The Palette/blue/Made in a lab or a works

Prussian blue

The first modern synthetic pigment, made by accident in a Berlin dye works, and the reason The Great Wave is blue.

Also called Berlin blue · Paris blue · Milori blue · bero-ai

Hex: #003153
RGB: 0, 49, 83
Pigment index: PB27
First used: c.1706
From: Berlin

What it is

Iron(III) hexacyanoferrate(II), Fe₄[Fe(CN)₆]₃, an iron cyanide complex. The cyanide is locked in the crystal and the pigment is not poisonous.

How it is made

A soluble iron salt is mixed with a hexacyanoferrate solution and the blue crashes out of the liquid immediately as a precipitate. It is filtered, washed and dried. That is the whole process, and it is so cheap and so easy that this pigment has never been expensive.

The original was an accident. Johann Jacob Diesbach was making a red cochineal lake in Berlin around 1706 and used potash contaminated with animal oil supplied by the alchemist Johann Konrad Dippel. Instead of red he got blue. The contamination was the reaction, which is the most productive mistake in the history of colour.

There is a nice bit of chemistry behind the depth: the colour comes from electrons moving between iron atoms in two different oxidation states, which absorbs almost everything at the red end. That is why it goes so dark without going neutral, a genuinely different mechanism from a mineral's colour.

Prussian blue, Synthetic pigment. Wikipedia's description, not ours.

The history

Before 1706, blue was either gemstone-expensive or unreliable. After it, blue was cheap. Prussian blue went across Europe within thirty years and quietly changed what a painting could look like, because a painter could now put a strong dark blue anywhere without a conversation about money.

Its second life is Japanese. Imported Prussian blue (bero-ai) reached Japan through Dutch and Chinese traders in the early nineteenth century and gave woodblock printers a blue that was strong, stable and printable, which their traditional dayflower blue was not. Hokusai's Thirty-six Views of Mount Fuji, The Great Wave included, is built on it. An accident in a Berlin laboratory is the reason the most reproduced image in Japanese art is that colour.

It also goes far beyond art. The blueprint is Prussian blue chemistry, the cyanotype process precipitates it on paper, which is why engineering drawings were blue for a century. And it is on the WHO list of essential medicines, given orally to treat thallium and radioactive caesium poisoning: it binds the metal in the gut so the body cannot absorb it. A pigment that is also an antidote.

Where it is used

Painting from the 1720s on, Japanese woodblock printing, blueprints, laundry blueing, printing inks, and the darkest part of most cheap blue-black ink. Picasso's Blue Period leans on it.

In a palette it is the most useful dark blue there is, and the most dangerous. It has enormous tinting strength (a trace of it will take over a mixture) and it makes blacks and deep greens that no other blue can. If your brand's dark navy is doing something slightly sinister and cold, this is the colour family it belongs to.

Go and look at it

The Great Wave off Kanagawa
Impressions in most major print collections
Imported Prussian blue, in two strengths, a pale wash for the sky and a deep printed key line for the wave. The whole image is a demonstration of one pigment's range.
Any cyanotype or architectural blueprint
Archives, and a garden-centre sun-print kit
The same compound, precipitated by light instead of in a vat. You can make one on a windowsill.

How it behaves

Made of: Iron(III) hexacyanoferrate(II), Fe₄[Fe(CN)₆]₃, an iron cyanide complex. The cyanide is locked in the crystal and the pigment is not poisonous.
Opacity: Transparent. It stains rather than covers, so it can be glazed over another colour.
Lightfastness: Good. Safe for anything that is not hung in direct sun for a century.

Enormous tinting strength. A little contaminates everything on the palette.
Destroyed by alkali, it fades in fresco and against lime plaster.
Some samples fade in strong light and recover in the dark, which is unusual and confuses conservators.

The numbers

All derived from the hex, never typed beside it

HEX: #003153; What a browser and a design file want.
RGB: 0, 49, 83; The same three numbers in base ten. Light, not ink.
HSL: 205°, 100%, 16%; Hue, saturation, lightness, how a person describes a colour.
CMYK: 100, 41, 0, 67; Arithmetic only. A real separation depends on the press, the paper and a profile.
LAB: 19, 0, -25; Perceptual. L* of 19 is how light this actually looks, which HSL gets wrong.

This hex is an approximation, and a page about pigment has to say so. Prussian blue is a substance. A ground mineral scatters light from thousands of separate grains at slightly different angles, and no single sRGB value is that event. It is a flat average of a photograph of it, and the real thing is outside what a screen can emit. Use the number to specify a screen colour. Do not use it to judge the pigment.

Where it sits on the wheel

Hue 205°

Its opposite, by the diagram

#003153

#532200

The arithmetic opposite, 180° round in HSL. Useful, and not the same as the opposite a painter mixing pigment would name, that wheel was built for paint.

Lighter and darker

Towards white

#003153

#26506D

#4D6F87

#8098A9

#B3C1CB

#D9E0E5

Mixing with white does not only lighten, it dulls and cools. This is why a pale tint of a brand colour so often looks chalky, and why a designed light shade beats a generated one.

Towards black

#003153

#002A47

#00223A

#00192A

#000F19

#00070C

Mixing with black kills the hue as it darkens. A better dark is usually the colour plus its neighbour round the wheel, which is what every painter does instead.

Can you set type on it

Handgloves

White on it
13.43:1 · AAA

Handgloves

Black on it
1.56:1 · Fails

Handgloves

It on paper
12.74:1 · AAA

Handgloves

It on ink
1.36:1 · Fails

4.5:1 is the floor for body text and 3:1 for large text, by WCAG. Treat it as a minimum you may not go under rather than as a target: plenty of pairs pass at 4.6:1 and are still unpleasant to read for a page, and the checker will not tell you that.