate answer, but it's worth noting that if it weren't for that, we'd have quite a bit more iron in the crust than we do. 
+matthew rappaport I'm pretty sure I've had longer posts hit WH in the past. G+'ers tend to like their long-form text more than you would guess. :)
Ha I remember all the TLDR's from posts way shorter than this one. Of course G+ doesn't measure how/why people interact with posts, a lot of people enjoy you and don't get a chance to read the post or like red barns. I wonder if any of that type data is coming at I/O. Is there even a character limit for Posts or Comments? 
+matthew rappaport I believe that there's a 1M limit on posts; I can't remember what the limit is on comments. Just to protect the system, really. 

+Tau-Mu Yi Thank you! 
The fact that Google+ lends itself to a bit more discussion and a bit more depth than Facebook or Twitter is probably the single biggest reason I'm active here and not on either of those.
Ha a 1 million character post . . any actual posts come close? 
This post has to have what 10 - 15,000 characters. 
Do we have to wait until May 15 for a new change here.. or.. no comment lol
Most excellent post +Yonatan Zunger .
Belongs in a book about the way the world works, as  only you can reveal.
One thing I never understood: why do the frequencies of light produced by our sun coincide with the frequencies of light that are transparent to our atmosphere?
+David Jao Huh. I never thought about that before, but it's rather interesting. I'm going to have to think that through for a while... I know that part of it is because molecular nitrogen and oxygen have very low electric dipole moments, but there must be a lot more to it. 

That may turn into another article at some point. 
Are we unaware of that of which we are unaware? IOW, perhaps the sun transmits frequencies we have not yet detected.
+Lisa Borel Well, it transmits a whole lot of frequencies, but its peak transmission is in the visible spectrum -- not coincidentally, since our eyes evolved to see its peak transmission. And in fact a lot of the IR and UV light it emits is absorbed by the atmosphere. (CO2 and O3, respectively) 

It is rather interesting that our main atmospheric gases -- N2, O2, and Ar -- are all transparent in the visible range, and that in "clean" air even the trace components don't give the air a color. I'm going to have to think a bit about why that's so. 
+David Jao Perhaps life couldn't have evolved where the source of energy doesn't reach its cradle and would have died out, if the light had stopped after it evolved. We don't know, if life can first evolve via geothermal energy or otherwise under clouded skies. Even if some forms of life could evolve in darkness, perhaps higher intelligence requires light at various points in its development, either for energy or for the quality of information signals in its environment to facilitate the kind of selective pressure that would produce an ecosystem of value chains where intelligence was necessary. These are just guesses to serve as starting points to a closer analysis. There could be other reasons.
+Lisa Borel +David Jao Incidentally, here's a graph of the complete spectrum of sunlight as it's emitted from the Sun, and as it reaches the surface of the Earth:

You can see that there is a good deal of absorption at all frequencies, but in the visible range it's fairly uniform, which means that the air doesn't impart much of a color to the light. (As opposed to in the IR range, where there are a number of sharp troughs due to absorption of specific colors of light by water, CO2, etc.; if we could see in the far IR, air would definitely have a color) 
How do we know that is the complete spectrum +Yonatan Zunger ? That is what I was getting at earlier? Perhaps there are spectra we have not yet detected.
+Lisa Borel That's very high-confidence data; we've done extremely thorough spectroscopy of the Sun. 

I'm thinking about this question more, and I think that the answer is actually just anthropic. The atmospheres of the Earth and Mars are fairly transparent in the visible spectrum, but the atmosphere of Venus isn't; in addition to thick clouds, it has an orangish color due to the various sulfur compounds. 

Part of this is also because of the definition of visible light, too -- our eyes are evolved to see the light that the Sun deposits on the surface of the Earth, not the light that it emits in space. If the sky were opaque to major bands, like it is in the IR, then we simply wouldn't have eyes that could see those bands particularly well. 

The sky could still have a color, if there were nontrivial features midway through the visible spectrum; given the way eyes work, they capture a range of colors, so if there were a narrow line missing, we couldn't just exclude it, and it would look like air having a color.

Which, in fact, it often does, especially if you live near Beijing or somewhere similar. 

So I think that to some extent this is just a case of us evolving somewhere where enough sunlight reaches the ground for life to function, and our eyes evolving to efficiently see the sunlight which does reach the surface, combined with a bit of luck that there aren't any major optical absorption lines from any of our "normal" atmospheric compounds right in the middle of that to give the sky a color. But that really does seem to be partially luck: there are many things that end up in the sky which do give it a color.
Mmmm. I used to print signs with Gripflex ink. $15/gal for black, green, blue, yellow,etc. $120.00/gal for red. Red is also the first color to fade outdoors. Look at some of those food posters at restaurants. The picture looks greenish yellow because all the RED is faded away. Look at any bumper sticker with red on it. I don't buy this theory.
+Philip Lang Those are very sophisticated binders and pigments: they aren't a simple, cheap, ochre red. To get the bright reds you see in a Pantone red, you've got to use things like Strontium-based pigments, which are a lot more expensive and less stable than iron ones. 
Ok, I'll buy that. Funny, my wife and I just had this discussion on our last road trip after passing the 47th red barn.
You should say something about why paint needs to contain a pigment at all, I think. Why not just the stuff you're calling a binder? Latex alone or oil alone is enough to seal out weather and actually attenuate a lot of UV that would deteriorate the wood of a barn, too.
ABL;BWR - A Bit Long; But Worth Reading ;)
+Yonatan Zunger - You should find a partner and make illustrated narrations of your texts - I bet they would become quite popular!
+Robert Kennedy Yeah, I was wondering about that while writing this. I'm not immediately sure why pigments were necessary at all -- except that it looks a lot nicer, and if you're going to take the effort of painting a barn with binder, you may as well put on actual paint. 

