Historically, the worst problem for the potters was to arrive at the correct firing temperature. If it got too warm, the porcelain melted and got warped. To stop this, was why Kaolin was added to the clay. Kaolin strengthened the porcelain clay and gave it a larger temperature interval during which it could become hard but still not get out of shape. On export / minyao porcelain and very early Ming porcelain, the kaolin content were low, probably by economic reasons. Overfired pieces then often melted and got thrown away, or was sold as warped. During all history there has been a "second rate porcelain" market going on in Jingdezhen, where somewhat failed porcelain has been sold at very low prices to private porcelain peddlers. At this market "shape" has traditionally been rated as a big failure. After having done some porcelain lifting and sorting myself I have realized that this might very well be because of transport economics. To ship economically one item must fit perfectly into one another or they will take up more space or actually cause the nearby items to crack. This in itself could motivate the expenses and trouble caused by the addition of Kaolin by a high volume ceramic industry like Jingdezhen.