Casting, Slip Casting

Casting, Slip Casting
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Slip casting is a specialized way of using plaster moulds for mass production of ceramics, especially for shapes not easily made.
. The word "slip" means a single clay or a mixture of clays and other materials that has been mixed with enough water to make it liquid, so that it can be poured into a plaster mould.
Slip casting is a fairly new process in the history of ceramics, as it only became practical after the invention of plaster of parts 200 years ago.
Forming pottery by pouring deflocculated (water reduced) clay slurry into plaster molds. In the process the absorbent plaster pulls water from the slurry and over a period of minutes a layer builds up against the mold surface. The slurry is then poured out and within a short time the item shrinks slightly and can be removed from the mold.

The sanitary ware industry produces the largest tonnage of products using this process. Water closets and sinks are made by casting porcelains in very complex many-part molds. Molds are heavy, cast sections are thick and take considerable time to release and extract from the molds. By contrast, fine delicate porcelain items can be cast very thin and quickly.
The technique is suited to the production of complex shapes, especially if with relief decoration and thin walls. Much modern fine factory porcelain is made by the technique. It is also commonly used for sanitary ware, and basins, and smaller pieces like figurines and teapots. The technique can also be used for small-scale production runs or to produce limited edition, one off objects, especially reproductions of antique dolls and modern porcelain doll-making.


But almost any type of powdered ceramic can be made into a slurry and cast. If the slurry can be suspended and deflocculated, it shrinks enough on drying and has enough strength to hold itself together as it pulls away from the mold, it can be cast. Even non-plastics like calcined alumina and silicon carbide can be cast by incorporated small additions of plasticizers and binders.
In industry, casting rate is important. Items need to cast quickly and extract from the mold quickly. Molds need to be dried rapidly. Much automation has been devised to speed these processes. In addition, much can be done to fine-tune the balance of permeability and plastic strength of a body. It is important to understand the role of each material in the body recipe and other options for each of those materials. A typical industrial casting body has much more kaolin, less ball clay and usually no bentonite.