The Imo cut deeply into the side of the Mont-Blanc, toppling barrels of benzol that spilled open. When the Imo reversed out, the grinding hulls shot sparks onto the Mont-Blanc’s deck, igniting the benzol. The ship lit up in flames. Captain and crew, realizing they were on a ticking bomb, abandoned ship while shouting warnings to boaters who had rushed to help put out the flames. The engulfed Mont-Blanc drifted toward shore.

War had transformed Halifax into a bustling place, and like any other Thursday morning, schoolchildren and workers hurried about their day. But the sight of the burning ship postponed the day’s commute. Transfixed and unaware of the impending danger, Halifax’s residents also drifted toward the shore.

Within minutes the flames detonated the picric acid. A massive explosion was felt as far away as Sydney, Nova Scotia, 200 miles to the northeast. In Halifax the blast knocked people off their feet, shattered windows, and shot shrapnel into the terrified crowds. Within moments a 52-foot tidal wave swept across three city blocks and washed over Halifax’s Richmond neighborhood. Everything within a 1.5-mile radius was destroyed or damaged. Approximately 1,600 people died instantly, with 300 more succumbing later. An estimated 9,000 people were injured, while more than 25,000 were left homeless—roughly half the city. The Mont-Blanc was obliterated.

William Barton, a guest at the nearby Halifax Hotel, staggered outside to see a “giant smoke cloud” rising above the rubble, the world around him suddenly leveled. “In ten seconds, it was all over,” he later recalled. Such instantaneous destruction wouldn’t be seen again until the advent of the atomic bomb. The explosion was among the greatest man-made disasters the world had witnessed. And of all the catastrophes involving picric acid, it just might have been the worst.

Picric acid’s origins did not foretell such a calamity. Rather, its arrival into this world promised a brighter future. But over time, scientists revealed its many faces. It was complicated, paradoxical even—possibly as paradoxical as the man who discovered it. 

Peter Woulfe was an eccentric with a reputation for blending brilliant science with mysticism. Born in Ireland in 1727, Woulfe lived in Spain and France before moving to London to continue his scientific research. His close study of minerals and the fabrication of novel glassware to advance his investigations earned him election to the Royal Society in 1767 and its most prestigious award, the Copley Medal, a year later. 

But Woulfe’s esteem among the scientific elite contradicted his peculiar way of life. He was known to invite friends to join him for his usual 4 a.m. breakfast, though gaining entrance required a secret knock. To cure colds, he would ride a horse-drawn mail coach from London to Edinburgh, a distance of nearly 400 miles, only to return immediately. In the laboratory he attempted to transmute base metals into gold and pursued the elixir of life. He ascribed his failures to a lack of good deeds and religious devotion, and so attached prayers and inscriptions to his apparatus to increase his chances of success. 

In 1771, amid one of his unconventional pursuits, Woulfe mixed nitric acid with indigo and observed the concoction turned silk a yellow hue. Woulfe gave a step-by-step recipe for making what would later be recognized as the first synthetic dye, named picric acid after pikros, meaning “bitter” in Greek, for its intensely bitter taste.

Woulfe would die long before this recognition, victim to his superstitious beliefs: during another of his curative trips to Edinburgh in 1803, he developed inflammation of the lungs. On returning home, he shut himself away in his chamber and died. 

About 40 years after Woulfe’s death, picric acid production experienced a turning point when chemists discovered it could be made from phenol, a chemical extracted from coal tar—the sticky and abundant byproduct of a booming coal gas industry that was fueling industries and lighting up streets.

When mixing phenol with nitric acid, three nitro groups replace three hydrogen atoms on phenol’s aromatic ring, converting the colorless chemical into the nitroaromatic compound picric acid, or trinitrophenol, with its signature yellow crystals. While the cheaper raw material made picric acid a common household dye in the mid-19th century, its commercial success as a textile dye was limited—its brilliant yellow just couldn’t stand up to sunlight. 

