Felt hats have been manufactured from small animal pelts since about the 14th century. The major method of separating the fur from the skin during the mid-19th century, used camel urine. Some hatters chose to use their own urine, instead. The active ingredient was nitrogen-rich urea.
Of those hatters using their own urine, any undergoing treatment for the venereal disease (now called STD) syphilis were found to produce, oddly enough, superior felt! It was eventually realized this was due to the mercurous chloride (HgCl) they consumed as medication. This led to the replacement of urine with orange-colored mercuric nitrate Hg(NO₃)₂ solution.
The fumes resulting from the vats of solution and the drying process contained dangerous levels of mercury, which chronically poisoned the workers. Chronic mercury poisoning leads to tremors, mood disorders—even dementia. Hence victims became known as “mad hatters.”
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