What is Meant by "Mad as a Hatter?"

Since the 1700s, hat makers sometimes experienced nervous disorders and odd behaviors to the point that they were accused of being mad. It occurred in observable patterns, but remained a puzzle at the time. What was going on?

The craft of hat making was brought to the Colonies by early European colonists. Raw materials were abundant here, and there was a huge market for hats in Europe, so the industry flourished. In fact, in 1731, King George II banned Colonial-made hats from being imported in order to protect the hat-making industry in England. Danbury, Connecticut was the center for hat making in the Colonies, and even though the city was burned by the British in 1777, the Industrial Revolution that followed independence brought the industry back to the rebuilt city in full force. Hat making changed from a cottage industry to factory production as mechanized factories were built.

In the 1800s, Danbury was the hat making capital of the world. At the industry's peak, there were 56 hat factories making over 5 million hats a year in Danbury. Felted fur was the most common material used in making hats. Part of the felting process included "carroting" which was so named for the orange liquid used to wash the furs. This orange liquid helped the fur separate from the skin, and it made the hairs lie together smoothly in the felting process. The orange liquid was mercury nitrate.

Workers were exposed to steamy vapors of the mercury nitrate, and of course they handled the felt many times as it was shaped and pressed. Hats were blocked with steam during several steps of the process, which released more mercury as vapors. Workshops were poorly ventilated; in the cold winter the heat from the washing and steaming of the fur was welcome. As a result, the highly toxic metal compounds accumulated in workers' bodies. Workers who had received long term exposure to the mercury nitrate came down with an ailment known as "The Danbury Shakes", which we now know was mercury poisoning. Those who suffered from the condition stumbled about with slurred speech and uncontrollable muscle twitches. Drooling, hair loss, loosened teeth, memory loss, and general confusion were other symptoms.

Even in later years, when mercury was no longer used in hat making, mercury poisoning was called "mad hatter syndrome".

Although it is dismal, this is the reason behind the phrase, "Mad as a Hatter".

Sources:
Textile history lectures

http://www.worldwidewords.org/qa/qa-mad2.htm