“The secret of Chartreuse has long been the despair of distillers, just as the natural blue of forget-me-nots has been the despair of painters,” reads an 1886 document referred to in a recent history of the company and order. Father Holleran spent five years overseeing the distillation process, ordering ingredients and planning its production schedules. When he departed the site in 1990, he became the only living outsider to know the liqueur’s ancient formula.

“It’s safe with me,” he said. “Oddly enough, they didn’t make me sign anything when I left.”

This trade secret is both a marketing coup and a potential catastrophe. “I really have no idea what it is I sell,” a Chartreuse Diffusion president told The New Yorker in 1984. “I am very scared always. Only three of the brothers know how to make it — nobody else knows the recipe. And each morning they drive together to the distillery. And they drive a very old car. And they drive it very badly.”

Beyond the two monks who now protect it, all the others — Carthusian or not — involved in the production of Chartreuse know only fragments of the recipe.

Inside the Grande Chartreuse, skilled monks receive, measure and sort 130 unlabeled plants and herbs into giant unmarked (or, in 2020, QR-coded) sacks. Then, at the distillery, five non-Carthusian employees work alongside two white-robed monks to macerate, distill, blend and age the liqueur. A computerized system also allows them to virtually monitor the distilling from the monastery.

Along its five-week distilling process, and throughout the subsequent years of aging, those two monks are also the ones who taste the product and decide when it is ready to bottle and sell. “They are the quality control,” said Emmanuel Delafon, the current C.E.O. of Chartreuse Diffusion.

The order owns the diffusion company almost exclusively, and works with the business’s secular employees, who carry out the tasks too foreign to the order’s hermetic vocation.

“It’s their product, and we’re at their service,” Mr. Delafon said. “They need it to maintain their financial independence. They trust us to make the link between monastic life and everything else.”