Slip casting is an easy way for making a variety of ceramic patterns; here are some directions for making a hippo design:
1. A clay model being prepared for molding. By using the simpler model, the process will be easier and you will get fewer parts to the mold. This one has been divided into two with a flat clay strip about two inches wide which is well supported with clay to take the weight of plaster A clay enclosing-wall can be added round the bedding strip to retain the plaster while it sets and a clay cone can be attached at the mouth of the animal to form the 'spare' of the mold, through which it will be filled with slip.
Buttons of clay are spaced on the clay bed to serve as keys, when fitting the two halves of the mold together. When the model is ready, plaster should be mixed and, just before it begins to set, poured over the model to a depth of about two inches, the same as the clay bed. Details which protrude can be detached and molded and cast separately to simplify the process.
2. When the first half has set hard, the whole job is turned over and the clay bed removed, leaving the model half bedded in plaster. Then the second half can be molded in the same way as the first. Before this, however, the plaster face must be treated to prevent the second batch of plaster sticking to it. This is done by soap-washing the plaster face, creating a greasy surface to which fresh plaster will not stick. When the second part has set hard, the two halves can be separated. The cylindrical spare piece can be removed at this stage.
3. Removing a cast from the mold. Before casting, the mold must be dried out and this should not be forced by heat. Molds can quite easily warp, and, unless tied together for drying, parts may be found not to fit later. For the casting, the bound mold should be filled with slip until the spare is full, allowing for the level of the slip to drop as water is absorbed by the mold and it should be kept topped up. Small molds like the ears can be cast solid by pressing in plastic clay, waste from the casting being used. The thickness of the cast can be judged by the clay lining the spare.
When judged right, the mold is allowed to drain completely into the casting slip and a note is made of the time taken from filling to emptying for repetition purposes. The drying of the cast should be watched, and, when shrinkage from the mold is apparent, the clay in the spare can be removed and the cast loosened by tapping the mold. The mold is then untied, the sections eased apart and the cast removed. It is still plastic and easily damaged at this stage and should be put on a clean bat to harden, the mold being bound for refilling or drying out.
4. Assembling and fettling the cast. Casting slip should be used for the joining together of molded sections immediately after they have been removed from the mold. Apart from attaching the separately cast parts of a model, there is considerable latitude for modifying the design while the clay is still in a plastic state. Figures cast in one plane can be changed into complicated positions which would be very awkward to mold. The final fettling or cleaning up of seam marks can be done when the model is dry.
Your slip casting hippo is now finish, you can now go on to exploring other pottery creations.
Sources:
W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics, John Wiley & Sons, Inc., 1976, ISBN 0-471-47860-1.
