This is the picture I was referring to when I showed this big bowl in 2006. The Secret Color Porcelain ware kept the secret of its origin until 1987. Then, archeological findings at the Fa Men Temple could prove that Mi Si Tsi (Secret Color Porcelain) was made in Yuezhou (Yue Yao, Yue kiln) near the Shanglin lake in the Zhejiang Province.
Celadon wares were made in numerous kilns, mostly in southern China, but also in Yaozhou (Shaanxi province). When a product is in high demand in China, it tends to get reproduced everywhere! (Sub sole nihil novi est). The reason Yue Yao was a secret is that the emperors didn't want ordinary people to know where their celadon wares came from. If everybody knew where to get emperor grade bowls, what's the point in being an emperor!?
But Lu Yu was close to reveal this secret when he noted in the Cha Jing, Chapter 4, that Yuezhou ware is first quality. Yuezhou ware is as fine as jade (the favorite jewel for Chinese). And, it would turn the cooked tea stew green again: even though Tang dynasty used green tea, it would turn red when cooked (which was how they made tea back then) through oxidation. The Yue Yao celadon would make the tea appear fresher, greener.
Bernard Leach makes a convincing case in "A potter's book" that Tang dynasty and Sung dynasty wares reached a perfection not found again since. They have greatly influenced Japan's tea culture. There, quality tea accessories are called Karamano, 唐物, literally Tang item. This is also why many Japanese accessories trace their origin in China. The japanese Cha Tuo (chataku) below is one of many examples of a Chinese tea accessory copied and adopted by Japanese.
Japan did itself, China and the world a great service by well preserving the Tang and Sung heritage. By building its tea culture on perfection, Japan could develop its own tea culture to the high degree we know now. This also enables us to find back to the historical roots of Chinese tea culture by using old Japanese accessories, which are close to their Chinese originals.