Slipcasting

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 in Europe as a technique for fine wares such as porcelain by the 1750s.

The type of clay body suited for slip casting differs from that for throwing. It is essential to make a good quality casting slip to get an intended result. The behavior of the slip will depend on multiple factors, including the types and proportions of water, clay, other chemicals, and deflocculant used; temperature, humidity and other local conditions; and the amount of energy involved in mixing the ingredients together to form a suspension. The process of changing a slurry from something thick and gooey to something thin and pourable that can be used in slip casting is called deflocculation. The process by which particles come out of the suspension is called flocculation.

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.

History

The technique was first developed in China during the Tang dynasty (618-917), but was relatively little used in China until recent times. It seems to have been reinvented independently in England around 1745 "reputedly by Ralph Daniels of Corbridge", and before long was very widely used in European porcelain factories, becoming the main forming technique for expensive wares by the end of the century.

Technical considerations

Accurately measuring the viscosity and specific gravity of a slip is important for determining whether it is properly mixed and ready to use. The specific gravity should be measured and adjusted before making adjustments to the viscosity.
The amount of time needed to form a casting will vary with the type of slip used in the casting.

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.

As a processing technique for ceramic laminates, a ceramic powder is often placed in suspension to form a slip with a high solids content (>60 wt%) as well as a very low viscosity value (<40 mPa). The suspension is poured into a porous mold. As the mold dries, a solid particulate layer is formed on the mold through deposition of the solids in the slip. A series of ceramic layers can be formed by changing the composition of the slips used in repeated castings. If the chemistry of the materials being used is well understood, the thickness of the layers can be controlled by varying the length of time involved in the casting.

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.

Slipware

Slip cast ware, objects that are formed using slip casting, should not be confused with slipware, pottery that is formed by any technique and then decorated using slip. The French for slip is ''barbotine'' (''coulée en barbotine'' means slip casting). As far back as the Roman empire, potters created what is termed "Barbotine ware" by using clay slip to decorate the surface of pots. "Barbotine pottery" is sometimes used for 19th century French and American pottery with added slip cast decoration, as well as (confusingly) 17th English slipware that is decorated with thick blobs of slip.

References

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