Casting

Slip casting is used to produce clay-based ceramics from clay-water slurries contain­ing 25 wt% water or more [26]. For casting slips, the water content is high enough so that all of the particles in the system are separated by free water. Most often, slip cast­ing requires no applied pressure, although many industrial shops have switched to pressure casting (slip casting with an applied pressure) to improve productivity and reproducibility. Slip casting requires a well-dispersed, stable suspension of ceramic particles and a porous mold, which is most often gypsum (hydrated plaster of Paris) [22]. When the slurry is poured into the mold, the pores in the mold draw water out of the slurry, causing particles to deposit on the mold surface (Fig. 12) [15]. When the cast layer has sufficient thickness, the excess slip is poured out, leaving a thin, nega­tive replica of the mold. The replica is partially dried in the mold until it pulls away

Casting

Casting

Fig. 11 Schematic illustrations of the processes of (a) jiggering and (b) ram pressing used for soft plastic forming of clay-based ceramics (a reprinted by permission of Addison-Wesley from F. H. Norton, Elements of Ceramics, Addison-Wesley Publishing, Reading, MA, 1952; b reprinted by permission of the McGraw-Hill Companies from F. H. Norton, Fine Ceramics, McGraw Hill, New York, 1970) [22,3]

from the sides of the mold and is rigid enough to be removed without deformation. Slip casting is used to form objects that are not conducive to jiggering or jollying because of a lack of radial symmetry or those with complex surface details such as decorative figurines. Slip casting is also used to form objects that are difficult to form by other techniques because of their size such as radomes, large crucibles, and large diameter furnace tubes [25]. Slip casting does not work as well with solid objects due to the problem of removing water uniformly and differential shrinkage [1].