Fine porcelain originated in China (that's why it's called china), and in the 17th century, Europe couldn't get enough of it. French potters would have loved to get in on that business, but they did not know how to produce porcelain. The Chinese weren't about to give away their secret techniques. The Jesuits sent missionaries to win souls around the world, and they were also into gathering knowledge from every culture they visited. So they assigned Francois Xavier d’Entrecolles to figure out how porcelain was made when he was sent as a missionary to China. D’Entrecolles had to learn the language, gain the trust of the Chinese, and then learn how to make porcelain. It took him ten years. Then he set it all down in a long letter.
But by the end of the letter, he has taught his interlocutor exactly what porcelain is made of, how those materials are mixed, separated, and purified, and how the resulting clay is rolled, kneaded, moulded, and fired. He has gone over special cases (extra-large pieces; glaze preparation; crackling) and speculated about how to reconstruct various techniques that the Chinese artisans considered “lost secrets,” including kia-tsim—a glazing technique in which illustrations appear on a bowl only when it’s full of water.
A modern reader comes away with a good understanding of the porcelain-making process, as well as an appreciation for the creativity on display. D’Entrecolles tells of porcelain ducks and turtles that float on water, and realistic porcelain cats with eyes that glow when candles are put inside. (Those were meant to scare rats.)
Did this satisfy his superiors? No, ten years of work on one project wasn't enough for them. Read the story of D’Entrecolles and his industrial spying at Atlas Obscura.