Information on MOULD-MAKING.

All the items I make are cast into dry plaster of paris moulds which I make myself. There is a very popular hobby of colouring slip-cast pieces, Hobby Ceramics, where the organiser does the casting using bought slip and ready-made moulds. These moulds are beautifully and very cleverly made in two or three parts. As explained on the previous page (Materials) I have always found it better for my own use to make a mould of a horse with all the legs and ears in place, or cast a traditional bath with the little legs attached. A horse mould can be made up of ten to thirteen parts. I think everyone can get their head around the idea of a two-part mould, left side, right side, with a hole between the parts at the top into which the slip is poured in and out, leaving a stiff skin of clay on the inside. A three-part mould is left side, right side and base. A bath mould is left side, right side, top (which sets the shape of the lip of the top edge) and base (which fits between the four legs.) A horse mould has left side, which may include part of the face, right side, front chest, under chin, under tummy which is the keystone wedge, under front leg, under back leg, and back or bottom, which is where the tail goes. This is a minimum of eight. The tail is cast in a two part mould and attached over the filling hole. Often there is another little bit which fits between the ears. The trotting pony, which is featured in the Horse pages, has thirteen parts because two legs are off the ground.

The small mould parts are laid onto one of the large sides, and the other large side is laid down onto the jigsaw, locking each part in place. The mould is held together with rubber bands cut from lorry tyres, four around the front to back, and four more (minimum) top to bottom. The Shire mould, when full weighs more than 25 kilos, and cannot be lifted by hand.  All I can do is tip it so that the liquid clay flows out gently while I blow air in through a tube to replace the slip, otherwise the suction can collapse the body.

Continued top right.

MODEL-MAKING for ceramic slip-casting.
A knowledge of how the liquid clay behaves is essential. It shrinks as the water is sucked out by the dry plaster mould. But even then it is soft and easily deformed. The mould parts must be drawn off in a straight pull. Therefore the original model must be conceived with the knowledge that there can be no undercuts to catch hold of the soft clay. I have taught people how to make models and the moulds they need to cast ceramic figures but much of the technique is a case of trial and error and learning from experience. It is not a skill that is taught in Art classes.

I have used many materials to make the original models. For the shapes required for simple dolls house china models, plaster or brass can be used. Both can be turned on a lathe. Plaster can be cut, shaped and sanded like wood. Pottery plaster shrinks as it turns from a liquid into a solid, and has the unfortunate ability to cling tightly to the model unless the angles are well designed. For a horse, I make an armature out of brass rod, pad it with string, plaster of paris bandage, and then more plaster to get to within an inch of the final surface. Then I apply plastic clay or modelling compound. Around this soft model of the horse I cast a silicone rubber mould, in several parts. Into the rubber mould I cast more silicone rubber, and then fill that rubber shell with a hard resin compound. Out of this mould comes a rubber horse, which is tough enough to withstand the handling that comes from making the working plaster mould parts.

The original clay models can be discarded because the rubber model is carefully preserved so that fresh working plaster moulds can be made when the first ones wear out. I reckon on getting 50 casts from a good plaster mould.

Has this information been of interest? Would you like to try it?
If so, there is a related set of pages illustrating the step-by-step making of a simple two-part mould, and then the seven steps to the making of the Shire Horse model. All this and much more besides is described and illustrated in my book, "Mould Making for Artisans".

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