In LACMA’s current exhibition, Fashioning Fashion: European Dress in Detail, 1700–1915 (closing March 27), one distinctive accessory for the well dressed gentleman is the top hat.
Smith and Company, Man's Top Hat, c. 1840–1860, Costume Council Fund
The manufacture of top hats is a labor-intensive and hazardous process. In the eighteenth century, the best hats were made from the fine underfur of the beaver. Other top quality furs were camel and vicuña. By the mid-nineteenth century, the beaver population had declined and a 100% beaver fur hat was rare; most were a blend of furs, incorporating rabbit, hare, or otter.
One key component in top hat manufacture is the felting of the fur, a long process of moistening, heating, and pressing the fur fibers into a dense mat. One way to speed up felting—and production—was carrotting, where fur pelts were soaked in a solution containing mercury salts. The chemical bath causes the scales to lift up, causing more fiber entanglements and making it easier to separate the fur fibers from the skin. One recipe called for three pounds of nitric acid, three ounces of mercury and seven and a half pounds of water. After the fur dried, it was shaved from the pelt. Whether working with the “fluff” dry or moist, the hat-making workshops were an environmental disaster filled with moist mercury-laden air and dry mercury-contaminated dust.
Hat-makers dip felt hats into nitrate of mercury solution. From W. Gilman Thompson, MD, "The Occupational Diseases," D. Appleton and Company, 1914
As far back as the eighteenth century, the harm that mercury-containing compounds could do to hat makers was well known. Afflictions included tremors (called “hatters’ shakes”), tooth loss and blackened gums, excessive drooling, muscle twitching, and a lurching manner of walking. So bad were the symptoms that it drove many a hat maker to drink. It’s no wonder that the phrase “mad as a hatter,” came to be. By the late nineteenth century, scientists and doctors began publishing their observations and suggesting workplace improvements such as increased ventilation, shortened exposure times, and the use of protective gloves and clothing. In the twentieth century, newer hat making processes replaced those that used mercury but by then the popularity of hats was in decline.
Peck and Company, Man's Top Hat, c. 1832, Costume Council Curatorial Discretionary Fund
S. Tuttle Hat & Cap Manufacturer, Man's Top Hat, 1840–1860, purchased with funds provided by Suzanne A. Saperstein and Michael and Ellen Michelson, with additional funding from the Costume Council, the Edgerton Foundation, Gail and Gerald Oppenheimer, Maureen H. Shapiro, Grace Tsao, and Lenore and Richard Wayne
Catherine McLean and Charlotte Eng, Conservation Center
Unframed The LACMA Blog 
Thank you so much for this information! I have a “fetish” for vintage hats and their history. I had heard about the mercury treatment but appreciate your more inclusive info. I definitely will make it out to LACMA to see this exhibit!!
Great research!
what was the newer hat making processes that replaced those that used mercury? and what did it include if not mercury? thanx!
How can you tell if a hat is covered in beaver felt vs. silk plush? I see many that say they are silk plush that look identical to the ones that say beaver… Can you tell by maker? I own one made by Dunlap, New York… was told it was from the late 1800s (probably late 80’s-90’s) and that it was beaver felt… doing research online I find it hard to tell the difference… I work at a Museum and am working on an exhibit about accessories of the Regency period (although the hat is not of that period, we are offering it as an example and having information about the differences in styling during of the period). I want to make sure I am identifying it properly. Can you offer me some insight please.