Pottery: Mold a Shallow Dish from a Clay Model

Hollow plaster molds are made from solid clay models, shaped to the inside section of the shallow dish. There must be no undercut or turning in at the lip, which would prevent easy release of the shallow dish from the mold.

Shapes which are circular in plan are made with plastic clay directly on the wheel head or a circular bat. Other regular and irregular forms will need to be modeled accurately by eye or by the use of a template on stiff clay. They are best done on a plaster bat, using a template to shape the clay model. The bat should be about half an inch thick and cut or molded exactly to the plan of the shallow dish.

To make a bat, a clay wall, carefully enclosing the plan drawn on a flat surface, serves as the mold into which plaster is poured and sets level. This bat is a guide for the template as it is moved round to shape the clay model. The template can be cut from a stiff card and an allowance should be made for the height of the bat; the bat itself should first be covered with thin paper so that the model can be easily removed onto a larger board ready for molding. Both in the modeling and later in the molding process the work is easier and more efficient if it is carried out on a turntable.

Before you start to mold the shallow dish a wall must be placed round the model to retain the plaster when it is poured over it. With molds for shallow dishes up to nine inches across this wall must be built about one and a half inches from the model to allow for the thickness of the mold. The space should be proportionally wider for larger dish molds. With circular ones made on the wheel, this space should be allowed for on the wheel head and the wall made simply by tying a strip of zinc round the head. For other simple shapes, the zinc is placed upwards of one and a half inches from the model and sealed with clay, but with irregular ones a strong clay wall is used. It should be high enough to clear the model by two inches since this will be the thickness of the mold in the middle. With a clay wash over the board or the wheel head to stop the plaster sticking, the model is ready for molding.

Plaster of Paris is mixed by adding the powder to the water in correct proportions. Too much powder will make a stiff mix which sets quickly without flowing and too little will make a weak mold. Plaster varies in quality and grade. With potters' plaster the proportions should be about twenty-five ounces to the pint. Practice and experience is the best guide, not only for this but also for the quantity needed for the job.

The water is put in a bowl or open jug and the powder sifted in evenly by hand so that the water is absorbed without lumps forming. When all the powder is soaked, the water is stirred, working smoothly by hand to prevent air bubbles from forming. Blending continues and after several minutes the mixture begins to thicken; air bubbles which have formed can be surfaced by tapping the bowl. When this thickening or initial setting has definitely begun, the plaster should be poured gently and evenly over the middle of the model while it is turned slowly to allow the plaster to level itself. The final setting and hardening is indicated by the heat generated through the chemical action of water on the plaster. When this is felt, the zinc or clay wall can be removed but the plaster is still soft enough for trimming with a steel scraper or hacksaw blade. Sharp corners should be rounded off because they chip frequently and because a mold will turn easily on the bench when being used for pressing if the bottom face is slightly curved. Cold water on the mold will release it from the wheel or board and the clay model.

It is important to remember that a chemical action takes place in the mixing of plaster and that adding water during the stirring to a mixture that is too thick will result in killing the setting and making it useless. Plaster, while being a very useful material in pottery, can be disastrous if particles or pieces get into clay since this causes trouble in firing. Cleaning up should therefore be thorough and waste plaster should be properly disposed of in the rubbish bin, not in the sink.

Molds will gradually dry out with use and should not be forcibly heated; otherwise they will soften or crack. They can be easily duplicated from each other. After being washed and lathered several times with a solution of soft soap, a plaster surface develops a greasy film which will not allow fresh plaster to stick. The mold, with the zinc strip tied firmly round it, can then be filled with fresh plaster. When this has set hard, the mold and the plaster cast can be separated, the cast being a copy of the clay model. This in turn needs soap washing before it can be used for making molds. It can also be used as a convex mold.

Deep or irregular molds may be difficult to separate from the cast. In the case of this shallow dish, the original mold may have to be broken, but the molds then made from the plaster model are more easily removed. The clay model should be lightly soap washed each time another mold is made.

Sources:

Peterson, Susan. The Craft and Art of Clay. The Overlook Press, Woodstock, NY, Second Edition 1996. ISBN 0-87951-634-8.