The bright-white sticks drop one by one into the whir and clatter of a weatherworn piece of machinery, where they are stamped with the most celebrated name in chalk: Hagoromo.
The early stages of the process look a lot like food production. The ingredients in what the company’s owner calls a “recipe” are dumped into a mixer originally designed for bread dough, and what comes out is fed into a kneader originally intended to make udon noodles.
Of the thick grayish mass that emerges, four ingredients are known: calcium carbonate, clay, glue and oyster shells. The other three are a secret. In a video posted to YouTube about the chalk, an American fan offers a guess as to one of them: angel tears.
Hagoromo chalk is a cult favorite of elite academics, artists and others around the world who praise it for its silky feel, vibrant colors, scant dust and nearly unbreakable quality. Mathematicians in particular are prone to waxing poetic about it, and buying it in bulk. The YouTube video, produced by Great Big Story, has been viewed more than 18 million times.
Despite its renown, Hagoromo is still produced on a relatively small scale, using custom-made equipment, much of it run by two laborers who are identical twins — a throwback in a high-tech era where interactive displays are replacing chalkboards.
The chalk has survived World War II and, nearly 70 years later, the closing of the Japanese company that originally made it. The coronavirus pandemic is the latest threat, hurting sales as education and other activities go virtual.
Hagoromo’s continued existence is an unlikely story that bridges Japan and South Korea, two countries that have had an uneasy, and often bitter, relationship ever since the war.
The company that produced the chalk for its first eight decades was founded in Nagoya, Japan, in 1932 as Nihon Chalk Seizosho. After its production building burned down during a World War II air raid, the company was rebuilt and chalk manufacturing resumed in 1947 under the name Hagoromo Bungu.
Production peaked in 1990 at 90 million sticks. But sales then steadily declined, eventually reaching half that level over the next two decades amid competition from whiteboards and newer technologies like so-called smartboards.
In 2014, Takayasu Watanabe, the grandson of the company’s founder, announced that Hagoromo would halt production, partly because of the industry’s declining fortunes and partly because of his own ill health. He did not ask any of his three daughters to take over the company.
As Mr. Watanabe was preparing to shut it down, he received a visit from Shin Hyeong-seok, who had been importing the chalk to South Korea for nearly 10 years. Mr. Shin sold the chalk through the company he started, Sejong Mall, named after King Sejong the Great, who in the 15th century created Hangeul, the Korean writing system.
Mr. Shin had discovered the chalk years before in Japan while investigating the workings of cram schools. At the time, he was teaching math part time while pursuing a Ph.D. and his dreams of becoming an architect.
“I went into the teachers’ lounge and remember being mesmerized by the fluorescent-colored chalks,” he said. “And when I started writing with one, I could not put it down.”
On his trip to see Mr. Watanabe, Mr. Shin presented what he called a “crazy idea.” He, a teacher and importer with no manufacturing experience, would take over production of the chalk in South Korea. Mr. Watanabe laughed.
But Mr. Shin kept pressing. “My pitch to him was that there are many things in the world that will disappear one day, but the best-quality item should be the last to do so,” Mr. Shin said.
Eventually, Mr. Watanabe came around. He gave his machines to Mr. Shin at practically no cost. The slightly younger of the two twins, Choi Eui-chun, an ethnic Korean born in Japan, moved to South Korea to work at Mr. Shin’s factory, and the older brother, Choi Eui-haeng, later joined him.
Mr. Watanabe visited three or four times to examine the installation and operation of the machines from his wheelchair. He died in July.
Since 2015, the chalk has been produced in a modest corrugated metal building along a four-lane rural highway 20 miles from the border with North Korea.
Many of the machines in the small factory, like the logo stamper, with its rust-flecked metal and old-style rubber imprinter, look more like museum pieces than cogs in a modern assembly line. At the end of the assembly line, the chalk is sorted into rows of 12 and then placed by workers into boxes bearing the name Hagoromo Fulltouch Chalk. “World Best Quality,” the boxes proclaim.
Seventy-two white pieces sell for $11.
Mr. Watanabe was called a traitor by some of his colleagues who accused him of seeking to transfer good technology to South Korea. The Japanese chalk association feared that Mr. Shin would produce an inferior, cheaper product and export it to Japan, undercutting manufacturers there.
Mr. Shin recalled that Mr. Watanabe had been untroubled by any of this criticism. He said that Mr. Watanabe eventually came to treat him as a son.
Takako Iwata, the second of Mr. Watanabe’s three daughters, who served as interpreter for Mr. Shin and her father, saw a grander ambition in their bond. “I hope my father’s chalk could help relieve Japan-Korea tensions,” she said. “That’s the hope, at least.”
She said she wasn’t exactly sure how Hagoromo had become so beloved outside Japan. “I guess people who came to Japan just kept on bringing the chalk back to their home countries,” she said. “When my father was still running the company, he did not know about this huge following.”
That changed a bit, though, in his company’s final months, when he received a flood of orders, including from American professors who hoped to buy supplies large enough to last 10 years or more.
David Eisenbud, the director of the Mathematical Sciences Research Institute at the University of California, Berkeley, said he had bought enough to last the rest of his life.
Dr. Eisenbud is a key figure in the chalk’s popularization in the United States. He was first introduced to it years ago during a visit to the University of Tokyo. “Everything about the chalk was exquisite,” he said. “I thought, ‘Chalk is chalk,’ but I was wrong.”
He later persuaded an acquaintance to import the chalk into the United States. (Mr. Shin now sells it to American buyers through Amazon.)
Yujiro Kawamata, a Japanese mathematician who introduced Hagoromo to Dr. Eisenbud, marveled at the turn of events.
“I happened to tell Eisenbud about the chalk, which was just a tool that was a part of my everyday life, and now the whole world knows about it,” Dr. Kawamata said.
Su-Hyun Lee contributed reporting.