La Gare Saint-Lazare, arrivée d'un train , by Claude Monet (1877)
My favorite color has changed throughout my life, cycling through the entire spectrum of visible light and beyond. I don’t remember when blue was the chosen one, exactly; maybe when I was 13 or so. After that, yellow, purple, orange, green, and pink occupied the top spot for various periods. Blue never made a comeback. I saw it as a banal, common color. After all, the sky is made of it, and the sky is everywhere.
Then I realized when compiling the tech tree that blue is the most fascinating color, because it is the hardest of the common colors to create artificially. You can’t just take a piece of the sky and put it into a painting. And blue pigments are fairly rare in minerals, plants, and animals. So blue had to be invented, time and time again, from 4000 BC to the 21st century. It is the most technological color, and I’m willing to claim that this is why it is usually, in science fiction and elsewhere, used to represent the future.
The story of blue starts with indigo. It is an organic dye made from plants in the Indigofera genus, which grow throughout the tropical and subtropical regions of the world. The first known traces of indigo dye come from the New World, in ancient Peru, 6,000 years ago, using Indigofera suffruticosa, or anil. In the Old World, it was known from Africa to East Asia, but became particularly associated with India (hence indi-go), where Indigofera tinctoria was domesticated. Indigo soon became a luxury, traded from India to Greco-Roman and then medieval Europe, where the same blue dye could only be made from a less productive plant, woad or Isatis tinctoria. Eventually the “blue gold” became an important colonial crop in the Caribbean and was part of the story of slavery, next to sugar, tobacco, and cotton.
A bucket of indigo in China ( source )
Before indigo was a thing in the Old World (that started circa 2400 BC), the Egyptians had already become obsessed with the color blue. Besides the sky, it was available in the form of semiprecious stones like turquoise and lapis lazuli, cobalt oxide (more on that later), as well as the mineral azurite, which they mined in Sinai and the Eastern Desert.
Azurite geode from Sweden ( source )
Azurite would later enjoy a fruitful career as the main blue pigment in European painting, but to the Egyptians it was costly, and besides it isn’t the most stable blue color: it degrades and fades when in contact with air. And so they created the first synthetic pigment in history: Egyptian blue. The oldest evidence of it is in a bowl dated to 3250 BC. Egyptian blue is a calcium copper silicate with formula CaCuSi 4 O 10 or CaOCuO(SiO 2 ) 4 . Its method of manufacturing, in a rare example of lost technology, was forgotten towards the end of antiquity, but has been plausibly reconstructed. It likely involved heating together quartz sand (silica) and some source of copper (either copper ores or scraps from the bronze industry), together with an alkali (like natron) and a calcium oxide (unintentionally added as impurities in the other materials).
Various Egyptian blue objects from the British Museum ( source )
In another cradle of civilization, a very similar story unfolded from about 800 BC. So similar, in fact, that it has been speculated that knowledge of Egyptian blue spread along the early silk road, all the way to China, where Han blue (together with Han purple) makes an appearance during the Zhou dynasty. Han blue has almost the same chemical formula as Egyptian blue, but replaces calcium with barium: BaCuSi 4 O 10 . It may also have been an independent invention, perhaps the work of Taoist alchemists and glassmakers. Its use declined after the Han dynasty, and few examples survive.
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