Part of it may have to do with protecting against deterioration due to visible light rather than UV, plus the fact that you can repaint in order to patch up defects, whereas if you were using a mostly transparent binder then it wouldn't cover things up as well. But there may be something more (probably something really obvious) that I'm missing.
"Did we just get lucky that the Sun is yellow" - nope, because the sun is green.

With an average temperature of 5800K using Wien's law we can determine that the peak wavelength produced by the sun is 500nm or 5000 angstroms with is green.

The sun appears yellow since the molecules in the atmosphere refract green light more readily than yellow.
That was a much better read than the texts I had to read in school. Strange thing is that I hated Quantum Mechanics while in school, but nowadays I find myself thinking about it all the time.
+David Jao I don' t know the answer to your question, but my guess is that gasses that would have colors in our sun's light would probably also react with the light. Those molecules would therefore have shorter lifespans than the non reactive molecules which would eventually become the predominant atmospheric components.

Just a hunch.
+Yonatan Zunger
What is the necessity of this comment? Whether a problem solved? Or there is no any problem and we just have a lot of time that should be spent. What is the main reason to see red? And what is this red from each person mind standpoint?
Someday a man asks God: Why watermelon is larger than the its tree? Therewith, an apple from tree that he was sitting its below fell on his head. He immediately said; Lord, you know better. If there was a watermelons instead of apples, I was dead.
So, I think dear Yonatan met up whit this kind of event for this comment. :D lol Sorry for this joke; Be happy!
What is this? Ted Kazinsky' s Main House?
Thank you for an educative information,shared with us,appreciative,hope to be able to read more of your educative topics,regards,Oyekunle Ajayi
What an excellent post!  This will be recirculated among my tutees, but deserves a many-orders-of-magnitude greater exposure, because it makes a whole range of scientific topics accessible and relevant.

I have always felt that even my best students, who dutifully learn about the D-block, the stability of nuclei, etc. don't really feel that they are learning something 'real'. It all exists on an abstract plane, mainly useful for passing exams. With this post, they have something to which to relate the abstract theory. 

So ... the next step: how about a 'textbook', or the online 21st Century equivalent thereof, with dozens of similar essays. They don't have to be comprehensive and cover all of physics and chemistry, just broad enough to touch on all the major areas. Make it a Google project, so it doesn't have to be done in Mr Zunger's spare time. 
You get "plus plus" as we say in Sweden, for pointing to the original B2FH paper +Yonatan Zunger. =)  THANKS!!!
cool. loving spectroscopy and astrophysics! please do post more!
I haven't read all the comments, but I remember reading something about colour and psychology in highschool, that suggested that barns were painted red because it counteracted an otherwise green/blue environment on the prairies, and humans needed that.
I can not remember why.  I shall have to dig out the article.

However, barns in Australia are almost never red.  I would have thought that the same economic forces would be at play here, too.
We do have red dirt, though.

I shall investigate further...
Also (random tangent, sorry) my association with red paint is ship yards.  Iron ships are typically painted in a burnt red paint which retards oxidation and other forms of corrosion.  Its full of heavy metals and is pretty nasty to get rid of.
 there's a housing development near my parents, on prime waterfront, which used to be a shipyard. They had to excavate three metres deep of the topsoil to get rid of all the nasty red sludge.
Now there's a great park there, built on a 'hill' of the stuff...
Very interesting & fun to read.
+Sakari Maaranen 
While it is may be that these organisms evolved in a different plenum than their current niche, chemotrophs currently fit a model for a possible organism that evolved "via geothermal energy or otherwise under clouded skies".
+Robert Kennedy
 Wood is particularly susceptible to environmental weathering as well as degradation by heat (infrared) and ultraviolet wavelengths of light.  Protection from UV and heat degradation is significantly improved by adding pigment to paint binder because the particles of pigment variously absorb, reflect or refract UV (and to a lesser degree, IR) light.  Additionally, not only does pigment provides improved direct insulation of wood from light and weather effects, the chemical composition of most pigments is a significant defense against bacteria and insects, the principle agents of biotic weathering.
Thank you for a post that is both educational and humbling. Educational because I thought the price of red paint was governed by the law of Marketing's 4Ps. Humbling because it reminded me of a quote from Carl Sagan - "To truly make an apple pie from scratch, you must first invent the universe".
Striving to Evolve the Human R+A+C=E
+D. Luria interesting... but like you say, we don't know how these chemotrophs first emerged. Even if abiogenesis can occur without much sunlight, it would still have to evolve to become intelligent to be directly relevant to this discussion. We don't know if those chemotrophs could develop intelligence in their dark environment. Such environment might not be dynamic and varied enough to generate sufficient levels of biodiversity in any reasonable time to produce ecosystems where advanced central nervous system capable of abstract thought would develop.
Nice explanation.

BTW, the Sun is white, not yellow. If you look at it in a neutral density filter, it's surprising how plain WHITE it appears.

The yellow tint is an illusion, I'm not sure what the mechanism is.
+Florin Andrei It has to do with scattering from the upper atmosphere -- the green band isn't reaching our eyes as much. The yellow isn't so much an illusion as the difference between the light we see at ground level, and the light we would see in orbit.
+Matt Austern
Yes and no. The older pigments are still cheaper - in the short term.

Barns, maintenance sheds, and other structures in which aesthetics aren't important, and cost is, are generally painted using zinc oxide paint, which is white. It's very durable and cheap; while it's not as cheap as old red paints, it's more durable so in the long run it's cheaper.

Most modern paints, though - especially for residential and commercial structures - use titanium dioxide as their primary tint. Like zinc oxide, it's white, but because of the size of the molecule, it's also kind of transparent and takes other pigments pretty well. It's also very durable - but quite expensive compared to other pigments.

