Blue

• 1. Introduction

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  Blue is the colour that hovers between violet and cyan on the visible spectrum, and it is also, by multiple surveys across Europe, the United States, China, Malaysia, and Indonesia, the single most popular colour among both men and women. That consensus feels almost too easy, too obvious. Blue is the sky. Blue is the sea. Of course people love it. But the story behind how blue got there, embedded in daily life from military uniforms to the robes of the Virgin Mary, from Nobel Prize-winning semiconductor research to the pigment that once cost more than gold, is anything but obvious. How did a colour so common in nature become so rare in ancient art? Why did whole civilisations struggle for millennia to make it reliably? And how did a dye discovered by accident in 1709 end up painting the wave paintings of Hokusai?

• 2. The Science of Blue Light

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  Rayleigh scattering, confirmed by Albert Einstein in 1911, is the mechanism that paints both the daytime sky and the deep sea the same colour. When sunlight passes through the atmosphere, oxygen and nitrogen molecules scatter the shorter blue wavelengths more widely than the longer red ones, sending more blue light to the human eye. The sea works along similar principles: water absorbs longer red wavelengths and reflects and scatters the blue back toward the viewer. The deeper an observer descends, the darker that blue becomes, and in the open sea only about one percent of light reaches a depth of two hundred metres.

  Isaac Newton placed blue among the seven colours of the visible spectrum in his first formal description, and he chose seven categories deliberately: he believed the number matched the notes of the musical scale, which he thought was related to the optical spectrum. He included indigo as a distinct colour between blue and violet, though today it would be classified simply as blue.

  Pure blues sit near the middle of the 450-495 nanometre range on the spectrum. Shorter wavelengths within that band push toward violet; longer ones lean cyan. The Tyndall effect, an optical phenomenon closely related to Rayleigh scattering, explains blue eyes: irises do not contain blue pigment at all. The stroma of the iris scatters short-wavelength light in a way that makes the eye appear blue to an outside observer, much as the atmosphere makes the sky appear blue.

• 3. A Colour Slow to Name

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  Lapis lazuli artefacts dated to 7570 BC have been found at Bhirrana, the oldest known site of Indus Valley civilisation, yet the linguistic category of blue arrived far later than the pigment itself. Research into the development of colour terms in natural languages shows a consistent sequence: languages gain words for black and white first, then red, and blue arrives last, often only once a culture can reliably manufacture blue pigments.

  Ancient Greek poets described the sea as green, brown, or the colour of wine. Blue appears several times in the Hebrew Bible under the word tekhelet, but it was not one of the four primary colours for Greek painting, as Pliny the Elder enumerated them: red, yellow, black, and white. The Romans did use the colour extensively, borrowing two words from foreign languages that eventually became the roots for all modern European names for blue. The word blavus came from the Germanic blau; azureus came from the Arabic lazaward. The modern English word blue traces back through Old French bleu to Old High German blao, meaning shimmering or lustrous.

  Several languages handle the blue-green boundary differently. In Japanese, the word 青 (ao) is commonly applied to colours that English speakers would call green, including the colour of a traffic signal meaning go. In Lakota, the word tȟó covers both blue and green, the two not being distinguished in older usage. Vietnamese uses xanh for both the colour of tree leaves and the sky. Russian, by contrast, goes in the opposite direction: it has two entirely separate words, goluboy for light blue and siniy for dark blue, with no single umbrella term.

• 4. The Long Hunt for Blue Pigment

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  Egyptian blue, the first known artificial pigment, was produced as early as around 2500 BC by heating pulverised sand, copper, and natron together. It was used in tomb paintings and funerary objects to protect the dead in their afterlife, and the Greeks imported it alongside indigo dye from India, which they called indikon. For thousands of years, these two sources, along with lapis lazuli mined in Badakhshan province in northeast Afghanistan, anchored almost every blue surface in the ancient and medieval world.

  Lapis lazuli had been mined at Sar-i Sang and other sites in that region since at least the seventh millennium BC. Ground into powder, it produced natural ultramarine, the finest available blue pigment through the Middle Ages and the Renaissance. Grinding more finely produced a lighter colour. In Italian Renaissance art, the pigment was reserved for the most sacred figures: blue was typically kept for the robes of the Virgin Mary. Ultramarine was more expensive than gold, and in 1616 Richard Sackville commissioned a portrait of himself by Isaac Oliver that included three different blues, with ultramarine used specifically for his stockings.

  The search for a cheaper equivalent drove real innovation. In 1709 the German druggist Johann Jacob Diesbach discovered Prussian blue accidentally during experiments involving heating dried blood with iron sulphides; he initially called it Berliner Blau. By 1710 the French painter Antoine Watteau was using it, and Prussian blue became enormously popular for wallpaper manufacture. Beginning in the 1820s it was imported into Japan through the port of Nagasaki under the name bero-ai, or Berlin blue, because unlike the traditional Japanese blue pigment ai-gami made from the dayflower, it did not fade. Hokusai used it in his wave paintings, as did Hiroshige.

