laster molds are used in ceramics to pour multiples of a single object with liquid clay (slip). Making a mold can be a daunting task, depending on its complexity. Mold making is a highly skilled and highly payed job, e.g. with large ceramics companies like Rosenthal or Royal Doulton. On the other hand, anyone can learn the basics of simple plaster mold making.
The first step in making your mold is to put together a recepticle for the plaster. This we will make by nailing the four boards together to make a bottomless box. The edges should not be flush, but actually sticking out. Make sure you don't nail them together too well, as you will be taking them apart later. Once the boards are nailed together, place them on a smooth surface, like a table-top, and wedge clay sausages into all overlapping seams. Make sure none are left out, as this will stop any plaster seepage. Now place your bowl shape flat side down into the center of the box. Push it down a little so that it sticks (moisten if necessary). Mark the height of the foot of the bowl with a marker pen on one of the inside boards. Mark a second line at about 50% higher.
Now you are ready to mix the plaster. Follow the instructions for plaster mixing on the bag. Make sure you don't breath in any plaster dust, as this can be health hazardous. Use a mask if necessary. Thoroughly mix the plaster in the bucket with your hands to get rid of any lumps. (Some peope may prefer to use rubber gloves for this, as the plaster is alkaline and may irritate the skin). You should have a minimum of 10-15 minutes for this, before the plaster starts to set. After this period, (but before setting), pour the plaster over the cup shape, right up to the second mark. Excess plaster can be left in the bucket to harden. This can be knocked out later.
Let the plaster harden for at least a couple of hours, better overnight. Then, using a hammer, carefully knock the boards apart. You now have a plaster mold with a positive form sticking inside. The clay bowl form will shrink on drying and then easily pop out - voila! you have a plaster mold for making multiple tea bowls, either by slipcasting or press molding.
Important safety precaution: make sure you do not breath in any plaster dust. If necessary wear a mask.