As for the other pigments, the ones that give your paint a color other than white? Those are still based on clay and various minerals. Ultimately, we're using the same pigments we've been using for, oh, at least the last 10,000 years.
+Yonatan Zunger Thanks, that was marvellous! 

Why the sky is blue isn't necessarily a short explanation either because it hasn't always been perceived as blue according to Guy Duetcher in his book "Through the Language Glass: Why the World Looks Different in Other Languages".

And why exactly do we have waves at the beach?
Sadly, this is wrong.  Nickel is a great counter-example.  Nickel is the fifth most common element on earth, but it is mostly in the core (with a lot of iron).  Only 0.008 percent of the crust is nickel (much of that in a meteorite strike in Ontario).  We somehow got lucky with Iron having a high prevalence in the crust.  So Prius batteries are expensive and rust colored paint is cheap.
Also, for some enjoyable - albeit not short - explanations of stellar aging, I suggest reading Stephen Baxter's books Ring and Raft. As novels, they're pretty "meh", but if you'd like some plot with your quantum mechanics, they're a good way to go.
Thought this are bloody american barns!
+Yonatan Zunger Red Ferraris vs red Hondas:  actually, silly but true.  Ferraris are made of aluminium and magnesium, whereas Hondas are mostly made of iron.
+Jas Strong Strangely, though, when I try to manufacture them in my linear accelerator, the production cross-section for either pp -> Honda or pp -> Ferrari is really low, much too small to account for the observed composition of the streets. 

Clearly it's a sign that there's an intelligent designer out there. ;)
Italian designer. Both Ferrari and Honda NSX.
+Topher Belknap There's a much more complicated story here which I didn't get into, which is crust abundance versus bulk abundance. You're right that Ni is fairly common (fifth by mass, seventh by number of atoms) in the Earth as a whole. It's the 14th-most abundant element in the solar system, and its higher concentration on the Earth has to do with the fact that most of the more abundant ones are gases (H, He, O, N, etc) which tend to escape small planets and end up dragged in instead by the gravitational fields of their larger neighbors. 

Within the crust, though, both Ni and Fe are a lot less common than you would guess from their total abundances. Fe is rarer than its total abundance because it's heavy and tends to sink -- a disproportionate fraction of the iron is in the Earth's core. Nickel combines an even higher density than iron with solubility in metals and a tendency to react violently with oxygen, all of which make it migrate even more aggressively to the core; the result is that there's very little Ni in the Earth's crust. 

There are quite a few issues one has to skip if one wants to keep under 3,000 words. :)
Interesting post, but I want to take issue with something. This post says that s and p bonds do not give rise to transitions in the visible. This is not true. The beginning of the wikipedia pigment article, cited above, refers to "chemical bonds of conjugated systems." These are hybrids of s and p and are the basis of organic pigments, which are not really all that uncommon, despite the reference to the rare imperial purple. They are not from plants or animals generally but are synthetic, and are used as dyes. A large fraction of human-colored objects around you are using organic molecules.
So... all that is essentially again saying that red paint is cheap? ;) Good article non the less.
+David Strubbe I was specifically referring to inorganic compounds, since (as I mentioned before) organic pigments are pretty much all going to be more expensive than the cheapest inorganic ones. Organic compounds can make a much wider variety of colors, because the flexibility of things like a carbon skeleton and hydrogen bonding which can adjust the shape of the entire molecule makes it possible to have quite precisely tuned band gaps.

