An article in the journal, Archeology, describes new evidence of a robust dye industry that endured on the Mediterranean coast for millennia. A dig in Tel Shikmona, south of the city of Haifa in Israel, yielded dozens of pottery vessels and shards covered with purple and blue stains. They also unearthed industrial pools and mounds of murex shells.
Earth’s Skies Are Violet, We Just See Them As Blue
Why is the sky blue? It’s a common question asked by children, and the simple answer is that blue light is scattered by our atmosphere more than red light, hence the blue sky. That’s basically true, but then why don’t we see a violet sky?
The blue sky we observe depends upon two factors: how sunlight interacts with Earth’s atmosphere, and how our eyes perceive that light.
When light interacts with our atmosphere it can scatter, similar to the way one billiard ball can collide with another, making them go off in different directions. The main form of atmospheric scattering is known as Rayleigh scattering. If you imagine photons bouncing off molecules of air, that’s a rough approximation. But photons and air molecules aren’t billiard balls, so there are differences. One of these is that the amount of scattering depends upon the wavelength (or color) of the light. The shorter the wavelength, the more the light scatters. Since the rainbow of colors going from red to violet corresponds with wavelengths of light going from long to short, the shorter blue wavelengths are scattered more. So our sky appears blue because of all the scattered blue light. This is also the reason why sunsets can appear red. Blue light is scattered away, leaving a reddish looking sunset.
But if that’s the case, why isn’t the sky violet? Sure, blue light is scattered more than red or green, but violet light has an even shorter wavelength, so violet should be scattered more than blue. Shouldn’t the sky appear violet, or at least a violet-blue? It turns out our sky is violet, but it appears blue because of the way our eyes work.
Color sensitivity of the cones and rods of the human eye. Credit: Wikipedia
We don’t see individual wavelengths. Instead, the retinas of our eyes have three types of color sensitive cells known as cones. One type is most sensitive to red wavelengths, while the other two are most sensitive to green and blue wavelengths. When we look at something and the light strikes our retina, the strength of signal from each type of cone allows our brains to determine the colors we see. These colors roughly correspond to the actual wavelengths we see, but there are subtle differences. While each type of cone has its peak sensitivity at red, green, or blue, they also detect light of other colors. Light with “blue” wavelengths stimulate blue cones the most, but they also stimulate red and green just a little bit. If it really was blue light that was scattered most, then we’d see the sky as a slightly greenish blue.
We don’t see the greenish hue, however, because of the sky’s violet light. Violet is scattered most by Earth’s atmosphere, but the blue cones in our eyes aren’t as sensitive to it. While our red cones aren’t good at seeing blue or violet light, they are a bit more sensitive to violet than our green cones. If only violet wavelengths were scattered, then we would see violet light with a reddish tinge. But when you combine the blue and violet light of the sky, the greenish tinge of blue and reddish tinge of violet are about the same, and wash out. So what we see is a pale blue sky.
As far as wavelengths go, Earth’s sky really is a bluish violet. But because of our eyes we see it as pale blue.
Purple in Peru
A few years ago, when my wife and I were traveling in Peru, we visited the village of Chinchero, high in the Andes. We stumbled on a weaving shop where you learned how the area’s alluring textiles were made.
We stood, watching five squatting Quechua Indian women. They were artists, weaving Andean llama, alpaca, and vicuña wool into dazzling fabrics famous for their vivid hues and striking designs.
As I watched them work their magic, I wondered, where did these vibrant pigments come from? We drew closer to see the details. The women placed a small heap of grayscale insects, called cochineal (pronounced co-chee-kneel), they had collected by hand from prickly pear cacti. One woman crushed the pile of insects with a pestle. Another poured some wood ashes on their pulverized bodies.
We gasped – the ashen powder turned red, then red-purple, and finally a radiant blue-purple.
A weaver of Chinchero showing us how she makes dyes. (Photo by P.Salber)
I closed my eyes, recalling the wonder I felt in elementary school when the teacher demonstrated the litmus test. We had just witnessed an elaborate experiment carried out not by chemists in their laboratory but by people who are one with their environment and are living its most intimate secrets. Whether they understood the molecular basis of the change, as laboratory chemists would, was irrelevant.
How on earth did they figure this out?
As my kids would say: AWESOME! Envision the Quechua learning that these insects produce color. Crushing one of them between your fingers stains them a bright red. The bodies of the dried female insects contain 12-16% carminic acid which is a vivid shade of crimson.
But, how did they learn to combine different additives to the crushed dried bodies of these insects in order to create different shades of the original color? They likely experimented, as scientists do in their laboratories.
Wood ash and other alkaline substances increase the pH of the mixture to create purple. Small amounts of iron can also be used to transform the red to purple. Adding an acid, such as lemon juice, produces a bright scarlet. This brings me back to my wonder when I learned of the litmus test.
The surprising role of the colors blue and purple in history
The ancients knew how to make the colors purple and blue
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- As early as 3000 years ago, the ancient Phoenicians made three major discoveries:
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- They gave us the alphabet we are using today.
- They discovered that by heating silicon oxide, found in unlimited quantities in the sands of the Mediterranean beaches, they could make glass.
- And, by extracting the secretions of the seashell Bolinus brandaris(also called Murex brandaris) found on the beaches of the eastern Mediterranean, they could make a highly-prized purple dye. The dye did not fade with time but instead increased in brilliance with exposure to air and sunlight.
The dye was called Tyrian Purple, after the Phoenician port city of Tyre. They also extracted another dye, Royal Blue, from a closely related species.
As we’ll see later, the process of getting the blue dye was not straightforward and was very laborious. Couldn’t the ancients find an easier way to get blue?
There aren’t many naturally-occurring blue materials
As Baruch Sterman, a physicist in Israel, explains that our eyes can only see an object as blue when it absorbs red light. This is something few naturally occurring materials do.
Stones and plants were among the handful of naturally occurring blue materials in ancient times, including:
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- stones, including lapis lazuli from what is now Afghanistan
- plants such as indigo that grow in warm climates like India and Africa
- woad (a plant of the cabbage family) that grows around the Mediterranean.
Ground-up lapis lazuli can be used to make paint, but not to dye textiles. Sadly, while indigo and woad dye fabric, they eventually fade.
Part of what made murex dye so valuable was that its colors remain brilliant. For example, 2,000-year-old pieces of murex-dyed wool found in caves near the Dead Sea are still vibrant today [REF]. Unlike the Andean women we observed in Chinchero, there are no Phoenicians around to explain how they did it.
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