Interest in the compound, however, did not fade. Over time, chemists had discovered picric acid had a less innocuous side. A strong acid, it readily reacted with metals and oxides to form unstable metal salts—“picrates”—that were highly explosive. But its utility as an explosive remained overlooked until a couple of key innovations.

In a series of publications between 1871 and 1873, chemist Hermann Sprengel showed picric acid “alone contains a sufficient amount of available oxygen to render it, without the help of foreign oxidizers, a powerful explosive,” proving it could be detonated. Building on this work, French chemist Eugène Turpin invented a process to melt and pour picric acid into molds, finally demonstrating its practical use as an explosive for artillery shells. Not long after, picric acid would reshape battlefields.

There are many properties to consider when choosing a new artillery shell filling:

1. It needs to deliver considerable power, enough to impart a lot of gas and heat.
2. It must be insensitive enough to withstand the shock of firing and impact, so it penetrates the target material before it detonates.
3. It should have a high density so that a large amount can be enclosed in limited space.
4. It must have a high explosive velocity to produce a significant shattering effect.
5. It needs to detonate fully and reliably by the action of a detonator, such as a fuse, which itself cannot be too sensitive to firing.
6. Finally, it must be stable enough for safe manufacturing.

Few explosives satisfy all these needs. Picric acid’s biggest failing was its instability; it posed a constant risk when handled. But its explosive power—10 times that of dynamite—overwhelmed such concerns. By the late 1800s it was the shell filling of choice for artilleries across the globe under different names and in slightly different formulations—Mélinite in France, Lyddite in England, Pertite in Italy, Granatfüllung 88 in Germany, and Ecrasite in Austria-Hungary.

In Japan “Shimose powder” was packed into shells used to bombard Russian warships during the Battle of Tsushima in 1905. The Japanese shells rained onto the warships, bursting on contact with a force and sensitivity that exceeded anything the Russians had seen before. The explosions tore apart steel plates and superstructures on the upper deck. Writing after the battle, Captain Vladimir Semenoff recalled, “Such havoc would never be caused by the simple impact of a shell, still less by that of its splinters. It could only be caused by the force of the explosion.” Stunned, he watched as the shells spread a fierce “liquid flame” across his ship, so hot it burned waterlogged boxes and the paint off steel. By the end of the battle three Russian ships had sunk and more than 12,000 Russian sailors had been killed or captured. Japan’s triumph marked it as a new world power. 

The success of picric acid paved the way for other nitroaromatic explosives, including trinitrotoluene (TNT), that would eventually eclipse it. When World War I erupted in 1914 picric acid was still a common armament; the French were particularly fond of it. Over the course of the four-year war, 1.5 billion artillery shells were fired on the Western Front alone—causing 60% of battlefield casualties in a war that killed 8.5 million soldiers.

To keep up with demand, the Allies turned to the United States to bolster their supply of picric acid and other nitrogen-based explosives. Munition plants sprung up along the Atlantic coast, and by November 1917 the United States was churning out 600,000 pounds of picric acid a month—a number that increased nearly 20-fold within a year.

With little oversight in place at these plants, laborers were left working in spaces that lacked ventilation, inhaling nitrous fumes that scarred their throats and lungs. Toxic yellow dust burned their skin and stained them head to toe, giving rise to the nickname “canaries.”

Alice Hamilton, a pioneer of occupational health, began tracking what was happening at the plants. She rooted out their secret locations by following the telltale trails of yellow and orange plumes spewing from chimneys, then tailed the unmissable canaries, who led her the rest of the way. Of the 2,432 cases of occupational poisoning listed in her 1917 report on 41 munition plants, nitrous fumes accounted for 1,389 and for 28 of the 53 worker deaths. Hamilton’s research spurred the growth of industrial toxicology, and her report helped establish industrial safeguards that are now commonplace, including requirements for proper ventilation, safety attire, set exposure limits, and health monitoring.

