Why Was Mercury Used in Hat Production?

Back in the day, hat makers experimented with different materials to improve the quality of their hats. Eventually, they found out about mercury through a coincidence and used it to produce excellent felt hats for over 100 years. However, it also led to some unexpected consequences.

Mercury was used in hat making to toughen the fur’s fibers and make them mat together more efficiently. The compound used to moisten the fibers was Mercury Nitrate Hg(NO₃)₂, and the process is called carroting. It produced a superior-quality felt, which in turn, resulted in higher-quality hats.

Using mercury in hat making led to so much suffering that we had to ban it eventually. Let’s discuss the full story behind the use of mercury and the mad hatter disease.

Dmitry Lityagin@123rf.com

The Curious Case of Mercury in Hat Making

The story of mercury being used in hat making is an interesting one. Around the 14th century, hats started becoming a famous status symbol for men. Milliner became a term to describe a trader specializing in importing women’s accessories from Milan.

By the 17th century, these fashion items’ mass production became necessary to meet the growing demands of Western society, and being a milliner became a lucrative business. During the 19th century, felt hats, in particular, became very popular in North America and Europe. 

Hat makers started experimenting with a variety of materials in style and production. They began using camel hair to prepare felt for hats. Soon, they found out that moistening the fibers with camel urine accelerated the entire process. So it became a common practice to remove fur from the animal with the help of camel urine.

But in France, people thought, “To hell with camel urine! We’ll just use our own urine.” So French workers started using their urine to moisten the fibers. It worked because the main component in urine is nitrogen-rich urea, which disrupts the fur’s chemical bonds.

Interestingly, they noticed that one particular workman among them was producing a superior felt consistently. There must be something magical in his urine, right? It turned out that he was being treated with a mercury compound for a disease named syphilis. He was taking mercurous chloride (HgCl) as medicine.

People associated mercury treatment of fibers with a superior felt, and rightly so. Eventually, people started using an orange-colored compound known as mercuric nitrate Hg(NO₃) in place of urine₂. This is how milliners started using mercury for hat making.

A Story of Human Suffering

Turning the fur into a finished hat was a complicated process. Usually, with the cheaper kind of fur, a solution of mercuric nitrate was brushed on the fur to toughen the fibers and allow them to mat together more easily. This process is called carroting because the fur would turn orange afterward.

Hat makers had to use beaver, rabbit, or hare fur to produce high-quality felt. Beaver fur has natural serrated edges, making this step unnecessary and a preferred fur for making hats. But given its scarcity and cost, cheaper furs that required carroting had to be used.

After carroting, the fibers were shaved off the skin and made into felt. The felt was later dipped in a boiling acid solution to harden and thicken it. The last steps were to shape the hat by steaming and then iron it.

During this process, hat makers typically worked in workshops that were not properly ventilated. Consequently, the vapors would concentrate over time. The workers would breathe in vapors of the mercury compound, which resulted in a build-up within their bodies, leading to mercury poisoning.

Mercury poisoning is also known as erethism, mad hatter disease, or mad hatter syndrome. It’s a neurological disorder that damages your brain and produces changes in behavior.

Its physical symptoms include loss of teeth, uncontrollable trembling, excessive drooling, loss of coordination, and slurred speech. Its mental symptoms are loss of memory, irritability, depression, anxiety, mood swings, and difficulty thinking clearly. 

In some extreme cases, the victims also experienced hallucinations, delusions, and extreme paranoia. Workers were suffering tremendously and could do nothing about it. Many of them also hid their disease because they would get on the manufacturers’ blacklist if they claimed compensation.

It wasn’t just mercury poisoning; hat makers also faced a more dangerous health issue, tuberculosis, which was the leading cause of deaths in the US back then. Hat makers were more susceptible to this disease in the close, steamy workrooms of hat-making factories.

Mad as a Hatter and Other Phrases

Danbury, Connecticut, was the hat-making capital of the world in the 19th century. It was also known as The Hat City. Fifty-six different hat-making factories produced over five million caps per year in Danbury. 

Understandably, mercury poisoning was widespread in this city. People referred to this ailment as The Danbury Shakes. Near the end of the 19th century, Hatter’s Shakes also became a term to describe the intense muscle spasms and tremors seen in hat makers.

Most of us are familiar with a character known as the Mad Hatter. This eccentric man was featured in Lewis Carroll’s 1865 novel Alice’s Adventures in Wonderland. He was named the Hatter and was referred to as “mad” in the story. This character is widely associated with the effects of long-term mercury exposure on physiology and behavior.

However, Carroll did not invent this phrase. The phrase mad as a hatter was commonly used when Lewis Carroll wrote his novel. The term came into use because the hat makers went mad. They experienced extreme paranoia and wandered about in a confused state, often being mistaken for drunks. Read more about the origins of the phrase here.

Switching to Hydrogen Peroxide

Mercury poisoning was noted in medical literature for the first time in 1860. For the next couple of decades, the Connecticut State Board of Health kept an eye on the effects of mercury. However, they didn’t take any action mainly because the carroting process only affected the workers and the general public’s health.

In 1919, a socially prominent doctor Alice Hamilton, MD, of Harvard University, studied the conditions of hat-making factories. She produced a report in 1922 detailing the effects of mercury poisoning. After intense objections from the hatters’ labor unions, another major scientific study was performed in the 1930s, and mercury poisoning in hatters was documented. 

Finally, on the 1st of December 1941, the governor of Connecticut officially banned the use of mercury in hat making. At that time, 10% of 22,000 US hat makers were suffering from mad hatter syndrome.

Mercury was then replaced by hydrogen peroxide (H2O2), which was much less dangerous and slightly less expensive than mercury. For many years after the ban in 1941, Danbury’s hat makers celebrated the ban on mercury every the 1st of December.

The last recorded case of the mad hatter disease was in the early 1940s. It is now common knowledge that hydrogen peroxide produces almost the same effect as mercury. Besides, it is also easier to acquire and readily available. Mercury is no longer used in the felting process in any hat-making factory around the globe.


We all love the character Mad Hatter from Alice in the Wonderland, but who knew there was an entire story of suffering behind that phrase.

Nobody would think hat-making could be lethal and cause so much suffering. Fortunately, we’re past that phase, and mercury is nowhere to be found in the felting process of hats. We hope that this article provided more insight on this topic.

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