Slip casting, or slipcasting, is a ceramic forming technique for pottery and other ceramics, especially for shapes not easily made on a wheel. In this method, a liquid clay body slip (usually mixed in a blunger) is poured into plaster moulds and allowed to form a layer, the cast, on the inside walls of the mould. The process usually takes at least 24 hours per piece. It gives very precise and consistent shapes, and is now the most common technique used for commercial mass-produced pottery, although it began as a technique for fine pottery, especially porcelain.
In a solid cast mould, ceramic objects such as handles and plates are surrounded by plaster on all sides with a reservoir for slip, and are removed when the solid piece is held within. For a hollow cast mould, for objects such as vases and cups, once the plaster has absorbed most of the liquid from the outside layer of clay the remaining slip is poured off for later use. After a period for further absorption of water, the cast piece is removed from the mould once it is leather-hard, that is, firm enough to handle without losing its shape. It is then "fettled" (trimmed neatly) and allowed to dry out further, usually overnight or for several hours. This produces a greenware piece which is then ready to be decorated, glazed and fired in a kiln.
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, very often the entire production. It is also commonly used for sanitaryware, such as toilets 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.
An additive with deflocculant properties, such as sodium silicate, can be added to the slip to disperse the raw material particles. This allows a higher solid content to be used, or allows a fluid slip to be produced with a minimum of water so that drying shrinkage is minimised, which is important during slip casting.
Slip cast ware should not be confused with slipware, which is pottery formed by any technique that is decorated using slip. The French for slip is barbotine (coulée en barbotine means slip casting), and "barbotine pottery" is sometimes used for 19th century French and American pottery with added slip cast decoration, as well as (confusingly) being the English term for a variety of slipware that is decorated with thick blobs of slip.