And even for inorganics, the statement isn't quite true -- once you go up high enough in the periodic table, the difference between p, d, and f bonds becomes smaller. (Which is why lead is the basis of various inorganic pigments, despite being in the p-block) But high up in the periodic table, the size of the nucleus makes elements rarer, so I decided to elide that particular bit. 
My house painter recommended against red trim saying that red paint fades faster than the other hues. 
Yonatan, I wonder if you have fully considered the role of Oxygen in your analysis. Most of the free oxygen on the planet is a product of photosynthesis (photosystem II), which is a great oxidizer of iron. I think free oxygen had a major role in the formation of iron oxides on the early Earth. The source of that free Oxygen is liquid water. If this is the case, the story of iron oxide is deeply tied to the evolution of photosynthesis and the delivery of water to the early Earth. While oxygenic photosynthesis also depends on energy from the sun, it seems possible that biological photochemistry and the delivery of oceans to the planet make this explanation more complex.
+Yonatan Zunger
I would hope that things left out would not negate the concept, which they seem to in this case.  Prevalence in the crust is the whole story of why iron is cheap.  It is a bit like the people who claim that hydrogen is the most abundant element in the universe, therefore fuel cells are a good idea.  Who cares how abundant it is, if it isn't abundant HERE?  The story of iron in stars is a great story, it doesn't need the tie to barn paint.
+Topher Belknap I don't think it negates the story, because while the crust abundance of iron is significantly less than its bulk abundance in the planet, it's still the fourth most common element in the crust, after only O, Si, and Al, none of which make good pigments for the reasons discussed.
+Matt Oliver I didn't go too much into the oxygen question, because there's a lot to say about the prevalence of oxygen on Earth. :) However, it doesn't directly affect the iron issue, because the large majority of the iron on Earth (and on Mars as well, for that matter) already appears in the form of oxides, especially iron (III) oxide. This was caused by the original chemical reactions as the Earth formed -- oxygen is the most abundant element by mass on Earth by number of atoms, and absent reactions like photosynthesis which keep replenishing the amount of free O2, its reactivity means that most of it is in the form of compounds with other materials. So the iron oxide (as well as calcium and silicon oxides, which are also very common) is actually where the oxygen lives when it isn't being liberated by photosynthesis.
Yonatan, I'm assuming that most of the iron oxides used in production today are from surface formations much younger than the onset of biological photochemistry (3.5 billion years). It might be true that most of the  oxygen atoms in iron oxides have not gone through photochemistry, but I would guess that most of the earths crust (what we actually mine) is younger than the onset of photosynthesis. Right now, there is new iron oxides being made everyday at hydrothermal vents by combining iron(II) with oxygen rich seawater (the source of the oxygen is oxygenic photosynthesis at the surface) (see Eventually, these new iron oxides will get scraped onto the continents via the Wilson Cycle. If it were true that we mined iron oxide that was older than 3.5 billion years old, then I think your original story holds, but I don't think that is the case. Also, you mention the prevalence of iron oxide on Mars. It does not seem like Mars is tectonically active in the same way Earth is, so if we ever paint a Barn on Mars, I think your original description of why it would be red is correct. I think the story on Earth is a bit more complex.
Does it matter if the barn is metal or wood?
+Nancy Gates This all started out on wood barns; painting protects the wood in various ways. (The binder keeps water out, the pigment keeps UV out, and some kinds of pigment also block mold and so on) Metal barns need to be treated as well, but they're also a much more recent innovation -- metal was pretty expensive until recently. :)
+Matt Oliver The iron we're mining is largely newer than the onset of photochemistry, and I'm not really sure how one tells whether any particular iron oxides got their oxygen atoms from the reaction of free iron with atmospheric (photosynthetic) oxygen versus from other sources. But one thing that makes me suspect that a lot of the iron oxides had non-biological oxygen sources is the reactivity of free iron: at the formation of the planet, most of the crustal iron already took the form of iron oxides, and unless something first ripped the oxygen off them, they would likely stay that way.
Thank you Yonatan for your explanation. Appreciate it.
We have metal board and batten down here. Is a primer grey with flaking spray over greys. :/ 
Nasa said there was most probably life on Mars, which means photosynthese. But the curiosity detected no Fe or Iron oxyde, while the upper surface is red. Rusty red. That's peculiar.. was that a mistake in their report here? I looked it up. No Iron oxyde..
Yonatan, I'm shooting from the hip, but my understanding is that the ancient earth was had a lot of iron(II) because of a highly reducing surface and ocean environment. Also, microbes played a pretty big role (both oxygenic and non-oxygenic) in "rusting out" the surface of the Earth, creating many of the large iron formations we mine today. The link I sent before explains how folks can tell how microbes were at work in early iron formations, and that major iron oxide deposits we mine have been microbially (oxygenic and anoxygenic) mediated. In summary from the science daily article ("This research not only provides the first clear evidence that microorganisms were directly involved in the deposition of Earth's oldest iron formations; it also indicates that large populations of oxygen-producing cyanobacteria were at work in the shallow areas of the ancient oceans, while deeper water still reached by the light (the photic zone) tended to be populated by anoxyenic or micro-aerophilic iron-oxidizing bacteria which formed the iron deposits"). If this is true, then the story of red paint isn't just a story about dying stars, but a story about early geomicrobiology, with oxygenic photosynthesis as a major player. Here is the link to the nature communications article
You missed my point.  It could easily have been that iron was as rare as nickel.  Then all that bit about nuclear fusion would lead one to exactly the wrong answer. Iron ISN'T plentiful and thus cheap on the crust because of nuclear fusion, iron is plentiful on the crust for some OTHER reason.  If everywhere had the iron availability of Japan, we would be talking about how expensive red paint is, without changing one bit about nuclear fusion.
+Topher Belknap I don't understand what you're saying. Iron is plentiful in the solar system because of being the endpoint of nuclear fusion; it's somewhat less plentiful in the crust of the Earth than it is in the Earth or the Solar System as a whole, but not that much less plentiful, and is still by far the most common transition metal. It's true that, if the laws of physics and chemistry were different, it could dissolve faster and be as rare as Nickel, but as that isn't the case, I'm not really sure what that has to do with anything.
Nickel is not at all rare, since iron and nickel dissolve in each other.
+Yonatan Zunger

Now we are getting somewhere.  What are the laws of physics and chemistry that make nickel rare, and iron plentiful?  That is what explains why red paint is cheap.
+Topher Belknap Well, to start with, iron is nearly 20x more common than nickel in the Earth. There are additional processes that make nickel sink more towards the core, and thus reduce its frequency even further in the crust, which I'm afraid I'm not familiar with -- you'd have to ask a geologist. But even if it weren't for that, iron would still be far more common.
Hmm.. wood that's more reddish is stronger, grow larger and is more sustainable in use.. Teak e.g. Same goes for human blood btw.. not exactly the color, but the amount of iron. 
Thank you for the post. I added a couple more points in my own take on the red barn question (

• The Great Oxidation Event, 2.45 billion years ago, during which the iron in Earth's crust and soils first began to rust.

• The fact that iron-oxide-rich dirt needs no processing other than grinding before use, which contributes to its comparative cheapness.

There's also the question of white barns, but that's a whole different story....
Nicola Twilley, I tried to point out the importance of microbial processes on the iron formations we currently mine in a previous comment on this thread. There is plenty of evidence that the iron we mine was precipitated by microbial processes (like the cyanobacteria). see 
Iron Oxides are relatively easy to get at ("cheap") because they are ancient sedimentary processes mediated by microbes. 
The story told by Yonatan is very interesting, but I think it is too simple to be considered the reason why iron formations are so easy to get at on the surface of the planet.
+Matt Oliver
Aha, I wrote my post before I saw your comment — that Science Daily link is great. I agree!
+Yonatan Zunger Hi.
I'm still on about the prevalence of iron vs other elements and your basic claim that iron is common, therefore red paint (Fe2O3-based) is cheap, therefore it was used long ago and now we have a tradition.

