Question:

How can I make a mould?

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.

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!

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.

Gang and Core

A gang mold is what the name implies, a gang of molds made into one. Chess sets are usually gang molds.
Core molds are very similar to gang molds except that they have only one pour gate that branches inside the mold

Sand Seams

After the bulk of seam lines have been removed with a tool, the next step is to sand the lines with a sander sponge.

Separation in mold

If a plate or bowl separates into two layers, either you opened the mold too soon or you drained it too soon.

Alternative

You don't have to mess with plaster to make molds. You can make your own from bisque. Fire the bisque at a high enough temperature to give it strength, but low enough that it is still porous so the clay dries and does not stick. To make a bowl mold, throw a solid piece on the wheel, smoothing the edges well with ribs, etc. When it is leather hard hollow out the inside until your edges are about 1 inch thick. Fire.

“cases”

Did you know that “cases” are the frames around a block used in the formation of ceramic molds?

Facts

A mold is like any other too in that it will wear out after prolonged use.
Molds are expensive and should be cared for properly like any other investment
If you have poured a mold a number of times and it shows wear it is possible to scribe some of the detail back by carefully carving with a sharp tool.