  In 1799 the French chemist Louis Jacques Thénard produced a synthetic cobalt blue that became immensely popular with painters. In 1824 the Societé pour l'Encouragement d'Industrie in France offered a prize for an artificial ultramarine; it was won in 1826 by Jean Baptiste Guimet, who refused to reveal his formula. In 1828, Christian Gmelin, then a professor of chemistry in Tübingen, independently found the process and published it, launching an entirely new manufacturing industry that eventually nearly replaced the natural product.

• 5. Blue in Power and Devotion

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  Between 1130 and 1140, the Abbe Suger rebuilt the Saint Denis Basilica in Paris. Suger believed that light was the visible manifestation of the Holy Spirit, and he installed stained glass windows coloured with cobalt. Combined with light passing through the red glass, the effect filled the church with a bluish violet light. The colour became known as bleu de Saint-Denis, and in the years that followed, more elaborate blue windows went into Chartres Cathedral and Sainte-Chapelle in Paris.

  The Roman Catholic Church formalised blue's association with Mary through a decree of Pope Gregory I, who lived from 540 to 601, ordering that religious paintings tell stories comprehensible to all viewers and that figures, especially Mary, be easily recognisable. When depicted alone, her costume was painted with the finest blue, ultramarine. When shown with Christ, a less expensive pigment was used, to avoid outshining him.

  Blue's elevation to royal status in France followed a different path. The coat of arms of the kings of France became an azure, or light blue, shield sprinkled with golden fleur-de-lis. The colour moved in a generation from near-absence in early medieval European life to the dominant colour of monarchy.

  In Judaism, the Torah commanded that a twisted thread of blue, tekhelet, be woven into the fringes of garments. Maimonides described this blue as the colour of the clear noonday sky; Rashi described it as the colour of the evening sky. The dye was extracted from a Mediterranean snail called the hilazon. Many items in the Mishkan, the portable sanctuary in the wilderness, including the menorah and the Ark of the Covenant, were covered with blue cloth when transported.

  In Hinduism, many gods associated with Vishnu are depicted with blue-coloured skin. Shiva is shown in a light-blue hue and is called Nīlakaṇṭha, or blue-throated, for having swallowed poison to save the universe during the Samudra Manthana.

• 6. Uniforms, Denim, and the Business Suit

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  Frederick William I of Prussia chose Prussian blue for his military uniforms in the seventeenth century for a practical political reason: the pigment was made from woad, a local crop, rather than indigo, which was produced in the colonies of Brandenburg's rival, England. That choice locked blue into the visual vocabulary of military power across Europe. In 1748 the Royal Navy adopted a dark shade for officers' uniforms, calling it marine blue, now known as navy blue. The militia organised by George Washington selected blue and buff, the colours of the British Whig Party. Blue remained the field uniform colour of the US Army until 1902.

  Blue jeans arrived through a different lineage. In 1853, Jacob W. Davis used metal rivets to strengthen blue denim work clothing in the California gold fields. The invention was funded by San Francisco entrepreneur Levi Strauss. The blue came from indigo, and by 1878 the German chemistry and dyeing company Badische Anilin- und Soda-Fabrik, known as BASF, had synthesised indigo under chemist Adolf von Baeyer. That synthetic product rapidly replaced natural indigo, eliminating vast farms that had grown the plant. It is now the blue of blue jeans worldwide.

  The early nineteenth century ancestor of the modern blue business suit was created by Beau Brummell, who lived from 1776 to 1840 and set fashion at the London Court. The gendered association of blue with males developed as a trend from the mid-nineteenth century, applying primarily to clothing, and became more widespread after World War II.

• 7. The Blue LED Breakthrough

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  In 1993, Shuji Nakamura of Nichia Corporation demonstrated high-brightness blue LEDs. Working in parallel, Isamu Akasaki and Hiroshi Amano of Nagoya University were pursuing their own development in the same field. Nakamura was awarded the 2006 Millennium Technology Prize for his invention. In 2014, all three researchers, Nakamura, Akasaki, and Amano, received the Nobel Prize in Physics for the invention of an efficient blue LED.

  Blue lasers took a separate path to wide availability. Lasers emitting in the blue region of the spectrum became accessible to the general public in 2010 with the release of inexpensive high-powered 445-447 nanometre laser diode technology. Before that, blue wavelengths were reachable only through DPSS systems, which are comparatively expensive and inefficient. DPSS systems remain in use by scientists for applications including optogenetics, Raman spectroscopy, and particle image velocimetry, because of their superior beam quality. Blue gas lasers continue to serve holography, DNA sequencing, and optical pumping, among other scientific and medical uses.