I'm so sorry about how poorly you were represented in the first place I read of this article on Smithsonian Magazine.
They took out all the subtlety and explanation that you add, as well as the fact that you are completely aware that iron is not the most common element in the Universe or the Earth's crust. ::sigh::

Could you add a translation of the final paragraph so that if a lay-person wants to skip your (awesome) explanation and quote you or soundbyte you they can do it correctly? I can just tell from a lot of the comments here and on the Smithsonian blog (and what their author "got" from this article) that it's gotten quite popular, and people are missing the point: 1) you must be a good pigment (i.e. using a d-block element). 2) You must be plentiful (i.e. highly abundant in the universe). Iron is the most common element that fits both these characteristics.

Because when we miss that first important detail because we were so caugh up reading the interesting stuff about stellar fusion so we forgot the first part, then we might as well be painting with aluminum oxide (elements are a bit more common in the Earth's crust) or soot. Which is boring, not a good, pigment.

I tried to read all the comments to make sure this hasn't been said, and it looks like you've managed to branch a couple of FASCINATING discussions on this topic.
I have a corollary for you, +Yonatan Zunger !   Why is the kitchen painted blue?   Because that's Mother's favorite color.   

Ok, Ok, not as great a story to get to the ending, but exceptionally useful life lesson, don't you think?

Great post, by the way.
Right, and why are manhole covers round?

Because manholes are round.
+Matt Oliver maybe a stupid question, but has their been also a test with pure oxygen, with absolutely no contamination of whatever nanobacteria with Iron that makes iron-oxide?
As someone who just recently bought Barn Red to paint an actual barn, this may have once been true, but it's definitely no longer the case. Barn Red is mixed up by a paint computer same as any other color.

For your next was-historically-relevant-but-isn't-today story, you should talk about why fences are painted white. Hint: whitewash.
What a great post Yonatan! and a brilliant sustained feed back you have generated! 
+Ellen Molenaar it is not a stupid question. Yes, iron oxide forms without microbes. +Yonatan Zunger is correct that no microbes are needed to form iron oxide, and that dying stars certainly provide (on average) plentiful iron to Earth. However, the key is that we don't mine the "average" concentration of iron on the planet, we mine what is locally concentrated on the Earth's crust. Because Earth is tectonically active, the crust has been remade many times over since the formation of the planet. In fact, some of our first radiometric dates of the Earth come from the Moon! When we look at the actual iron formations we mine, they show that it was microbial processes and sedimentation that have locally concentrated the large iron oxide formations we actually mine (thus making them "cheap"). These processes are far removed from a dying star, and are thought to be related to the "Great Oxidation Event" mentioned by +Nicola Twilley . +Yonatan Zunger argues that iron oxide is cheap because dying stars provide a lot of iron to the Earth. This is true; but imagine for a minute a scenario in which iron oxide is plentiful, but totally inaccessible  It would not be cheap, nor be used in common paint. One example of this is gold. There is a lot of gold in the ocean (20 million tons according to NOAA), but it isn't concentrated in any one place to make it easy to mine. In my opinion, +Yonatan Zunger argues that average abundance of iron oxide delivered by a dying star is enough to make iron oxide cheap for paint. I do not think this is correct because we do not mine the average abundance of iron oxide on the Earth. We mine highly concentrated deposits of iron oxide, whose mechanism of deposition is likely driven by microbes, and only distantly related to dying stars. 
+Matt Oliver Ah! Now I see what you were getting at. Yes, this is absolutely true; biological processes have a lot to do with how iron gets concentrated at the surface. (But I don't know much about them so I won't say more) There's also the fact that iron oxide gets even more concentrated at farms because farms have iron tools which rust; farmers used to clean off that rust and mix it in with oils to make the paint.
+Yonatan Zunger thanks for understanding. Sorry if we were talking past each other. I would argue that rusting tools is also biologically mediated, since the free oxygen in that reaction is produced photosynthetically. This however, is a much messier cause for red paint on barns compared to your post about dying stars. I perceive that the story you wrote between the everyday red barns and distant dying stars gives a nice sense of scope and connection between the everyday barns we see and the magnificence of a dying star. It is hard to make that same connection with a single cell organisms most people associate with disease.
+Matt Oliver It all still boils down to the frequency of iron within the Earth -- if it weren't for that, all of these biological cycles would use something completely different, and that atmospheric oxygen wouldn't be rusting it. It's just another step in how iron ends up in easily-available locations.
Thanks for the explanation, Matt.
And that is only what men can see with our eyes, I mean the frequencies of elements in colors. Closing them and your body begins to detect them. We call that dreaming or illusion.. I am not sure about that though. I think eyes are more ment for focussing on a task, the rest we missed is processed at night. Einstein knew that. Anyway, our universe is electric and vibrating!
Highly circumstantial argument, +Yonatan Zunger.  Barns are not painted red due to cost (the marginal cost of other paint colors is low; you would instead use a black paint or a white paint, as they are cheaper than red paint) .  It's because red is a highly contrastive color against both green (summer color) and white (winter color).  Geez, any farmer knows this from trying to find their barn in a snowstorm!
+Yonatan Zunger No, it does not still depend on the average frequency of iron within the Earth. It depends on the high concentration of iron at the surface, which we actually mine, which has nothing to do with dying stars. Dying stars may have provided the source material, but so did the big bang, or any other antecedent cause. You are confusing the concepts of mass and flux at a surface boundary. You may be correct that the average mass of iron oxide in the whole Earth is a result of dying stars, but we don't mine (flux) the average. We mine the near-surface, high concentrations of iron oxides that have been concentrated through the joint process of tectonics, microbial metabolism, and sedimentation.
Consider liquid water. The average concentration on the planet is much much lower than the local concentration at the surface. The availability of liquid water (and therefore price) to humans depends on the surface concentration, not the average concentration on the planet.
My concern is that you have written a nice article that has been widely, and uncritically accepted because of your science background and your visible platform of +Google. Your article has a lot of interesting science facts on iron oxide and dying stars, but your basic hypothesis that iron oxide is cheap because of dying stars does not match the observations. You have written a beautiful story, but that is all it is. Simple stories in science are beautiful, simplistic stories are misleading.
+Yonatan Zunger Hadn't heard the bit about farmers making their own paint from rust, but it makes some sense. If true, though, then the use of iron oxide as a pigment for barn paint is due not only to its plentifulness but to iron's suitability as a material for tools. If for instance copper were harder than iron rather than vice versa, presumably it would be used for tools even though there's less of it available; in that case would barns be, say, Egyptian blue? Then a complete physics-based explanation of barn color must include a discussion of why iron is a better tool material than the alternatives.
+Richard Holmes There are a lot of aspects to that, but part of it boils down to the fact that iron is plentiful as well as being hard. One of the key changes from the Bronze Age to the Iron Age was that the prevalence of metal tools went up sharply, simply because iron is so much more prevalent than copper and tin.
+Richard Holmes Check out the "Age of Iron" episode of Neil Oliver's BBC2 "A History of Ancient/Celtic Britain" for a very interesting look at the economic impact of the conversion from bronze to iron.  Iron is not only more readily available, it is worked very differently.  A broken bronze implement has to be smelted down and re-cast.  Iron can be welded, re-forged, etc.

