If you've read or watched an Alice in Wonderland story, you'll have heard of the Mad Hatter and seen his unusual behaviour. I hear the phrase “mad as a hatter” all the time, usually applied to myself. But at least I'm not suffering from chronic mercury poisoning.

Back in the eighteenth century, it was an occupational hazard of felt hat-making. And felt hats were, of course, all the rage.

Orange mercury nitrate was considered a necessary ingredient: it got smeared over the surface of the furs and shaved off once it had dried, allowing furs to be merged and the hairs to stand to attention. This was called carrotting. It worked pretty well. Really, it was a shame that mercury nitrate was a neurotoxin that severaly poisoned the hatters.

Sometimes this was through direct ingestion: whilst they were painting on the nitrate, the hatters would actually lick the tips of their brushes to sharpen the tips – much like painters who then got lead poisoning. Even if they didn't do this, however, the next stage of the process would allow them to breathe it in: as they shaved off the hardened mercury nitrate to produce the finished product, it would incidentally vaporise into a thin dust.

Mercury poisoning is not nice. Hatters suffered from all kinds of symptoms, ranging from confusion and emotional distress to reddening, shaking and muscular weaknesses. Eventually, it would probably kill them.

It wasn't until 1898, in France, that somebody realised it wasn't a good idea to allow people to do a job that slowly poisoned them, and passed a law to prevent it. The fad spread until mid twentieth century, when even the Americans stopped using mercury for hats.

It took a good deal longer for chemists in labs to stop picking it up in their hands and playing with it, of course.