Fur was readied for use in hats by separating the skin and hair through a process known as felting or carroting. Prior to the 17th century, urine was used, but French hat makers discovered that mercury – first in the form of mercurial urine from hat workers who consumed mercury chloride to treat syphilis and later in the form of the mercuric salts such as mercuric nitrate – made the process easier. Hat makers exposed to prolonged inhalation of the mercury vapours that were released upon exposure to heat developed mad hatter disease – or mercury poisoning.
