The molding of vases is interesting to see. To cast and mold clay are techniques probably as old as potting itself. Kaolinitic clay molds was used already during the Shang dynasty (16-11th c. BC), for casting bronzes.

In the Chinese porcelain production an important improvement was made around 1949, when the Chinese porcelain potters modernized the process by starting to use vertically divided molds made of gypsum (plaster of Paris) instead of the earlier, horizontally divided bisqued ceramic molds.

As I understood it when I got this explained for me, the properties of these two types of molds are different so that the absorbtion of moist are more rapid with plaster molds, the details could be cast more sharp and the match precision in between the sections of the molds are better since there occurs no warping in plaster.

The method of slip casting vases is simple to understand, once you have seen it being done.

The original shape of the item which is to be made by molding, is solid and made of polished steel. The plaster mold is then cast in two halves around this steel shape. When the mold is ready to be used it is separated in two halves and hollow where the steel shape was.

Liquid porcelain clay is then just poured into this hollow area inside the plaster mold. The plaster absorbs the moist in the clay closest to its walls.

The surplus liquid clay is then almost immediately poured out of the mold, and after a while the mould can be opened and put into the sun to dry before next use.

The thickness of the walls of the cast piece are decided by how long time you wait before pouring out the surplus clay slur. After some finishing the vase is ready to be decorated, glazed and fired.