Question:

How do I slipcast ceramics?

Opening the mold

After the mold has been drained of all its excess slip, the mold has to sit and dry. The mold pulls the water out of the clay and dries the piece out.
If you leave a piece in the mold too long, it will dry out too much and crack.
If you don't leave it in long enough, it won't let go of the mold and will probably tear or it will collapse under its own weight.
When I pour, I run a fan across the mold to help dry things out. Six to eight hours later, I go back and take the pieces out.
Larger pieces take longer to dry -- as much as a couple of days.
You can tell if a piece is ready to come out of the mold if it has pulled away from the sides of the pour hole.
When it is time to open the mold, remove the bands from around the mold and gently pull the mold apart. If the piece is dry enough, it should release immediately.
Try to pull the mold halves straight away from the piece since the clay is still soft and can be easily scarred or marred. The piece should handled carefully since it so soft.
When the piece comes out of the mold it will be a dark gray. Set the piece up and let it finish drying until it turns a light gray.
After the piece has dried to a light gray, it is ready to clean and fire in the kiln.

Procedure

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.

How to

How to Slipcast Ceramics
Slipcasting is a widespread ceramic technique, suitable to the manufacturing of multiples. A plaster mold is required.
Here's How:
If your mold consists of several pieces, tie them together with thick rubber bands or something suitable to the molds size.
Stir your slip thoroughly with a clean stick, but without introducing air bubbles into the liquid.
If you have a very large bucket of slip, 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 in subsequent casts, depending on the moisture content of the mold.
Depending on the thickness of your clay walls and the moisture content of the mold, the ceramic object will be dry enough to remove after several hours.
As the clay will shrink, whereas the plaster will not, the ceramic form will shrink from the mold and 'pop out', if the form allows it to.

Tips:
Make sure you have enough slip to fill your mold.
Have an extra empty bucket handy.
Clay and plaster do not mix -- make sure not to get plaster into your slip!

Slip

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 liquified 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.

Description

Molds for slip casting are made of plaster. This is because the mold must absorb water in order to form a clay body.
Slip refers to the liquid clay. Clay comes in many forms; the texture and chemical content of the slip dictate what type of clay body it forms.
Slips with a very fine texture form porcelain, which is white and extremely delicate and must be supported in the kiln during firing.
On the other end of the spectrum is stoneware, which has a very coarse texture and must be fired at a very high temperature.
Ceramic slip, which is also a clay, is what I work in and have the most experience with. It falls in the middle as far as its texture.
Putting your hand in a bucket of slip is like slipping your hand into a muddy ooze, which feels much like the mud pies we all used to make as children, (only with a much finer granular texture, anyway).