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Yes, mercury was used in the production of hats, particularly in the process of making felt for hats. The mercury compound, usually mercuric nitrate, was used to separate animal fur from their pelts and to make the fur fibers mat together more easily during the felting process. This process was known as "carroting" and it involved treating the fur with an orange-colored solution containing mercury compounds. Workers who handled this process were often exposed to mercury vapors, leading to mercury poisoning and symptoms such as tremors, speech problems, emotional instability, and hallucinations, which gave rise to the term "mad hatter syndrome." The use of mercury in hat-making was banned in the early 1940s in the United States.
