Making the perfect red  pigment is surprisingly hard.

We can nail the color, of course. Bright red is already out there  in paintings, mugs, and Ferraris.

But the ideal source of red isn’t just vibrant. It’s durable. And non-toxic.

All at the same time. We have found a pigment like this  for many other colors in the rainbow. But so far, we have yet to find that  ultimate red, either in nature or in the lab.

It turns out, much of the science behind  creating specific colors is still unknown,   which makes engineering new colors hard. So scientists are trying a bunch of different   approaches to making that perfect  red and seeing which one sticks. And one of them doesn’t use any pigments at all. [♪ INTRO] To precisely define red, I first have to  explain a little about how colors work.

Ultimately, colors are created by our brains,   based on electrical signals that  come from the backs of our eyeballs. But it starts with light. Light is made up of electromagnetic waves that we   often depict as sine waves  with different wavelengths.

Humans can only see the wavelengths  between roughly 400 and 700 nanometers. 400 is the violet end of the rainbow. 700 is the red. Anything outside that range doesn’t  play well with the biology of our eyes,   and is essentially invisible unless  technology helps translate for us. Now, light itself can appear to have a  color, depending on the exact blend of   wavelengths that are involved and how  they compare to generic white sunlight.

But much like we’re going to focus  on light instead of the human brain,   we’re going to focus on light interacting  with an object we perceive as having color. One main way we get color is through  special molecules called pigments. When light hits a pigment molecule, some  combination of wavelengths gets absorbed,   effectively removing them from the world.

The rest gets reflected and  produces the actual color. Exactly which wavelengths are absorbed has  to do with the pigment molecule’s electrons. Specifically, how much energy the electrons  can absorb to change where they’re hanging out.

We’ll get into a little more  detail later, but for now,   the important thing to know is each wavelength  of light has its own corresponding energy. To get a true red, a pigment  must absorb all wavelengths   except those in the red part of the spectrum. Roughly 620 to 700 nanometers.

If any other wavelengths sneak  past, they’ll muddy the hue. Now, everyone who’s had the misfortune of eating   a Red Delicious Apple knows  red pigments exist in nature. And humans have been using some of  them for centuries, if not millennia,   in paintings, textiles, and ceramics.

But there’s usually something that  keeps them just shy of perfection. For example, red can come from  organic materials like carmine,   made by grinding up female cochineal beetles. Carmine’s been used for thousands of years  in the Americas in paintings and textiles.

And it’s still used today in red  food dye and some red cosmetics. But carmine, like all organic reds,  fades over time when exposed to light. Life, even in death, is fleeting.

So to get something that can truly last,   people have tried pivoting away from life  to instead make red pigments out of rocks. Red ochre, for example, is a  pigment made from an iron oxide   mineral called hematite…which  basically is a block of rust. Unlike organic reds, red  ochre doesn’t fade over time.

In fact, cave paintings from 20,000 years  ago that used red ochre still look red. Well, red-ish. Red ochre’s Achilles heel  is that it isn’t truly red.

To me it looks kind of like  rusty reddish orange-ish but   scientists can also confirm that  it’s not true red quantitatively. If we measure which wavelengths it reflects  instead of absorbs, we can see that the   reflected wavelengths include oranges and  yellows, leading to that brownish tint. There’s another red rock pigment  called vermilion that’s also   been used for forever in art, and  it’s actually quantitatively red.

Vermilion’s made from cinnabar, a mineral  found near volcanoes or hot springs,   and we can also make it from scratch ourselves. Yay chemistry! And yay Pokémon for teaching  me how to pronounce those things.

But vermilion’s durability is hit-or-miss. Sometimes it’s fine. Other times, it chemically reacts to light  and chlorine in the air and turns black.

Definitely not red. What’s worse, vermilion’s chemical  make-up includes the element mercury,   which is highly toxic. So unfortunately, we can’t depend on nature for   a good red that both stands the  test of time and is poison-free.

So instead, we’ve been trying to make  red ourselves using inorganic materials. But making specific colors from scratch is hard. There are lots of variables that play into getting  the pigment molecule’s electrons to absorb the   right wavelengths…such as the types of atoms  in the material, and the distance between them.

And right now, we aren’t quite  able to predict exactly which   color will come out of different  combinations of these variables. The only way to know is to just  make it in real life and see. But scientists do at least know several general  mechanisms behind how inorganic materials produce   color, and they’re following these different  leads in search of the elusive perfect red.

Which I will get to, right  after I show you this quick ad. Hey SciShow viewer! You’re learning a lot right now.

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To get started with Brilliant’s tutor for  free, click the link below or scan the QR code. You can upgrade to Premium to unlock all courses. And right now, SciShow viewers can save 20% off  an annual subscription at brilliant.org/scishow One lead involves semiconductors,   which as the name suggests, are  conductors only some of the time.

When all their electrons are stuck  close to their home atomic nuclei,   the whole thing acts as an insulator. But if you give the electrons a  sufficiently large energy boost,   they’ll move a little further out,  and flow freely between atoms. The semiconductor switches  from insulator to conductor.

The amount of energy that’s needed to  make this switch is called the band gap. And semiconductors can absorb any  wavelengths with energy above that gap. In fact, this is why vermillion is so red.