When the bronze economy in Britain collapsed it was almost 200 years before the iron economy really became established--and with it a very different social and political order.
+Yonatan Zunger Sure, but prevalence notwithstanding, we wouldn't be using iron for tools if it were a bad material for it.
+Richard Holmes It's very telling though, that the tools used to mine the ores of the bronze age were not, themselves, made of bronze, but of stone, bone, and wood.  Bronze tools were as much (or more) status objects as they were items of utility.  The same cannot be said for iron.
Probably the most common bronze age tool is the bronze axehead and by the time of the iron age's beginning, they had become, essentially, currency, completely divorced from their utilitarian purpose.  Viz. the hoards of unused axeheads that have been uncovered from the collapse of the bronze economy:
+Yonatan Zunger Getting even more interesting.... excellent researched responses by you to  comments has made this discussion a great science resource for high school students studying chemistry, physics, geology and  biology.
It's only Einstein that can solve d questions within d given time frame. I shall be interested in the solutions of the posers. Thanks. 
Good rearch.i would have says it has low reflection to the eyes.
You know, that's all very fascinating, but it doesn't  truly have anything to do with why barns are painted red. It's distracting but baseless to try and say we paint them red simply because  the element closest to the cutoff point for normal fusion reactions in stars forms a red compound with oxygen.

It's all neat nuclear astrophysics until you start trying to use it to explain why humans do arbitrary things. I'll tell you what, barns are painted red coincidentally. It's not the stars fault we chose to use iron oxide in our paint.
+Peter Brydon Congratulations, you failed to grasp the entire point.

There are reasons for the things people do, often economic ones. Economics are governed by abundance, which is determined in this case by physics.

"Coincidence" means you don't know what's really going on.
+Richard Holmes  (However this silly tag thing works)

Hydrogen is the most abundant element in the universe, iron is not.  The fact iron requires a dying star is what makes heavier elements and the elements like it more rare.  Spectroscopic analysis of the milky way galaxy shows that there are only 1,090 ppm of iron present, with  739,000 ppm or so of hydrogen. If it was about abundance we would be using hydrogen or other lighter elements for everything. The most common elements in the universe are hydrogen, helium, and oxygen. 

I know a fair bit about nuclear astrophysics myself, I've taken astronomy, physics, and a fair bit of nuclear chemistry. Coincidence can also mean  unrelated data. Such as "Oh, we use this because X but this is also Y. Y is coincidental because it has nothing to do with why we use it."

The real reason they use red paint? I'm going to wager it's simply that it was available and did the job. It has nothing to do with how the elements were produced. It has to do with the properties of the compounds and whether or not they had an easy enough way to get and use them. Trying to act smart writing an article like this, talking about the nuclear physics behind it  and then  pretending that's why we use iron is just stupid.  It honestly doesn't make sense.

We use things because we find uses for them and have them available in sufficient amounts. We don't choose to use things just because they're this magical cutoff element when a star approaches its demise. 

I failed to grasp the entire point? Please, I'm a chemical engineering major at UCSD. I'm only annoyed that he wrote a nice science article on readily available information like this he could have copied from wikipedia and other articles and then lowered himself to a stupid claim in the end.  I think you missed my point, honestly. lol
+Yonatan Zunger this was a link I found on gizmodo and then on Smithsonian as pointed out above.. quite impressive. This is actually the first time I had a reverse lookup for something I was reading up on related to science!
Tim Tan
"they tend to be much more expensive for the simple reason that there are a lot more rocks than there are animals and plants"

This doesn't really make sense when in terms of pervasiveness the most commonly synthesized pigment on earth is chlorophyll  (hell, in a way it basically makes itself! the evolutionary optimization led to a much shorter loop than recovering iron)

The whole narrative of the pieced would make more sense if it asked "why paint" instead of making a leap of faith from why color X to why iron. As it is, it's good science promo for joe 6 pack, but somewhat masturbatory for serious inquisitors.
+Tim Tan The volume of iron oxide in the Earth's crust is much, much higher than the total volume of all the plants on the planet. 

