Tian qing ni 天青泥 has taken on a legendary status for many Yixing teapot enthusiasts. Said to be the finest variety of zini (Yixing purple clay), most of the information available about tian qing ni can be found in sources that rely on ancient references to the clay as well as surviving examples of antique tian qing ni teapots and the raw ore (such as the examples found today in the Yixing Ceramics Museum).
Sky Blue Clay?
The name tian qing ni can be translated as “sky blue clay.” Some have claimed that the name refers to the colour of the fired clay and that tian qing ni teapots should be blue. This is not true. Blue teapots are never made of original ore zisha of any type. Green and blue teapots have other elements mixed into the clay to give them these colours, for example molüni teapots.
Another more plausible explanation for the name, is that it refers to the colour of the original ore before it is processed. Examples of tian qing ni ore show it to be mostly blue-grey blue-purple-grey before it is processed and fired.
As for the fired clay, according to the classic 阳羡茗壶系“Yanxian Famous Teapot Series” written during the Ming Dynasty, “天青泥，出蠡蜀，陶之变暗肝色…” “Tian qing ni from Lishu; after it is fired it is a dark liver colour…” surviving examples of antique tian qing ni teapots, such as those from the Yixing Ceramics Museum, show a dark red-brown clay. Some small yellow flecks are also sometimes visible on the surface of the fired clay; these are traces of lipini 梨皮泥 a kind of lüni 绿泥 found together with tian qing ni ore. Tian qing ni is fired at temperatures between 1160-1210C and has a shrink rate of around 8%.
Historically, tian qing ni has been mined from the 蠡蜀大水潭 Lishu Dashuitan mining area, on the southeast side of Huanglong Mountain. Today the original mining location is under water.
According to the book 宜兴紫砂矿料 by 朱泽伟, Tian qing ni is classified as zini and is notable for having relatively little mica and a fine and somewhat sandy texture. Like other zini, the processed clay is known for having good plasticity, is easy to shape and fire, and is said to improve the taste of tea. Its properties, when used for brewing tea, would not likely be that different from other kinds of zini - muting undesirable flavours in tea such as "smokiness" and other harsh notes.