Cup with curved rim and qingbai glaze, Northern Song, 11th century, Jiangxi, or perhaps Fanchang county, Anhui, height 7.3 cm.
The word used in the business for a perfect shape like this is "finely potted"; and she is: she shows off that which is best about Chinese porcelain (thanks to the world's best kaolin): it is very pliable when soft and extremely hard once fired, making it possible to produce very thin, fine shapes.
Qingbai wares were made over a fairly long period of time, from the early Northern Song period to the middle of the Yuan period. Their evolution follows a trajectory familiar for many types of wares: the earliest examples are finely made and take their inspiration from already popular wares made at other kilns (...); then there is a period of growing confidence, when the potters develop and perfect their styles -- the classical period; then attention turns to maximizing profit and methods of improving production, such as upside-down firing; and, finally, quality starts to drop off and an attempt is made to cover the deficiency with extravagant decoration and novelties.
Of course, art historians of Chinese pottery are interested in this trajectory model because its automatic application helps them "date" pieces (rather dubiously, I should think); but there is something convincing about it: it may seem to fit many other art forms -- consider the trajectory of Italian painting between 1400 and 1750. There are probably important psychological and economic reasons why this should be so; it bears thinking about, does it not.
But enough theorizing. Look here: the superiority of porcelain over plastic no longer lies in color and shine; but it still lies in the feel of things (porcelain is heavy, hard and cool); and -- in texture. If you look closely at the piece you can just make out (despite my bad photography) that the color of the qingbai glaze is not as smooth to the eye as its surface is smooth to touch. Rather, if you look very closely you will see that the color looks like curdled milk: with more or less uniform-sized fat bits of white swimming in a somewhat darker shade of white. (Click to enlarge).
In this, qingbai is like Lucknawi emrboidery: white on white. (You see a fellow in a shirt and it looks kind of crumpled, like he forgot to iron it. But then you look closer and -- oh my god -- it is covered with a field of tiny flowers!) There is a moment of wonderful surprise; and the pleasure of knowing that we have seen something most will never do.