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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.

Hump/Slump molds/Masters

Plaster makes great slump and hump molds because the clay doesn't stick. And you can make them in a variety of ways. Remember to use mold release on the object, before pouring the plaster.
You can pour plaster into objects you have around the house such as bowls and platters.
You can pour plaster into a bisque item.

Mold Release/Agents

When using Murphy´s Oil Soap or soft soap sponge on the soap with a sponge full of hot water, rinse the sponge in hot water, and rub again. Repeat this process at least 5 times. You will see the water start to bead up on the plaster surface. Once this happens, repeat twice more for insurance. Some more unusual methods of mold release are using shaving cream which is said to leave a lovely waxy film when dry. Or using a thin clay slip; the plaster absorbs the water and leaves a film of clay as a barrier.

Filling a crack

If you are filling a crack in greenware with slip, be sure to work the slip into the cracks as deeply as possible to form a solid bond. You may find using a stiff brush will help, using a back and forth movement of the bristles.

Bisque Molds/slump and hump

Bisque also makes great slump and hump molds. These can be thrown, or handbuilt, or molded from a found object. You can use found objects (mixing bowls, platters) directly by coating with vegetable oil, PAM, WD-40, etc. or by covering with plastic wrap or newspaper. Usually you will use these as slump molds, as the insides of your bowls and platters have the nice curvature you are looking for.

Making a mold with a master

Another way to use plaster is to pour it into and around existing objects. Every time you pour, you will be making a reverse of the object you are pouring into or around. You can pour plaster on top of leatherhard clay. For example, say you carved a relief design into a slab of clay, and you want a mold of it so you can make exact duplicates of that design. First you need to surround your slab with something to hold the plaster.

Mold lines

If you have a mold line that is difficult to remove from a piece of greenware, dampen the piece slightly. It will come off easier and lessen the chance of breakage

Air bubbles

Air bubbles (pinholes) in greenware can come from air in the slip from stirring too briskly or pouring too rapidly.
Air bubbles on rims of pieces can be reduced by rocking the mold while pouring.

Mixing airless plaster

Mix. You want to make sure that you aren´t adding air to your plaster, or you will get bubbles which will cause problems later. So if you use an electric mixer make sure to keep the blade deep in the plaster. Or, just take your hand (a glove is useful) and place it at the bottom of the bucket, and slowly move your hand back and forth across the bottom of the bucket. This slowly wets all the particles. When you can draw a line in the plaster and it doesn´t immediately flatten back out, the plaster is ready to pour.

Bisque Molds

1. Bisque: You can make press molds out of clay and bisque them. These work quite nicely and clay doesn't stick to them. Carve the inverse of your desired design into leather hard clay. For example, if you want raised lettering, carve the letters out of the press mold and the inverse will be raised. Remember to make your design about 10% larger than desired, to account for shrinkage.

Pottery Plaster

Buy plaster. When casting molds, Pottery Plaster #1 is best because the particle sizes are small and will capture detail the best. But if you can´t get this, normal Plaster of Paris will work.
Put on a dust mask when handling dry mixtures of this type.

One-piece molds.

The simplest mold to cast is the one-piece variety. There is no need to band it. It is easily drained and, in most instances, the casting comes free without difficulty.

Water-plaster ratio

There are actual suggested ratios of plaster to water for various plaster mixes, and some people use those. But most people I know use some rule of thumb similar to the one here, 2/3 water to 1/3 plaster to achieve the correct consistency.

Recognizing Under Deflocculation

Recognizing Under Deflocculation
If the slip is gelling after a few minutes or livering while mixing, more deflocculant is probably needed. Be careful not to add too much; this is a common mistake and will mean you will have to make more of the powder mix and add it and more water to counterbalance the oversupply of deflocculant. If the slip does not settle out overnight, then you can rest easy, it is not over deflocculated.
If the slip has not thinned after an addition, then there is already enough present. Sometimes a very small addition of water will thin the slip dramatically.

Fill handles

You should always fill the handles of cups with pouring. If they do not fill by letting excess slip settling into them, paint them full with a brush.

Dis-assembly

Before casting a multiple-piece mold, take it apart and examine it so you will know which piece should be removed first after the slip has been poured and the greenware is ready to be removed. To help you remember the disassembling order in a multiple-piece mold, use a waterproof marker to number the pieces. If there is something special that you must know about a particular before casting it, such as “drain with straw”, mark the outside of the mold with a waterproof marker. Once you have examined a new mold for any breakage and are ready to pour. Look it over one more time to see if it has any special problems you will have to cope with such as a small pour hole, undercuts, tricky draining etc.

Modifying Molds

Get More Shapes by Modifying Molds
You can modify the profile of a plaster mold after it has been made, and before it is dry. Say you want a rounded bottom hump mold, but only have mixing bowls with flat bottoms. Pour the plaster into the mixing bowl, and when it is has set but is still rather wet, put it on your pottery wheel (flat side down). Center, and trim the bottom into a round shape with a trimming tool.