LENHAM POTTERY MODELS
making high-fired semi-porcelain models since 1969
Additional information on pottery mould making: making a simple two part mould, page three of five

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Pouring the plaster for the first part.

Pouring the first part of the plaster mould.

This picture shows the wedges in place holding the mould walls firm.

Potter's plaster for ceramic slip casting should be mixed in the proportion of 70 parts weight of water to 100 parts weight of dry plaster. Even if the plaster is new, sieve it into the weighing container with a coarse (flour) mesh. With experience, one can judge the volume of water needed,(calculations at the bottom of this page) and sift the weighed plaster onto the surface of the water. Allow it to sink of its own weight and soak for one or two minutes before mixing. Stir gently  with a kitchen tool, preferably the sort with 'fingers' or coils which breaks up any lumps, for two minutes. Allow to stand and shake or jiggle the jug to persuade the air bubbles to rise. When a finger trailed across the surface leaves a mark, it is time to pour slowly from one corner of the mould so that the creamy liquid flows across the model, leaving no air bubbles trapped in corners. Jiggle the mould again to persuade the plaster to settle level on the (level) bench.

 

After the clay bed has been removed.

The plaster can be engraved with the name of the model and the part number when the surface gloss has gone but before the plaster sets hard. This is the plastic stage. Do not attempt to trowel or otherwise modify the plaster after crystallisation has begun. The plaster will heat up as the chemical reaction continues and the larger the part, the higher the temperature.

Do not attempt to disturb the mould until the heat generated has gone. When it is cold, remove the walls, peel off the clay bed, trying not to disturb the model, wash the plaster and soak it well in clean water. Then cut the natches and wash away the chips of plaster.

Cut natches into the first part of the plaster mould.The round blue marks are natches, or the dimples that provide the locating recesses for the second part which will have pimples. It is sensible to make a practice of putting the dimples assymet- rically so that it is impossible for the mould to be assembled incorrectly. It is obvious in the case of the banana which way round the second part should go, but may not be so obvious on a round model. (See, later, the apple mould.)

Do not cut the natches too deep.

The bowl of a steel teaspoon makes a good 'chisel' for cutting natches, but do not make them too deep. They are locating dimples for the next part which will have the corresponding pimple, but they should not be press-studs, locking the mould parts together.

MORE PLASTER CALCULATIONS: 65 parts of water will give a heavy, dense plaster which will not absorb water very well. (I use the term 'suck', as in, "it has little suction.") 75 parts of water to 100 parts of plaster will give a soft, absorbent mould which will wear easily and may break, especially when damp and held unevenly. If 70 parts of water happens to be 700 millilitres= 700 grammes, and you put into the jug 100 parts by weight of plaster= 1000 grammes (1Kilo), the volume will not increase to 1700 millilitres, because a large part of the volume of the plaster will be air.

The volume will be about three fifths larger than the original amount of water. 700/5 = 140. Then 140 times 3= 420. Then 420+ the orginal 700= 1120 millilitres or just over a litre.
Most people can estimate the volume of liquid in a plastic drinks bottle. They come in cylinders, I know, of a half a litre, 75 cl (bottle of wine) and one litre. But we get milk today in square bottles, and this makes it easier to judge the volume required to fill the mould. I try to make a note of the volume of water needed for a particular mould part. A simple table will give me the weight of plaster required.

Donald E Frith, in Mold Making for Ceramics, (ISBN 0-8019-7359-7), is the expert writing on this subject. He is also good on slip-making.

The next page is boxing up for the second part.