From the mid-17th to the mid-20th centuries, a process called "carroting" was used in the manufacture of good quality felt for making men's hats. Rabbit skins were treated with a dilute solution of the mercury compound mercuric nitrate. The skins were dried in an oven until the thin fur at the sides turned orange- hence the name of the process. Pelts were stretched over a bar in a cutting machine and the skin sliced off in thin shreds, causing the fleece to come away entirely. The fur was blown onto a cone-shaped colander, treated with hot water to consolidate it, the cone peeled off and passed through wet rollers to cause the fur to felt. These 'hoods' were then dyed and blocked to make hats.
This toxic solution and the vapors it produced resulted in widespread cases of mercury poisoning among hatters, which may have been the origin behind the phrase "mad as a hatter" and the character of the the Mad Hatter in Alice in Wonderland.
The United States Public Health Service banned the use of mercury in the felt industry in December 1941.
