The term “Mad as a Hatter” refers to someone who is behaving in an irrational way. The origins of the term come from 19th Century England. Hatmakers used felt that was treated with toxic chemicals such as arsenic, lead, and mercury. Symptoms included confused speech, personality changes and tremors.
Modern day methods of hatmaking have changed, making it a much safer industry.
Examples of handmade wool felted hats.
Felting basics and how to make felt hats
More About Felt making:
How to Make Handmade Felt
You can make felt in your bathtub.