On October 14th, 2015 we learned about

The cryptic compounds of Egyptian blue pigments

Blue is the most popular color in the world, despite the fact that it’s historically been one of the hardest colors to manipulate. Minerals used in paint (and pigments in animals) are most often in “earth tones” such as browns, reds, and tans. There are naturally occurring blue minerals, such as lapis lazuli found in Afghanistan, but that hasn’t always been readily available to the world. Ancient Egypt found a way around this difficulty though, creating a blue compound that has been lighting up the art world ever since.

Building their own blue

By 2600 BC, Egyptian artists had come up with their own recipe for blue pigment. They didn’t record it directly, but an account from a Roman writer in 100 BC described it as sand, copper and natron. The natron was a source of sodium, like sodium carbonate, and the sand likely provided some lime for calcium. The big catch was that these ingredients had to be cooked at 1472 to 1652º Fahrenheit while maintaining the correct levels of oxygen in the reaction. That probably was no small feat in a time where modern thermometers weren’t even available, although by 1500 BC Egyptians were working with both heat and chemistry to develop glassblowing, so cooking some pigment wasn’t an impossibility.

Shining bright under red light

The pigment created was widely used, eventually being shared with other cultures. In addition to its use in Egypt, the same blue formula has been found on Greek and Roman sculptures, as well as in paintings as recent as 1524 by Giovanni Battista Benvenuto. While creating the pigment wasn’t easy, finding it has been much simpler thanks to one more key property of this blue: it’s extremely luminescent under near-infrared light.

The combination of durability and infrared luminescence has put a new spotlight on Egyptian blue. Aside from its contributions to art, it may also prove useful in future engineering efforts, including biomedical imaging and security inks.

Source: Egyptian blue: more than just a colour by Paul Brack, Chemistry World

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