It doesn’t take long to realize that John B., the man at the heart of the captivating true-mystery podcast S-Town, is sick. Decades of restoring antique clocks in the desolate, rural town of Woodstock, Alabama (Shit Town, as he affectionately calls it), had left its mark on John B. McLemore, both psychologically and physically. By the end of the series, it seems highly likely that the illness in both his mind and body were caused by the same thing.

Caution: Many S-Town spoilers ahead.

In the podcast, hosted by This American Life producer Brian Reed, listeners learn that McLemore’s work with clocks led him to master the antiquated art of “fire-gilding,” an illegal chemical process that involves melting mercury and gold together to leave a rich, textured layer of gold on other metals. Throughout the podcast, McLemore shows signs of mood swings, anxiety, depression, and suicidal thoughts, and could very well be a long-time sufferer of Mad Hatter disease — a colloquial term for one form of mercury poisoning.

At the end of the second episode, we learn that McLemore commits suicide by drinking cyanide, and the rest of the seven-part series is dedicated to exploring the details of his life that might have led him to that moment. His brilliant work as a clockmaker — and ultimately, the harmful practice of fire-gilding — plays a huge part.

Fire-gilding (or “mercury gilding,” as it’s sometimes called) has been used by artists to apply a gold layer to objects for over two millennia — and in a largely unsafe way. When combined with molten gold, mercury, which is liquid at room temperature, turns into an oily substance that glides onto the surface of the object to be coated in gold. But to ensure that only gold and not mercury is left behind, the entire object has to be heated to 357 degrees Celsius, which is the temperature at which mercury evaporates into a gas. This leaves behind a thin layer of gold — meaning the mercury ends up in the air.

A 19th-century gas mask used for avoiding the fumes from fire gilding.

If, like McLemore, you’re not working with a professional ventilation system and other safety equipment, the fumes from that elemental mercury (that is, mercury in its metal form) are going to end up in your lungs. Over long-term, repeated exposure, this can be very dangerous. As a 2012 review in the Journal of Biomedicine and Biotechnology explained, elemental mercury vapor, which the lungs and other mucous membranes (wet organs, like the eyes and mouth) absorb readily, ends up in the brain in “considerable” amounts.

Heating liquid mercury releases harmful vapors, which causes metal to accumulate in the brain and other parts of the body.

There, it has serious effects on the nervous system. It’s also known to cause problems with the bone marrow, the reproductive system, and the heart, which it reaches through the bloodstream. It isn’t readily pushed out by the body, so it accumulates; treatment for toxicity involves swallowing a “chelating agent,” which collects mercury like a magnet and excretes it through the kidneys (to limited success). Miners are the most susceptible to this sort of poisoning. As the study authors point out, the “burning of metallic mercury on the gravel promotes the separation of gold, a process called amalgamation, which causes emission of large amounts of mercury vapor that is inhaled immediately by the miner, since they do not use appropriate personal protective equipment.” Antique clock tinkerers too, it seems, are also at risk.

“As if time gives a shit.” #STown, Chapter III

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People exposed to fumes from elemental mercury can show a range of symptoms: Some will feel flu-like, others will lose fine motor control, some lose sensory perception in their eyes and nose, and some get heart palpitations. Other times, it manifests as depression, anxiety, paranoia, and thoughts of suicide — the same symptoms that collectively make up Mad Hatter disease, which Reed suggests, at the end of the series, that McLemore has been suffering from for years. Technically, Mad Hatter disease is caused by a different form of mercury than that McLemore dealt with — it’s named for hatmakers in medieval Europe who were said to go “mad” after working with inorganic mercury salts, not metallic mercury — but the detrimental health effects of long-term and large-scale metallic mercury exposure are so broad that the cause of McLemore’s condition is hard to doubt.

Ultimately, however, McLemore is never officially diagnosed with mercury poisoning. His autopsy didn’t test for mercury, but that’s not surprising, as the element doesn’t stay in the body for very long. Reed notes that “John also had physical symptoms that are consistent with mercury poisoning as well as behavioral ones.” Specifically spontaneously vomiting, an enlarged brain, and congestion in the lungs, all of which were noted in the autopsy results. To know for sure, the mercury levels of John’s workshop would have to be tested, and the Burt family (the family referenced at the beginning of the podcast and the new owners of the land), wouldn’t allow Reed to get samples from the property, which also houses John B.’s incredible hedge maze. Despite the overwhelming evidence that mercury poisoning led him to his suicide, his final cause of death, like so many other aspects of his rich and complicated life, will forever remain a mystery.

Photos via Brian Reed/Twitter