I didn't go into why paint, but the short answer is that it protects the wood: the binder from water, the pigments from UV, and some pigments (including iron) from fungus and other parasites as well. 
Tim Tan
Surely the volume of biomass on earth is sufficient (and readily available for literal plucking) for the purposes of coloring. Evidently we can even consume loads of the stuff everyday without running out. The point here is that abundance is hardly an adequate argument in itself, which is why expanding on the other virtues of paint and specifics of pigmentation in its manufacture rather than astrophysics would help better advance the general case you seek to make (ie. why are barns red, or generally why object X is color Y). Otherwise the question becomes "why aren't all cheap things red".
Off the subject, but why are REE elements so highly fractionated in crustal rock that La is often enriched by a factor of >100 compared to CI chondrites?
+Yonatan Zunger I'm really annoyed by this post.  and after some basic research, I was able to confrim +Oleg Zabluda's conclusions:

1) barns aren't painted red more than other colors
2) milk-based paint was cheaper and easier to work with, which is why most barns were painted white, not red

It might be better next time to write a post that is simple:
TL;DR: barns are painted white because the universe exists

and then provide a very short series of simple pointers to evidence.  Your post is a nice walk through cosmological physics, but the conclusion is fundamentally wrong; you wasted my time.
+David Konerding Congratulations. You have just written a classic Internet comment! You score points for:

* Nitpicking details (was white more common than red, historically?) without regard for whether they're relevant to the thread of the post or not,
* Making blank assertions (I know this is true!),
* Maintaining a smugly patronizing tone which demonstrates your intellectual superiority to everyone else on the thread, and
* Ending on an angry note of entitlement. (You wasted my time!)

To reach the next level of Internet commenting, you should try some combination of:

* Respond angrily about how I'm ignoring the obvious correctness of your statement, and providing a deluge of data that nobody cares about; 
* Respond angrily about how you're being silenced and ignored, and/or your freedom of speech is being infringed;
* Make a rant, here or elsewhere, about how my not paying attention to a word you say is proof that I'm a terrible person, should never be trusted, and am everything that is wrong with {Google+ | Googlers | Humans as a whole}; or
* Just go for the Godwin.

I think that you have already reached the level where you're ready to start to compete over at 4chan. Maybe you should try it out over there!
<wishes for daily double>
:: blink :: Usenet flashbacks... they told me this would happen if I wasn't careful about my internet consumption.
I just got redirected here by a commentor on another thread - why do I always miss the best posts? My revenge is this: I am hereby resurrecting this comment thread from the dead. See, done. Some nice storytelling here, BTW. 
Interesting. OK red paint is cheap but why did they paint the barns in NA when in other regions they are left with a natural wood finish which would appear to be even cheaper? Wood might be expected to rot, at least in rainy old Europe, just as much as here. I'm guessing fashion was a big part of it but maybe due to scale of the market paint was cheaper here than in other markets? Maybe labor was cheaper here relative to farm income?
+Owen Brunette Paint helps protect wood -- it seals it against pests, and the pigment protects the wood from sunlight as well. Painted buildings rot far slower, especially where there are problems like rain.
+Owen Brunette​​​​ old wisdom has it that if wood is allowed to air freely, sun's UV light will kind of cure a thin layer of its surface, turning it gray, but the wood inside stays healthy and won't rot, even for a hundred years. The gray layer is sensitive though and you will not get this effect if you paint it even once, because it will fall off with the paint. So if you paint it once, then you must paint it again and again. Also other surface finishing methods may spoil the surface. I'm not an expert on this, but just repeating what I have heard.

They also say that overhanging eaves and a high enough stone foundation are necessary for this to work. Then the gray wood should be more durable than when painted. I'm not sure if this is still true with modern paints. (Also, I have some Tikkurila shares, so use their paints, if you are going to paint anyway. ;)

The type of wood and local climate may be relevant. I'm guessing that this may be different in a humid climate with wood-destroying insects around the year. Is anyone aware of long term studies on this? It would take decades to properly test this, because it may be difficult to determine how existing structures have been treated over the decades.
Barns are red because before paint was available they used a mixture of milk honey and cow blood which was available after they slaughtered their livestock. History not physics
+Michael Meskis​​ and cow blood is red because... Oh yes. Iron. So physics doesn't play a role in the fact that around 99% of species on earth evolved an iron based oxygen transport despite the fact that other, rarer elements are more efficient. 
This is a good explanation of why red paint is so cheap, but doesn't address the reason that barns are painted red.

That reason is driven by history, sociology, and economics (but not in the way you might think)

The earliest builders of "little red barns" and log cabins in the United States were Swedish immigrants:

Swedes painted their cabins and outbuildings with a colour called, "falun rödfärg," falu red in English.  This colour was a byproduct of the copper mines in Falun:  The copper content of the paint helped to preserve the wood.

The bulk of Swedish immigrants ended up in the "North Country" of Michigan, Wisconsin, and Minnesota, where there were a lot iron mines, producing a lot of tailings.  Therefore, it was easy to get a similar paint to the one they had used back home.

King Magnus IV had a personal financial interest in the mines and to further that interest, he decreed that all houses be painted red with the paint produced by the mines.
+John Maxwell Hobbs good point. I have actually visited that mine. On a slightly tangential note, they used a lot of ox hide for ropes and made sausages of the meat, falukorv ("Falun sausage").
+Sakari Maaranen I love falukorv! (jag gillar falukorv), although they don't make them from exhausted mules anymore.  I have been to the mine as well, and that's where I learned the story of King Magnus IV and the sale of red paint.
Thank you everyone - brilliant all across the board - I've learned so much reading
Well, this explanation does not fit on my G+. Red paint is cheap.
+Yonatan Zunger​​ I have read about half of it so far and will read the rest later.

