The Accident That Made Prussian Blue: How a Ruined Red Dye Colored a Wave, a Blueprint, and an Antidote

Around 1706, a paint maker in Berlin named Johann Jacob Diesbach was trying to make red. He wanted a batch of cochineal lake, the crimson dye ground from dried insects that dyers and painters had used for centuries. The usual recipe called for potash, iron sulfate, and cochineal. Instead of red, the batch came out a dense, inky blue.

The best-known explanation comes from the chemist Georg Ernst Stahl, writing in 1731, a quarter of a century after the fact. In his telling, Diesbach had used potash supplied by the alchemist Johann Konrad Dippel, who had used it in making his "animal oil" from blood and bone, and the contaminated potash reacted with the iron sulfate to form iron ferrocyanide, a compound nobody had synthesized before. No other known source puts Dippel in the story, so treat the details as likely rather than certain. Either way, Diesbach had not discovered a better red. He had stumbled onto the first modern synthetic pigment, and one that would end up on a Prussian infantry coat, a Japanese woodblock print, an astronomer's darkroom trick, a crayon box, and a pharmacy shelf for radiation poisoning.

That pigment is Prussian blue, and its three centuries of side jobs are a decent argument that a good color doesn't stay in one lane.

A ruined batch of red

Diesbach's mistake would have stayed a curiosity if it had stopped at "unexpectedly blue." What made it matter was who noticed and how fast word spread. The earliest surviving mention is a letter from the naturalist Johann Leonhard Frisch to Gottfried Wilhelm Leibniz in March 1708. By 1710, Frisch had announced it in Miscellanea Berolinensia, the journal of the Prussian Academy of Sciences, and he was already promoting and selling it across Europe.

Chemically, it's iron(III) hexacyanoferrate(II), a deep blue lattice built from iron and cyanide ions locked together in a stable crystal. None of that mattered to the painters who bought it. What mattered was that it was blue, it didn't fade, and it was suddenly available without shipping lapis lazuli from a single mountain range in Afghanistan. The name wasn't even settled at first. Outside Prussia, dealers sold the same pigment as Berlin blue or Paris blue, and all three labels described one chemical compound coming out of the same handful of workshops.

Cheaper than a king's ransom, blue enough for an army

Before Prussian blue, a painter who wanted true blue had bad options, even though Egyptian blue had been cooked from sand and copper thousands of years earlier. Azurite turned green when wet. Smalt and woad faded within years. Indigo washed out. Ultramarine, ground from lapis lazuli mined almost exclusively in Badakhshan, held its color, but it cost more than gold by weight, which is why Renaissance painters reserved it for the Virgin Mary's robe and little else, and why a patron paying for a painting in ultramarine was, in effect, paying for a stone imported across half of Asia.

Prussian blue undercut all of it. It was synthetic, so supply wasn't hostage to a single mine. It was stable, so it didn't need the careful handling ultramarine demanded. And it was cheap enough that by the early 18th century, it had become the standard uniform color for Prussian infantry and artillery regiments, giving the pigment the name history remembers.

That's the part of the story people usually stop at: an accident that saved painters money and gave an army its look. It's the smaller half of what happened next.

The wave that needed a new blue

Prussian blue reached Japan by the 1820s, carried through Dutch and Chinese trade routes into a country where woodblock printers had been limited to indigo and dayflower blue, both soft, both prone to fading in sunlight. Japanese printers called the new pigment bero-ai, "Berlin blue," and it changed what a print could look like.

Katsushika Hokusai used it prominently in The Great Wave off Kanagawa, printed around 1831 as part of his Thirty-six Views of Mount Fuji series. His printers didn't simply swap Prussian blue in for indigo. They layered the two, printing indigo beneath Prussian blue to deepen the color without losing its brightness, then let Prussian blue carry the highlights and crests where the wave curls into claws over three small fishing boats. The pigment held its intensity for decades, which is a large part of why The Great Wave still looks vivid in museum prints nearly two centuries later, while older indigo-only prints from the same era have often gone pale.

