Mercury was used in hat making to treat animal fur before it could be pressed into felt. Hat makers brushed a solution of mercuric nitrate in nitric acid onto rabbit or hare pelts, which chemically altered the fur fibers so they would mat together more tightly during the felting process. This practice lasted roughly 300 years and poisoned generations of workers before it was finally banned in 1941.
The Carrotting Process
The core technique was called “carrotting,” named for the orange tint the mercury solution left on the fur. Workers painted a mixture of mercuric nitrate and nitric acid onto the tip ends of the fibers while the fur was still attached to the animal skin. The chemical broke down sulfur bonds in the fur’s protein structure, roughening and curling the individual fibers so they would interlock more easily when compressed.
After carrotting, the treated fur was shaved from the skin and gathered into loose mats. These mats were then repeatedly heated, pressed, and rolled with moisture to shrink and tighten them into dense felt. Without the mercury treatment, rabbit fur in particular was too smooth and slippery to felt well. The mercury essentially gave the fibers microscopic barbs that grabbed onto each other, producing a stronger, more uniform material prized for high-quality hats.
How the Secret Spread Across Europe
Mercury entered hat making in 17th-century France, where it quickly proved superior to older methods of preparing fur. The French kept the technique a trade secret for decades. It only reached England after the revocation of the Edict of Nantes in 1685 forced French Huguenots, many of them skilled tradespeople, to flee the country. They brought the carrotting process with them, and it spread rapidly through English and eventually American hat-making centers like Danbury, Connecticut.
Why It Made Hatters Sick
The danger wasn’t primarily in handling the treated pelts. It was in breathing. Multiple steps in the felting process involved heating mercury-treated fur in poorly ventilated workshops, which released mercury vapor into the air. Elemental mercury is toxic mainly through inhalation. Once breathed in, mercury vapor passes easily from the lungs into the bloodstream and accumulates in the brain, kidneys, and nervous system over time.
Workers also had direct skin contact with the mercury solution during carrotting, and the vapor could be absorbed through the eyes. But the lungs were the primary route. A hatter spending years in a steamy, enclosed workshop was inhaling small doses of mercury vapor day after day, building up a chronic toxic burden that eventually produced severe symptoms.
Symptoms of Mercury Poisoning in Hatters
The characteristic illness became known as “erethism” or “mad hatter disease.” Hatters commonly developed tremors, starting in the hands and eventually spreading to the limbs and eyelids. The tremor could become so severe that workers could no longer perform fine tasks. Slurred speech was common, along with irritability, shyness, and depression. Some workers became so anxious and withdrawn that they could barely function socially.
These neurological symptoms gave rise to the phrase “mad as a hatter,” which was a familiar expression well before Lewis Carroll wrote Alice’s Adventures in Wonderland in 1865. The condition was widely recognized within the trade, yet workers had few alternatives. Hat making was a skilled profession, and mercury-based carrotting produced felt that customers and manufacturers demanded.
How Mercury Use Finally Ended
Despite centuries of documented illness, regulation was slow. In France, where mercury was first used, hat making was considered notoriously dangerous from the start. In the United States, the Public Health Service eventually conducted studies in hat-making factories, documenting the health damage with clinical data and photographs of affected workers. These findings led to a 1941 agreement between labor unions, the hat-making industry, and the federal government that banned mercury from all hat-making processes in the U.S.
The ban was driven partly by practical wartime needs. Mercury was a strategically important material during World War II, and diverting it away from hat factories freed up supply for military applications. Hydrogen peroxide-based alternatives had already been developed and could achieve similar results in the carrotting process without the toxicity. The combination of public health evidence, labor pressure, and wartime economics finally ended a practice that had been poisoning workers since the 1600s.