Since I am very fond of "Alice in Wonderland," the question had often
crossed my mind in the past. This time, as it tried to cross, I
intercepted it and forbade it to continue until I had investigated it.
Below are my findings.
In times past, a mercury solution was commonly used in the felting
process. Mercury nitrate was used in processing the animal hair that
is used in making felt. It caused the fibers of the fur to separate
from the pelt and to mat together more readily. This is called
"carroting" (a name that might draw the interest of the March Hare).
"The most famous Mad Hatter, of course, is the one from the Mad Tea
Party in Alice in Wonderland, the partner of the March Hare. Both mad,
of course. But Lewis Carroll did not invent the phrase, although he
did create the character. The phrases 'mad as a hatter' and 'mad as a
March hare' were common at the time Lewis Carroll wrote (1865 was the
first publication date of Alice). The phrase had been in common use in
1837, almost 30 years earlier. Carroll frequently used common
expressions, songs, nursery rhymes, etc., as the basis for characters
in his stories.
The origin of the phrase, it's believed, is that hatters really did go
mad. The chemicals used in hat-making included mercurious nitrate,
used in curing felt. Prolonged exposure to the mercury vapors caused
mercury poisoning. Victims developed severe and uncontrollable
muscular tremors and twitching limbs, called 'hatter's shakes'; other
symptoms included distorted vision and confused speech. Advanced cases
developed hallucinations and other psychotic symptoms."
The Straight Dope: What caused the Mad Hatter to go mad?
"Hatters really did go mad. The chemicals used in hat-making included
mercurous nitrate, used in curing felt. Prolonged exposure to the
mercury vapors caused mercury poisoning...
The popular top hat of the time was made from beaver fur, but cheaper
ones used furs such as rabbit instead. A complicated set of processes
was needed to turn the fur into a finished hat. With the cheaper sorts
of fur, one step was to brush a solution of mercurous nitrate on to
the fur to roughen the fibers and make them mat more easily, a process
called carroting because it made the fur turn orange. Beaver fur had
natural serrated edges that made this unnecessary, one reason why it
was preferred, but the cost and scarcity of beaver meant that other
furs had to be used.
Whatever the source of the fur, the fibers were then shaved off the
skin and turned into felt; this was later immersed in a boiling acid
solution to thicken and harden it. The acid treatment decomposed the
mercurous nitrate to elemental mercury. Finishing processes included
steaming the hat to shape and ironing it. In all these steps, hatters
working in poorly ventilated workshops would breathe in (elemental)
HgTech: Mad Hatter
"The felt hat industry has been traced to the mid-17th century in
France, and it was probably introduced into England some time around
1830. A story passed down in the hat industry gives this account of
how mercury came to be used in the process: In Turkey camel hair was
used for felt material, and it was discovered that the felting process
was speeded up if the fibers were moistened with camel urine. It is
said that in France workmen used their own urine, but one particular
workman seemed consistently to produce a superior felt. This person
was being treated with a mercury compound for syphilis, and an
association was made between mercury treatment of the fibers and an
On December 1, 1941 the United States Public Health Service banned the
use of mercury in the felt industry in this country. Although it has
been suggested that the expression 'mad as a hatter' and the character
portrayed in Lewis Carroll's Alice in Wonderland may have other
origins other than mercurialism among hatters, few can resist making
this apocryphal analogy."
Dartmouth Toxic Metals Research Program: Mercury
"Danbury Connecticut has always been known as 'The Hat City'. It was
the hat making capital of the world in the 19th century. At the peak
of the industry, five million hats a year were produced in 56
different factories in Danbury. A process called 'carroting' was used
in the production. Carroting involved washing animal furs with an
orange-colored solution containing a mercury compound, mercury
nitrate. The colorful solution facilitated the separation of the fur
from the pelt and made it mat together smoothly. Workers would often
be exposed to mercury vapors in the steamy air. (reference)
Many hatters with long-term exposure, particularly those involved in
carroting, got mercury poisoning. Mercury poisoning attacks the
nervous system, causing drooling, hair loss, uncontrollable muscle
twitching, a lurching gait, and difficulties in talking and thinking
clearly. Stumbling about in a confused state with slurred speech and
trembling hands, affected hatters were sometimes mistaken for drunks.
The ailment became known as 'The Danbury Shakes'. In very severe
cases, they experienced hallucinations....
Some hatters eventually died of mercury poisoning. 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 hatmaking ceased. Processes to mat felt
that didn't include mercury were developed."
Connecticut Sea Grant: The Mad Hatter Mercury Mystery
My Google search strategy:
Google Web Search: mercurialism hatters
Google Web Search: mercury mad hatter "used for OR as OR in"
Google Web Search: mercury carroting