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Mad as a Hatter

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Most people are familiar with the Mad Hatter from Lewis Carroll's Alice's Adventures in Wonderland (1865) either through reading the classic tale or watching one of the many film and stage adaptations. 

The Mad Hatter: Have I gone mad?
Alice: I'm afraid so. You're entirely bonkers. But I'll tell you a secret. All the best people are.
 Mad Hatter Illustration by John Tenniel 1865
"The Mad Hatter" by John Tenniel, 1889
The eccentric character has rather dark origins though there is some debate over Carroll's inspiration and his interpretation of "mad." Is the Mad Hatter angry or insane?
Most of the evidence, literary and historical, support the latter. Many hat makers in the 18th and 19th century were suffering from longterm exposure to mercury, which is a neurotoxin. Known as "erethism" or "mad hatter syndrome," the disorder is characterized by behavioral changes such as depression, delirium and memory loss.
Beginning in the 1730s, mercuric nitrate ( Hg(NO3)2·2H2O) was used in the processing of fur fibers into felt. It was brushed onto the animal pelts, where the chemical would break down keratin proteins in the fibers and make them more suitable for felting. The mercuric nitrate was used on rabbit and nutria pelts. Beaver pelts did not need the chemical as the fibers are naturally fine and easy to felt. As the beaver population was hunted to near extinction, the use of alternative fibers by hatters increased as did the usage of mercuric nitrate and the number of hatters suffering from mercury poisoning.
According to the V&A Museum Blog: "Animal skins were immersed in a dilute solution of the compound, causing the fur and the pelt to separate and mat together. It was then dried in an oven causing the edges of the skin to turn orange (often called "carroting"). The skin could then be cut off in thin slices, consolidated, rolled into felt, then dyed and formed on a block to make a hat."

 

Antique bottle of mercury nitrate
Hatters working with the felt hat bodies were exposed to mercury vapors as they applied steam in the blocking process and also absorbed it through the skin. It was a hazardous occupation! In addition to the neurological symptoms mentioned in association with the Mad Hatter, there were other ailments such as trembling hands, which became known as the "Danbury Shakes" or "Hatters' Shakes."
Danbury, Connecticut was the center of hat making in the USA by the 1850s. They called themselves the "Hat City of the World." In 1880, Danbury produced 4.5 million hats. Of course, all of those that were felt likely contained mercury. The practice of using mercuric nitrate in hat production continued even though it was a known health hazard. It was cheap and effective. And workers' rights were not recognized until the early 20th century. The practice was finally banned in the USA during World War II. In the book, Fashion Victims: The Dangers of Dress Past and Present, the author cites a 1940 American Public Health Report: "11% of felt hat makers at 5 factories in Connecticut suffered from mercury poisoning."
The toxic practice was used in England through the 1960s. Yikes!
Mercury is persistent: it stays in the felt over time - and in the environment around former hat-making factories. What does that mean for you as a milliner or someone who handles vintage hats?
If you are purchasing deadstock rabbit fur felt hat bodies manufactured prior to 1960, I recommend testing them for mercury content. You can buy kits online. Museums use a XRF Analyzer, that  determines the elemental composition of materials. If you are unable to test, steam and block outside and wear gloves. Use the same protocol when steaming vintage hats for restoration or repurpose.
If you are a collector of vintage hats, occasional wear or handling should not be an issue. If it's a favorite felt hat worn frequently, I'd recommend testing and if it comes back positive for mercury, add a lining to serve a a barrier between your head and the hat.

 

Antique hat at the V&A, mercury content
 Antique hat in collection of V & A Museum, London
Safety labels indicate mercury salts in textiles.

All that being said, I personally own many vintage felt hats and wear them occasionally. I handle them with care and wash my hands afterwards. I brush and steam them outside. And as a milliner, I have worked with vintage deadstock felt bodies. The majority date to the 1960s -1990s and are mercury-free. Anything older is tested. By the 20th century, the connection between carroting felt and mercury poisoning in hatters was indisputable. The practice was being phased out prior to the 1941 legal ban. It is much more likely that 19th century felt hats will test positive for mercury when compared to 20th century examples. Be especially careful with older hats.

carroting Danbury shakes environment fashion history felt hats hat making mad hatter mercury

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