The Tragedy of Prussian Blue

When we look at the paintings of the old masters, there is a little blue or bright yellow, green in them. Earthy reddish tones prevail here, and these bright colors, especially blue, were reserved for wealthier clients, their portraits, and for paintings and frescoes that were made for the church.

It’s not that the old masters didn’t like blue. But let’s say with da Vinci, we see how he saved blue on the Mona Lisa, but used it for the robes for the Last Supper. Nevertheless, artists such as Raphael, Botticelli and Titian continued to use large amounts of ultramarine pigment in their large works. There is an anecdote about Michelangelo that he did not finish one of his paintings – The Burial of Christ – because he did not have the means to buy enough blue paint.

Why? Well, the blue pigment was extremely expensive. The blue color was mainly based on the pigment ultramarine, which is a powder of the mineral lapis lazuli, an extremely expensive thing that is not available everywhere – the deposits are mainly in distant Afghanistan and Iran. There is also the ancient Egyptian blue – cuprorivaite, which is formed by the reaction of limestone, sand and minerals containing copper, then indigo.

Only when, at the end of the 19th century, the synthesis of artificial pigments began, and some cheaper blue colors were obtained, artists began to use this color more boldly and generously.

There are many shades of blue and they are really fascinating. And each one has its own story. The story of the Prussian Blue has a touch of sadness and chills in it.

Around 1706, a new blue color was accidentally discovered in Berlin. Johann Jacob Diesbach worked in the laboratory of the alchemist Johann Konrad Dippel trying to make the red pigment, carmine.

Diesbach used an extract of crushed insects from which a red pigment is obtained, iron sulphate and potassium oxide (potash) to create cochineal red (an order of light dyes). One batch of products unexpectedly turned pale pink. When he tried to concentrate the mixture, it turned purple and then dark blue. He was looking for a potash seller, Johann Konrad Dippel, an alchemist. Together they realized that the reaction occurred because the potash was contaminated with bone oil, i.e. the blood of animals

They quickly realized that blue was stable and easy to make, and that meant it was worth a fortune because good, stable blue pigments were rare and expensive.

Artists quickly adopted the new shade and it became known as Prussian blue or Berlin blue. You can see it in any museum: Gainsborough used it – remember the famous painting The Blue Boy , as well as Picasso. This pigment is found in Van Gogh’s Starry Night and is one of the pigments used in Hokusai’s famous print The Great Wave off Kanagawa.

 

Scientists, of course, wanted to know exactly what this beautiful colored substance was, so for many years they researched the chemical composition of Prussian blue, trying to discover the chemical components that create this color.

The French chemist, Pierre-Joseph Macquer, eventually managed to break the pigment into two parts in 1752: an iron salt and an unknown acid. The new acid was essentially what we know today as hydrogen cyanide.

It was isolated from Prussian blue in its pure form in 1782 and characterized by the Swedish chemist Carl Wilhelm Scheele, it was first named Blausäure (literally “blue acid”) due to its origin in Prussian blue, and became popularly known in English as Prussian acid. Cyanide, a colorless anion formed in the process of making Prussian blue, is named after the Greek word for dark blue, κύανος, kyanos . Prussian blue is also called Berlin blue and even Paris blue, and its chemical composition is iron (II, III) hexacyanoferrate (II, III).

The cyanide ion is simple, it has one carbon and one nitrogen atom. And it’s poisonous. In our cells, it interferes with the use of oxygen, which leads to people suffocating even though their blood is full of oxygen.

It became the main subject of crime novels, for example it is the main subject in Agatha Christie’s novel Sparkling Cyanide. Unfortunately, cyanide did not remain only a means of poisoning and crime in novels. Cyanide was used as a poison gas in the First World War. It was also used to execute people in gas chambers. In Germany it was developed into Zyklon B, the poison used by the Nazis in the extermination camps in the concentration camps.

But there is another side to the story. Prussian blue is also a medicine, on the WHO list of essential medicines. Although the derivative of Prussian blue is a poison, the pigment itself acts as an antidote in case of poisoning with some heavy metals, primarily oral thallium and radioactive cesium. In medicine and biological sciences, Prussian blue is used to stain histological and pathological preparations.

Read more: story about yellow cadmium pigment