Vermillion is a semiconductor! But for an example of a purely  synthetic semiconductor pigment,   we can also look to cadmium red. This pigment combines cadmium, sulfur,  and selenium to make a brilliant red   that’s incredibly durable, leading to  its use in ceramics, plastics, and cars.

But cadmium is, say it with me, toxic. So researchers are looking at  another type of semiconductor,   called perovskites, in the hopes of  achieving a non-toxic cadmium red. Perovskites are a class of  materials that all feature   a specific ratio of three different elements.

One of their selling points is that  they’re resistant to high temperatures,   making them suitable for being fired in ceramics. To get a proper red, a perovskite needs a band gap   that allows the material to absorb  all visible wavelengths besides red. But tuning the band gap isn’t straightforward.

There’s no magic knob labelled  “band gap” you can adjust. Some researchers are approaching  this challenge by playing around   with different combinations  of elements and their ratios. So far, they’ve been able to create a  whole range of reds this way, but sadly,   none of them are as vivid as cadmium red.

Meanwhile, other scientists are  investigating metal oxides as yet   another potential solution to our pigment problem. Now, not all metal oxides can work as pigments. In fact, most can’t, because  of their chemical structure.

In an atom, generally speaking,  every electron has a designated   region of space that it’s allowed  to metaphorically bounce around in. This space is called an orbital. For a metal oxide to give off color,  electrons living in one specific type   of orbital must be able to jump to  another version of that same orbital.

But physics doesn’t normally allow that. It can only happen if the molecule’s structure  has a specific kind of geometric asymmetry. But in the rare case it has that, an electron  will make that jump, both absorbing the   appropriate wavelength of light as per usual,  and also reflecting the complementary color.

One chemist named Mas Subramanian has been trying   to use asymmetric metal oxides  to make the perfect red pigment. He stumbled upon this approach in 2009, when one   of his grad students accidentally  made the bluest blue pigment. It’s called YInMn blue, due to its  composition of yttrium, indium, and manganese.

Since then, he’s made a range  of other “perfect” colors by   swapping out different chemical elements. In a 2024 paper, he took a stab at making red  by creating a metal oxide centered around a   metal chromium ion, which has a similar chemical  structure to the manganese ion in YInMn blue. This specific chromium ion, however, requires  a super low oxygen environment to exist.

Like, what you’d find on the Moon low. So it took some work to wrangle the  new chemical together, but he got it! Except after all that work, the resulting pigment  wasn’t quite red…but rather reddish-magenta.

So we still haven’t really been successful with   inorganic materials in getting a truly red  pigment that’s also durable and non-toxic. But there’s still time and hope. While we wait for more news out of these camps,   other scientists are tackling the red  problem in a completely different way:   Using a method for producing color  that gets rid of pigments entirely.

Unlike pigments that produce color by absorbing  light at the atomic scale, structural color   produces color at the macroscale, using the  structure of an object’s surface to scatter light. Here’s how it works. When a ray of light hits the surface of  a structural color, some of it reflects.

No surprise, there. But then, that reflected wave can interact  with another bit of reflected light. In some cases, two interacting  waves will perfectly align,   which winds up reinforcing their color.

Other times, they're perfectly out of sync,   canceling each other out so the  corresponding color can’t be seen. Light waves can also exist in the  middle of these two situations,   with partial reinforcement  and partial cancellation. One example of structural color is  the iridescence on a soap bubble.

The soapy shell isn’t the exact same thickness  all the way around, so depending on exactly   where light hits the bubble, different  colors get reinforced or cancelled out. This produces a whole rainbow of colors. But of course we aren’t looking  for a rainbow right now.

We need something a lot more fancy and  fine-tuned than you can get with one of these. In a paper from 2022, one team of scientists tried   making a structural red by stacking  together layers of silver and silicon. The idea was to carefully eliminate all  non-red colors by using each layer to   get different wavelengths of light to cancel out.

And it worked really well! They got a nice bright red…but it  only looks that way at certain angles. Specifically, within 60  degrees of a straight-on view.

This angle-dependent behavior is one of  structural color’s primary pain points. Getting wavelengths to exactly line up to cancel   requires knowing exactly how they  travel through space to your eyes. And if you change where you stand,  that changes the whole calculation!

Another team of physicists  tried to tackle this angle   problem by using microscopic capsules  of nanoparticles suspended in water. Each nanoparticle had a polystyrene  core, surrounded by a hydrogel shell. The physicists did a bunch of  calculations and experiments   to figure out the best size and spacing of  nanoparticles to get red in all directions.

And according to the math,  everything seemed to work. But in reality, instead of  red, they ended up with pink. Turns out, they made an assumption in  their calculations that wasn’t true:   that all light hits the surface  of these particles once.

In reality, some light rays did only reflect once. In which case, red light was successfully  enforced while the other colors were canceled. But for some light, after that  first bounce, it kept bouncing,   shifting the alignment of some non-red  waves so that they no longer canceled out.

Together, all these extra colors added  a white hue, turning the red pink. While it’s not at all like accidentally  turning your sheets pink by washing them   with a stray red sock, the feeling  of disappointment is surely similar. The perfect red may elude us for now, but there’s  plenty of imperfect options to hold us over.

And whether scientists find the eventual answer   in YInMn blue’s cousin or a  suspension of nanoparticles,   I’m sure there’s at least one Ferrari fan who’s  gonna say it still doesn’t look quite right. No shade to Ferrari! I don’t know F1.

I don’t watch cars! [♪ OUTRO]