Design considerations for simple two part moulds.
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This illustrates why the apple - as a model - could not be divided into a two part mould from north to south. When cast in soft clay slip, the apple, as seen in the right-hand picture, would be locked into the mould. There are undercuts at stalk and blossom end. The first page showed that the dividing line would have to go around the equator. Then, if you wanted an apple without an enormous worm hole in the side, you would have to think of a way of sticking a patch over the filling hole. Or, see below.
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If you do make an enclosed void of any model, it is essential to provide a relief hole - just a pin-hole will do - so that the trapped air can escape as the clay shrinks, expands when heated, and shrinks again after firing. See below, in the centre of the orange. |
I did make the moulds of apples and oranges as well as bananas, for the commission referred to in the introduction. |
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Don't forget the relief hole, seen in the centre. |
This
orange was made in a two part mould, but cast as two separate halves
and stuck together while still in the mould. Details below. | Moulding
an Enclosed void. An alternative way of moulding for an enclosed void like the orange or the apple, is to have the break line around the equator, and make a two part mould in much the same way as the banana mould. In the case of the orange, I used a round piece of plaster as the base, and flexible vinyl flooring as the cottle, wrapped around the base and the clay bed - twice around to prevent leaks. Cast the stalk end. Take off the cottle and remove the clay bed, trying not to disturb the model. Instead of deep dimples for natches, I cut shallow scoops out of the plaster around the circumference, two close together and one on the other side. Then I cast the second half. However, there was no filling hole. Instead I put a temporary cottle around each half of the mould, and cast clay slip into each part of the open mould. In fact, I was treating it as two one-part moulds. Pour off the excess slip after the necessary number of minutes (20 - 30 for the orange). When the slip no longer looks glossy, but before it is really ready to come out of the mould, remove the temporary cottle, and cut the waste off leaving a smooth edge. continued right... |
Hatch the edges and wet them, paint the clay with slurry (slightly stiffer than normal slip), and drop a tablespoon of slip into each half. Immediately, put the two halves of the mould together, matching the ridge and recesses carved into the plaster, and shake the mould so that the liquid slip inside flows over the join. Then put a rubber band around the mould. The cast can be allowed to dry to leather-hard before unmoulding. In shrinking, it will pull itself away from the plaster mould. Immediately it is out, push a needle through either the stalk or blossom end, as shown above, to make the relief hole. You may even hear the air rush in as the needle is removed. A sphere, like an orange, needs to be supported in the kiln on three points. Pottery suppliers sell all sorts of kiln furniture, but small 'pips' are quite easy to make. The essential is to have a firm base and one thin long cone pointing up. Make them of fire-clay or stoneware clay if you have it. Fire them first, before using them in the glaze firing. Balance the sphere on three of these 'pips'. The thin points will break off rather than damage the piece. If a mark shows, rub it off with a fine white grind stone. |
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