In Chinese Porcelain in the Collection of the Rijksmuseum Amsterdam, The Ming and Qing Dynasties by Christiaan Jorg and Phillip Wilson (The Rijksmuseum, 1997) Christiaan Jorg states that
"Chinese soft-pate porcelain, which is different from European soft-paste, originated about 1700 and became popular in the second quarter of the 18th century as part of the export assortment.
Unlike ordinary porcelain, it is not translucent and often has a creamy-white appearance. The glaze is often finely crackled as a result of a difference in cooling between the glaze and the body. The later is made of a white-firing clay, called huashi, 'slippery stone', the use of which is documented in the reports of 1712 and 1722 by the Jesuit Pere d'Entrecolles. As the clay was expensive, soft-paste pieces are usually small and thinly potted. They are also well-painted, as the body is particularly suitable for detailed drawing.
Besides this 'true' soft-paste, there are pieces with an ordinary porcelain body and a coating of 'huashi' clay, which gives the same effect".
My theory is that this 'huashi' clay or "slippery stone" is plain and simple kaolin that stiffens the porcelain paste and fires very white. For anyone who have ever handled Chinese kaolin clay there are almost no other things on this planet that are more 'slippery' then unfired kaolin clay. Handling a lump of this material is similar to handling a jellyfish. The name slippery stone would seem very appropriate, in particular since Kaolin is not a technical term, but the location (Gao ling, High Ridge) from where it was mined until at least the end of the Ming dynasty, where other sources were found.