I was born and raised on a farm, and I remember this same question coming up in elementary school. The reason given was that red allows you to see the barn better in fog.

I never really debated that idea, but I never really fully believed it either. On extremely foggy days, working outside away from buildings, I know where I am relative to the farm buildings, and how to get back. Plus, the red barn will usually not be the first building I will see anyways.

Anyways, that's not why I am posting. I am posting because you can go farther!

If it truly is "because red paint is cheaper", then one could rightfully say that the cheapest solution is to not paint the barn at all.

So this answer would not be related to nuclear fusion anymore, but it would related to some other cool physics. This would ask the question of "why should I paint my barn?" Where you answered the question of "why did I choose the color red for my paint?"

By the way, I just noticed this original post is 165 weeks old. Haha
+Christopher Rucinski​​ that's why traditional barns in Finland aren't usually painted. As long as the roof has long eaves and the building has a stone foundation, the wood won't rot but will naturally develop a gray surface layer that can protect it even better than paint. Iron sulphate treatment can be used to speed up the graying process, but it also happens naturally by the sun's ultraviolet rays.

It doesn't matter how old threads are when they are on timeless topics. We could continue a two hundred year old discussion, had the Internet been available back then, with compatible systems of course. That's pretty much how science works as well.
+Yonatan Zunger​​ that's probably true - and it also depends on the type of wood. I would expect it to work in a range of climates, but not sure what would be the limits. This is not something I have studied extensively myself, so I cannot really comment on the specifics.
Yes, I was basically talking about the science of rotting, weathering, and the science of natural and artificial protection. 
from now on, google should just return "because stars die" to every question.... no one can refute it, and it'll be way cheaper than employing thousands of engineers.... onwards and upwards!
Beautiful article--thanks! I would argue that the evolution of eyes sensitive to visible light on (water soaked) Earth comes as much from the absorption v. freq. properties of H2O, and the deep notch right in the visible. See below (from J.D. Jackson "Classical EM")
"(There are also organic pigments, such as the Imperial Tyrian purple made from the snot of the Murex snail, but not as many, and they tend to be much more expensive for the simple reason that there are a lot more rocks than there are animals and plants."

This may have been true a long long time ago but, we have lots of cheap organic pigments that come from petroleum. Abundance isn't the only factor in price, some organic pigments are much more potent (you need less of them to get nice saturated colors). Many of them are really lightfast too.

"the real color is determined by the d-electrons of whatever attaches to it: red from iron, blues and greens from copper,"

eeep. We can get iron ochres that come in a variety of colors (yellow, green, black, violet even blue). Color is complicated: particle grind, hydration, impurities etc. all affect color. Granted, I'll bet that red/brown ochres are the most common.

The real reason why barns are painted red is might initially be due to price determined by abundance (although some sources suggest that blood was added to the linseed oil mix, blood is otherwise very expensive iron); but price is always a very complicated function that includes consumer behavior and preferences.
+mittimithai Yes, but if I started getting into the complete physics and chemistry of color, or how 20th-century petroleum extraction changed the economics of dye manufacture, this post would have gotten way the hell too long. :)
This thread. It will not die. :)
didn't realize this was so old :)
In fairness it's a really interesting topic.
I've always thought barns were painted red to stand out against a generally green environment...therefore being a functional choice to create a kind of landmark that someone could spot far off.
Ken Lim
This is a very late response, but my PhD Candidate son sent me a link to the old Smithsonian article referring to this page...

Congrats on a great wide-ranging analysis, with good [but overly-long & detailed] explanation of stellar nucleosynthesis.
HOWEVER, the end answer is probably totally wrong!
Believe it or not, I actually looked into this old fallacy sometime last century.
I’ve heard the red paint is cheaper answer many times before, but I’ve never seen any discussion or proof of that.
Every paint I’ve ever seen is priced on quality levels, NOT color! Price is absolutely inconsequential, since there are many pigments, oils, paints varnishes, & stains that are cheaper, more expensive or easier to apply or have better durability than the original linseed oil-based recipes that pioneer farmers used to protect their barns from the elements. Also, most red paint is NOT formulated with iron oxides which are primarily red-orange, not "barn-red"

The answer is clearly a cultural/traditional thing, since only American barns are mostly red. Barns are painted all kinds of colors in other countries. Even in America, red is most prevalent in New England farms and get less common as u go West.

Another factor missed is that from basic mineralogy, different compounds can appear wildly different colors with different impurities, qty & ratios & locations in the crystal lattices. Also, grinding stuff to make pigments actually results mostly in white powders, regardless of the color of the rock. (which is why the “streak test” is a simple, valuable field test in mineral ID). Finally, it’s now well-known that color can arise from nano-structured surfaces not dependent on electron shells at examples of it are insect shells, bird feathers, butterfly wings. {yes, I realize this last factor has nothing to to with barn color]

So, the original question is based on a bogus premise/implication...
that barns are painted red—some are, but most are not.

+Ken Lim Colonial red paint used an iron oxide pigment, hence a better premise might be that "where barns are painted red, the choice of red paint may have been motivated by its inexpensive, local production from iron oxide." In fact, price can explain a lot of other pigmentation too. There's rather more dark blue paint than people are accustomed to employed in Charleston, SC (my home town), but it makes quite a bit of sense when you realize that the colony was founded in part to grow indigo which tints paints as readily as it dyes pants.
+Ken Lim
 There are quite a few areas where barns are usually painted red.

In Sweden, there is a song:

"röda stugor tåga vi förbi"

Roughly translated as:

"red the barns that we are passing by".

Except, "stuga" is more like hut or cabin than like barn.
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