Next time you run out of ultramarine blue, remind yourself that there was a time when the ultramarine blue pigment was so expensive, no one could afford to buy too much of it! Johannes Vermeer, for instance, used so much ultramarine to paint Girl with a Pear Earring that he put his family into debt. And art historians believe that The Entombment was never finished because Michelangelo couldn’t afford to buy more ultramarine blue.

A fluke accident, however, changed everything for painters in search of an affordable blue pigment. The year was 1703. A colour maker in Berlin, Johann Diesbach, was busy mixing up a batch of standard red lake paint, made from cochineal, iron sulfate and potash. The poor man apparently was rushing to complete a customer’s order. In his haste, he grabbed some contaminated potash. To his surprise, the mixture turned blue instead of red.

Diesbach wasn’t sure what had happened, but he loved the colour. Today we know that there must have been some animal blood in the potash, which contained iron. The resulting chemical reaction produced iron ferrocyanide—a synthetic chemical that we’ve known ever since as Prussian Blue.

Our intrepid colour maker tried to keep the formula a secret but before long, his customer figured it out and started selling the colour himself. The new blue was an instant hit. No wonder. It was an intense blue, with great transparency and strength. Only a small amount was required to impart a strong tint to other colors. It also was a tenth of the price of ultramarine blue, and if you ran out, it was easy to make more.

By 1710, Diesbach’s blue was being used by so many artists in the Prussian court that it began to be known as Prussian blue.

As its popularity spread through Europe for both oil and watercolours, painters loved it. It’s unique properties allowed painters to work more spontaneously as well as mix a wider variety of colours on their palettes.

Artists such Monet, Constable, Gainsborough, Van Gough and Picasso were inspired to use Prussian Blue in their work. Prussian blue is the “blue” in Picasso’s famous “Blue Period”, the blue that Van Gough used in Starry Night, and the blue that makes Blue Boy—blue. It then spread to Japan, where it was used by Hokusai in his famous woodblock painting The Great Wave off Kanagawa.

Prussian Blue is credited as being the first truly synthetic color. A couple of years after its discovery, Diesbach created a less intense version, known as Antwerp blue. However, art historians eventually discovered that both colours tended to fade over time. In the 1930s, a substitute blue was created out of copper phthalocyanine. It was named by the company that created it: Windsor Blue ( green shade). By the way, “Phathalo Blue” is the same colour.

Some artists still prefer Prussian Blue. It is less staining than Phthalo and more transparent. If you’re one of them, take heart: fading is limited when Prussian Blue is mixed with other colours and UV coated glass is used when framing. In addition, certain brands promote better lightfastness in their formula (so read the labels).

Even if you’re not keen about Prussian Blue when painting, there’s still lots of reasons to love the pigment. Because of its light-sensitive properties, 19th century astronomer John Herschel used Prussian blue to produce the world’s first blueprints. Better still: Prussian blue is an effective chemical treatment for radiation poising (always good to know.)