Slipcasting

he paths to making a ceramic object are many: wheel throwing, hand building, coil building, press molding and last but certainly not least slipcasting. Although we may not think about it, slipcasting is the most widespread use of a ceramic technique, due to industrial manufacture, which tends to use this technique, which is most suitable to the manufacturing of multiples.

Once you have made a mold (see Making a Simple Plaster Mold), you can make hundreds of casts of the same object with relative ease. Molds do deteriorate with use however, and start to degenerate after a couple of hundred uses (more or less, depending on quality).

To slipcast a ceramic object we need so-called 'slip', which is liquid clay. Slip comes in various flavors, as do other clays: earthenware, midfire & stoneware. Slip is not just powdered clay thinned down with water, but rather a liquid clay with special additives. These additives, such as sodium silicate, keep the slip liquefied with as little water content as possible. This has the effect that the slip will be fairly viscous after standing in the bucket for a while. After some vigorous stirring it will become quite thin.

Things to watch out for when slipcasting:

make sure your mold is dry
make sure you have enough slip to fill your mold
have an extra empty bucket handy
place ample newspaper on your working surface
clay and plaster do not mix! -- make sure not to get plaster into your slip
prevent any foreign matter (e.g. earth) from getting into your slip
be aware that slipcast greenware has little green strength

Procedure:
Stir your slip thoroughly with a clean stick, but without introducing air bubbles into the liquid. If you have a large bucket, pour an amount into a smaller container, which will hold enough slip to fill your mold. Pour the slip through a sieve into your dry mold. (Straining the slip eliminates any lumps from getting through, which would otherwise stick to the wall of your pot.) You will be able to observe the slip adhering to the mold and thickening. At this stage it is possible to gently knock the sides of the mold (let's say with the ball of your hand or with a rubber mallet) to free any air bubbles which may be trapped in the clay. These will rise to the surface.

As the clay wall thickens, water is absorbed by the plaster and the level of the slip will drop. Continue pouring small amounts of slip into the mold to top up the level to the top edge. When you think that the correct wall thickness is reached, pour all the slip out of your mold back into your bucket. It is possible to time the period the slip stays in the mold, but this will vary depending on the moisture content of the mold. The molds moisture content will increase with every cast, thus actually lengthening the time of the cast. After a number of casts, depending on the thickness of the mold and its original moisture content, it will become impossible to proceed, and the mold will have to be dried out. After prolonged use, a white substance may crystallize from the mold on drying -- this is a normal reaction due to the additives in the slip and need not be of concern.

Depending on the thickness of your clay walls and the moisture content of the mold, the ceramic object will be dry enough to be removed after several hours. As the clay will shrink, whereas the plaster will not, the ceramic form will shrink from the mold and 'pop out'. Be gentle when removing the still moist clay object from the mold, as deforming might still be possible. Fast drying methods (e.g. microwaving) usually won't work, as the slip has little green strength and is prone to cracking. Having said this, you may then proceed to decorate and fire the work as you like.