HOENGSEONG, SOUTH KOREA — In the relative cool of midnight, Seo Seok-gu’s workers break the seals on the kilns and, using long steel hooks, drag out what remains of the oak logs that were inserted a week before and have by now turned into smoldering charcoal. It gives off a hypnotic glow Mr. Seo calls “my flower.” The temperatures inside the clay domes reach up to 1,400 degrees Celsius.

By dawn, the work is done, and the kilns are allowed to begin cooling down.

Later that day, Mr. Seo’s ovens are packed with something new: people, who huddle inside where the temperature still hovers around 200 degrees Celsius (nearly 400 Fahrenheit), so hot that synthetic clothes are banned because they can melt. For two days the kilns provide heat baths for visitors. Then, another cycle of charcoal production begins.

“They come from all round the country to crawl into my kilns,” said Mr. Seo, 73, the head of Gangwon Oak Charcoal, South Korea’s largest complex of charcoal kilns.

On a recent rainy day, Yang Eun-ja, 53, a homemaker from Seoul, sat inside a kiln with her knees pulled up to her chest. A towel was draped over her hair to protect it from the intense heat, while beads of sweat rolled down her face.