The pigment isn't one fixed shade. Depending on how it's mixed and diluted, iron ferrocyanide runs from a pale, almost gray blue through the near-black tone that shows up in the deepest folds of Hokusai's wave. That range is why it worked as both a highlight color and a shadow color inside the same print.

A cameo in a Starry Night

By the time Vincent van Gogh painted The Starry Night in 1889, Prussian blue was no longer novel. Multispectral pigment mapping done by researchers at the Rochester Institute of Technology and the Museum of Modern Art found that van Gogh reserved it for a supporting role: the green of the cypress tree in the foreground likely combines Prussian blue with a brown or yellow pigment, while the sky itself relies on synthetic ultramarine and cobalt blue.

It's a small credit compared to the sky everyone remembers, but it's consistent with how the pigment had settled into painters' kits by the late 19th century. Less a headline color, more a dependable mixer that could push any green toward something darker and cooler.

The blue that learned to draw blueprints

Prussian blue's strangest second act had nothing to do with painting. In 1842, English astronomer John Herschel was looking for a cheap way to copy his notes and diagrams. He found that paper coated with ferric ammonium citrate and potassium ferricyanide turned a deep, permanent blue wherever light hit it, while the areas kept in shadow washed out clear. He called the process cyanotype, and the chemical reaction it relies on produces Prussian blue directly on the page. Herschel was already reshaping the vocabulary of the new medium around him. He's the person credited with coining "photography" itself, along with "negative," "positive," and, decades later, "snapshot."

Botanist Anna Atkins took Herschel's process further than he had. In October 1843 she published British Algae: Cyanotype Impressions, laying seaweed specimens directly onto treated paper and exposing them to sunlight to create ghostly white silhouettes against a field of Prussian blue. It's considered the first book illustrated entirely with photographic images, made by a woman working alone, years before photography had settled on a standard process. Atkins kept making them for two decades, building out albums of algae, ferns, and feathers that read today as some of the earliest photograms in existence, all in the one color the process could produce.

The cyanotype process stuck around for a more practical reason. Through most of the 20th century, architects and engineers used it to copy technical drawings cheaply and reliably, because the process needed no darkroom and no expensive materials, just sunlight and treated paper. Every "blueprint" is named for exactly what it looked like: white lines on a field of Prussian blue.

A crayon retires, a poison antidote is born

In 1958, Crayola renamed its "Prussian Blue" crayon to "Midnight Blue," more than two centuries after Diesbach's accident first put the color in a paint jar. The change came from teachers, who reported that children could no longer place the reference. Prussia had stopped existing as a state in 1947, folded into other territories after the Second World War, and a color named after it meant nothing to kids who'd never heard of it. The pigment didn't change. The label did.

Then, in 2003, the same compound resurfaced somewhere no one in Diesbach's workshop could have predicted: the pharmacy. The FDA approved Radiogardase, a Prussian blue capsule, as the first medical countermeasure for people internally contaminated with radioactive cesium-137 or thallium. Taken orally at 3 grams three times a day, Prussian blue binds those metal ions in the intestine, trapping them before the body can reabsorb them and speeding their exit through normal digestion. The main side effects are constipation and an upset stomach, mild costs for a drug built from the same iron-cyanide lattice that turned Diesbach's dye batch blue three hundred years earlier, now sitting in emergency stockpiles in case of a radiation exposure event.

Prussian blue is one of several nineteenth-century colours with a darker side; the arsenic green is another. A compound doesn't usually get to be a war uniform, a Japanese woodblock print, a photographic process, a children's crayon, and a federally approved antidote. Prussian blue managed all five because none of those uses had anything to do with why it was invented. Diesbach was trying to make red. He never got there, and neither the Prussian army nor Hokusai nor a poisoned patient three centuries later would have any reason to mind.