Where does "mad as a hatter" saying come from?

Mercury was introduced into hat making in France in the 17th century. It was a notoriously dangerous profession. The manufacturing of hats from fur entails a process known as felting, in which the hair was cut from pelt (usually that of a rabbit), laid layer upon layer on a conical mould, and pressed and shrunk with the steam of hot water. The advantage of using mercuric nitrates in felting was that it made the outer stiff hairs on the pelt soft and limp, and twisted and roughened them so that they packed together more easily. In Britain the process came to be known as carrotting because treatment with mercuric salts turned white fur reddish brown.

The pelts were dipped into mercuric nitrates in poorly ventilated rooms and so common were the symptoms of mercurialism that terms such as "the hatters’ shakes" and "mad as a hatter" were used in everyday speech.

In 1805, John Pearson coined the term Erethism which was used to encompass the manifestations of mercury toxicity: excessive timidity, diffidence, increasing shyness, loss of self confidence, anxiety and a desire to be unobserved and unobtrusive. The victim had a pathological fear of ridicule and often reacted with an explosive loss of temper when criticized.

Erethism
(G. erithismos, irritation) — an abnormal state of agitation or excitement. [Steadman’s Medical Dictionary]
The psychotic symptoms of mercury poisoning had been described in the Eighteenth Century, when mercurial ointments were used in the treatment of syphilis: a night with Venus being followed by a lifetime with Mercury.

Business boomed because everyone then wore hats! Hats were indicators of gender, occupation, social status, season, interests, and personality. Abraham Lincoln's famous stovepipe hats were made of beaver felt. At the peak of the industry, five million hats a year were produced in 56 different factories in Danbury.

"Sea of Hats", circa 1919, courtesy of the Danbury Historical Society.

Better to suffer than give up trade.

In 1934, following intense objections from hatters’ labor unions, a major scientific study was performed and documented mercury poisoning in hatters. Processes to mat felt that did not include mercury were developed, and by 1943 all use of mercury in hat making ceased.

Currently, dentistry is a profession exposed to mercury. Dentists have intensely objected to removing Hg from their trade, similar to the hatter’s union in the beginning of the 20th century. Mercury toxicity is a significant health hazard to the dentistry profession. But, where hatters were endangering only themselves, dentists are putting their patients at risk also.

How Mercury Causes Brain Neuron Degeneration

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5-minute video, copyright University of Calgary 2001

Smoking Teeth = Poison Gas

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"Alice’s Adventures in Wonderland"

The term "mad hatter" was used in the portrayal of a character in Lewis Carroll’s "Alice’s Adventures in Wonderland". The Mad Hatter was an eccentric extrovert always obsessed with time. His watch, which he shook and looked at constantly, told only the day not the hour; it was two days late, a result of the March Hare’s putting butter in the works. This was an unending regret to him, as was his quarrel with Time, who, so he told Alice, was him not it. But his portrayal as a "mad hatter" was not accurate medically.

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