An ordinary mixture of clay and water cannot be used as casting slip as the clay particles tendency to draw together into groups, electric attraction, leaving the water at the top and heavy sludge at the bottom. In order to disperse the clay particles, so that each particles is floating by itself, sodium silicate is used as a media to electrolyte or deflocculant the clay particles. The electrolyte has an effect to causing the clay particles to repel one another and floating individually in the water rather than clinging together in groups. After casting slip is poured into a dry plaster mould, capillary action of the plaster will absorb high proportion of water from the slip, a layer of clay wall will be formed in the inner surface of the mould. The thickness of the clay wall is determined by the length of time of the slip remain in the mould, then pour out the excess slip leave it to set then the casting object can be removed from the mould.
It is the beginning of everything. The object to be cast should have a smooth surface, as it will be easy to remove from the mould.
A cottling board as its base. Find the centre line of the model. My model is quite easy to find the centre line because it has a seam line I can follow. Cover half of the model with soft clay along the centre line.
DON’T FORGET THE “SPARE”, it is an extension of the mould for pouring slip into the mould. Then place cottling boards around the model, tie it tightly with string. It is very important that to seal all the edges with soft clay in order to prevent the plaster leaking. Weight right amount of plaster and water, mix well, skim away all the bubbles on its surface then pour it into the mould. It takes about 30 minutes to completely set. After the first half of the mould has set, cottling boards and soft clay can be removed. Make several natches on the joining surface on the mould in order to easier to locate the other half of the mould when doing the casting. Soap wash the joining surface for three times. To do the other half, place cottling boards around the mould and seal all the edges with soft clay again. Then weight right amount of plaster and water as same as the previous procedure.
I took nearly the whole first term to finish this doll and the head is not done yet. I think I will throw the head on the wheel instead of making a mould because I can’t completely remove its hair.
slip casting can perfectly pick up complex and fine details of the object.