Prussian Blue

What color is Prussian blue?

The story of Prussian blue

Prussian blue was the first color anyone made in a laboratory, and the man who made it was trying to make red.

Johann Jacob Diesbach was a color maker in Berlin. Somewhere between 1704 and 1706 a batch of his cochineal red lake, made from cochineal, iron sulfate and potash, came out pale pink, and when he tried to concentrate it, it went purple, then deep blue. The potash had come from Johann Konrad Dippel, an alchemist who once tried to trade his elixir of life for Castle Frankenstein, and it was contaminated: with blood by one account, with Dippel's animal oil by another. The iron and the contamination made iron ferrocyanide, which is blue. The color was in letters to Leibniz by March 1708, was being called Preussisch blau by August 1709, and stayed a trade secret until John Woodward published the recipe in 1724.

Berlin blue, its other name, spread fast, because it was cheap and the alternative was ground lapis lazuli. Pieter van der Werff used it in an Entombment of Christ in 1709, the oldest painting we know of with it in, and the Prussian army put it on its infantry coats. It also named a poison: in 1782 Carl Wilhelm Scheele isolated hydrogen cyanide from it, so the acid is called prussic acid, and the word cyanide itself comes from the Greek for dark blue. The paint is harmless; the cyanide is locked to the iron and stays there.

The most famous use of it is Japanese. Printmakers there had been working with indigo, a plant dye that faded quickly, when Prussian blue began arriving from Holland in 1820 and then in bulk in 1829. They called it bero-ai, Berlin indigo. Hokusai, about seventy and broke, used it through his Thirty-six Views of Mount Fuji: the first ten prints are among the first Japanese prints to carry the new blue, and The Great Wave off Kanagawa followed late in 1831. Hokusai's best-known work is, among other things, an advertisement for a Berlin chemical.

Photography borrowed it next. In 1842 John Herschel found that paper brushed with an iron salt and potassium ferricyanide turns blue where sunlight hits it, and the blue that forms is Prussian blue itself; he called it cyanotype. Anna Atkins, a botanist friend of the Herschels, laid seaweed onto the treated paper and in October 1843 self-published Photographs of British Algae, the first book illustrated with photographs. From 1872 the same chemistry copied architects' drawings, which is why a plan is still called a blueprint, and it stayed standard until the 1940s.

Doctors use it too. Swallowed, Prussian blue binds thallium and radioactive cesium, which put it on the World Health Organization's list of essential medicines. What you buy today is the same iron blue Diesbach stumbled on, PB27, and the world makes about 12,000 tons of it a year, mostly for ink. I reach for phthalo blue more often, which arrived in 1935 and does the same deep-blue job without fading in tints or browning on chalky grounds. I keep a tube of Prussian blue anyway. Nothing else gives that inky, slightly green dark that looks like the sea at night.

Pigment facts

These describe the pigment in general. Every brand's tube behaves a little differently, so test yours on scrap paper.

How to mix Prussian blue

You can get close to Prussian blue with phthalo blue and a touch of black, but not all the way: the real thing is duller and inkier, and heads toward black when you pile it on. What it does better than any other blue is greens. With a warm yellow it gives deep, natural leaf greens; the old recipe was Prussian blue and gamboge.

Its other job is darks. With burnt sienna it makes a near-black that still reads as color, and it was the blue in William Payne's original shadow mix long before Payne's gray came in a tube. Go easy. A brush tip colors a whole puddle, and once it is on the paper it is staying there.

Using Prussian blue in watercolor

Use it where you want depth rather than brightness: the sea at night, a pine wood, worn denim. It is transparent, so even at full strength it has light behind it, which is what separates it from black.

Glazing suits it. A thin wash over a dry ochre wall or burnt sienna roof pushes it into shadow without turning it gray. Dropped into a still-wet yellow it makes instant leaf green, with edges you cannot get on a palette.

It pairs with the earths: burnt sienna for darks, yellow ochre for greens that look like real plants, Venetian Red for a brown-black with warmth in it. It does not need white; water does that.

One warning I learned the hard way. It stains. A stroke in the wrong place is there for good, so put it down late in a painting, and rinse the brush twice before you go back to a yellow. My first ruined set of leaves came from one dirty brush.

Prussian blue vs phthalo blue (and indigo)

Phthalo blue is the color that took most of Prussian blue's jobs. It is a copper compound, launched in 1935 as Monastral Blue, and in my experience brighter and steadier: no fading in thin tints, no fuss about chalky paper. Side by side, phthalo is clear glass and Prussian blue is ink. Both stain like mad.

Indigo is the older comparison. It is a plant dye, the blue of new jeans and the blue Japanese printers used before Prussian blue arrived, and on paper it faded, which is exactly why the import took over. Prussian blue is greener and darker; indigo is softer and more violet-gray.

If you want one deep blue for darks, phthalo is the sensible modern choice and Prussian blue is the romantic one. I use both. If phthalo blue is a swimming pool, Prussian blue is the harbor at night, and indigo is the jeans you wore to get there.

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Mel, founder of Tobio'sI make portable watercolor kits and I named the company after my cat. I write these pages for the beginner I used to be, one pigment at a time.