Dr. Mavis first had her suspicions after seeing the workshop and pile of hats, and
Olivia's description of her hallucinations made it clear. In the time that William
Pritchett lived, mercury was used in the process of hat making. William, Holly and
Olivia died of mercury poisoning from prolonged exposure in the attic workshop.
The symptoms include shaking, cramps, fever and hallucination. Mercury poisoning
was common among professional hat makers, which is where the phrase "mad as a
hatter" comes from.