Visitors to the Degas, Impressionism, and the Paris Millinery Trade may be surprised to learn that hat making was a dangerous business, due to the introduction of mercury to the process of preparing fur pelts in the seventeenth century. Fur was readied for use in both men’s and women’s hats through a process known as felting or carroting, in which animal hairs were removed from their hide, and exposed to heat, moisture, and mechanical pressure, causing them to mat together into felt. Prior to the seventeenth century, the skin and hair were separated using urine, 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 hairs softest and most pliable, enabling them to felt together with greater ease. By the end of the seventeenth century, this process had spread to England as well.

Although mercury was useful in the formation of fur felt, prolonged inhalation of the mercury vapors that were released upon exposure to heat, or in some cases the ingestion of the element by workers hoping to further increase the pliability of their felts by putting the mercury-containing materials in their mouths, resulted in mercurial or so-called “mad hatter” disease. This disease affected the workers’ gastrointestinal and central nervous systems. Notable symptoms included tremors, irritability, timidity, and mental instability. The condition eventually gave way to the well-known proverbial expression, “mad as a hatter,” by the second quarter of the nineteenth century.