Innovations in high explosives and other weaponry had the paradoxical effect of bogging down the war. Soldiers had little chance of gaining ground against their enemy’s rapid-firing artillery. Instead, infantries dug deep trenches and waited for an opportune time to attack. Soldiers idled for days in the muddy, wet, and vermin-infested ditches; the tense monotony and gloom burst randomly by the deafening explosion of shells.

The trenches dealt unique physical problems alongside the barrage of mental stress. A hideous condition called trench foot afflicted more than 75,000 British soldiers and 2,000 Americans. What first appeared as tingling, itching, and numbness in the feet, was the skin and tissue breaking down from prolonged exposure to cold and wet conditions. Gradually the foot would swell, sometimes to the knee, before gangrene set in. If left untreated, it was deadly. To remedy this condition, doctors painted the skin with a 1% solution of picric acid.

As baffling as it sounds, picric acid, the explosive wreaking death and destruction on and off the battlefield, was also healing soldiers.

The first studies of the physiological action of picric acid took place in 1827, when its toxicity was studied in dogs. Sometime in the 1850s, it reappeared under the name carbazotic acid; a Dr. Bell of Manchester, England, reported successfully using it to treat intermittent fever that came with malaria. Picric acid offered a cheap and effective malaria cure for British soldiers in India for whom quinine had lost its effectiveness. By 1868 Scientific American was touting the surprisingly safe medicinal value of a substance better known for blasting through ironclad warships: “while the alkaline picrates are endowed with such formidable properties, they also possess others which are useful for the alleviation of human misery.”

Doctors prescribed it to relieve various ailments, including anemia, headaches, and diarrhea. Although there were reports of patients’ skin and eyes turning yellow, by the late 1800s picric acid was in general use as a drug in England and other countries.

Picric acid offered topical benefits too and was commonly found in household medicine cabinets and first aid kits as a remedy for first- and second-degree burns. Wrapping a wound in a bandage soaked in diluted picric acid provided both antiseptic and analgesic benefits, preventing pus and relieving pain. F. E. Tulley was so thrilled by the results of the “modern treatment” that in 1898 he wrote to the Journal of the American Medical Association extolling its virtues. “The persistent use of a weak solution of picric acid,” Tulley wrote, had eradicated the “shock and septicemia” associated with severe burns. He urged other doctors, especially those “who have discarded the old-fashioned methods,” to try it.

This new practice caught the attention of a few colleagues, including T. F. Brown, commanding officer of the Australian Auxiliary Hospital in Egypt, who a few months into World War I tested the treatment on more than 3,000 wounded soldiers returning from the disastrous Gallipoli campaign. He observed picric acid to be “four times more potent than carbolic acid” in killing bacteria, such as Streptococcus and Staphylococcus, and could do so within two minutes. It was more effective and less irritating than iodine, Brown found, and its soothing properties reduced demand for aspirin and morphine. Won over by the results, he recommended “picric acid be stocked by all dressing stations, clearing hospitals, and field ambulances if not already in use.”

Picric acid’s popularity wouldn’t survive World War I. Its volatility and the availability of safer alternatives doomed it as both a weapon and medicine. Find a bottle in an old first aid kit and you would be better served calling the bomb squad than dousing a wound.

But the story of picric acid is not only one of an unstable chemical. Its history weaves dual realities—one benevolent; the other destructive—often seen in chemistry. The same story could be told for elements, such as chlorine, and more complex compounds, such as DDT. Like those substances, picric acid’s narrative has been shaped as much by human ingenuity and intent as chemical nature, an idea beautifully illustrated in a tale from Muhu, a small island in the Baltic Sea.

Legend has it that in the 1930s locals began salvaging picric acid from the Russian World War I warship Slava, which sank in the shallow waters between the island and mainland Estonia. Back on land, islanders used the recovered picric acid, once meant for bombs, to dye their handmade skirts an eye-catching yellow, sparking a postwar surge in the skirt’s popularity.

Scientists too reclaimed the multifaceted chemical by returning to its origins. These days picric acid has earned a new place in medicine—as a histological stain.